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Mars Express Mission

Spacecraft     Launch    Mission Status     Sensor Complement    Ground Segment    References

Mars, our most Earth-like planetary neighbour, beckons. Its pristine and diverse surface, equal in area to Earth’s land surface, displays a long and fascinating history, punctuated by impact events, volcanism, tectonics, and aeolian, fluvial and glacial erosion. A century ago, astronomers believed they were witnessing the last attempts of a dying martian civilisation to cope with the devastating effects of climate change. The notion of an intelligently inhabited Mars was later dispelled, but the expectation that simple life forms could have survived persisted. Today, after sending robotic missions to Mars, our view of the planet retains some striking similarities to those earlier romantic conjectures. 1)

We know from orbiting spacecraft that Mars has undergone dramatic climatic and geologic changes. Water coursing over its surface in the distant past left dramatic evidence in deeply carved channels and fluvial networks. Yet today we find the planet is cold and dry. There is no evidence so far that life exists there now, but primitive life during Mars’ warmer, wetter past is a real possibility. So, mysteries remain: how did our Earth-like neighbour arrive at its present parched, cold and almost airless state? Did life evolve and then die out? Did it leave a fossil record? Last but not least, can the changes experienced by Mars teach us something about the dramatic changes being predicted for our own planet?

These and other questions have spurred scientists and engineers to meet the enormous challenge of sending missions to Mars. A Mars-bound spacecraft must survive journeys of more than 6 months, approach the planet from just the right angle and at the right speed to enter orbit, and then operate successfully to return valuable observations. Some missions have failed, but the successes have more than repaid the effort and risk. Our knowledge about Mars has grown dramatically with every successful visit. Four decades of space-based observations have produced more information and knowledge than earlier astronomers with Earth-bound telescopes could have imagined.

Mars Express is a space exploration mission of ESA (European Space Agency), Europe’s first mission to the Red Planet. Mars Express is so called because it was built more quickly than any other comparable planetary mission. Beagle 2 was named after the ship in which Charles Darwin sailed when formulating his ideas about evolution.

The Mars Express mission is dedicated to the orbital (and originally in-situ) study of the interior, subsurface, surface and atmosphere, and environment of the planet Mars. The scientific objectives of the Mars Express mission represent an attempt to fulfill in part the lost scientific goals of the Russian Mars 96 mission, complemented by exobiology research with Beagle 2. Mars exploration is crucial for a better understanding of the Earth from the perspective of comparative planetology. The mission's main objective is to search for subsurface water and deploy a lander onto the Martian surface.

It carries seven instruments and deployed a lander, Beagle 2. The lander was lost during its attempt to reach the planet’s surface but the orbiter continues its highly successful on-going global investigation of Mars and its two moons, Phobos and Deimos.

ESA provided the launcher, orbiter and operations, while the instruments were provided by scientific institutions through their own funding.

Scientific objectives:

The Mars Express orbiter is the core of the mission, scientifically justified on its own merit by providing unprecedented global coverage of the planet, in particular of the surface, subsurface and atmosphere. Beagle 2 was selected through its innovative scientific goals and very challenging payload. The combination of orbiter and lander was expected to be a powerful tool to focus on two related issues: the current inventory of ice or liquid water in the martian crust, and possible traces of past or present biological activity on the planet. The broad scientific objectives of the orbiter are:

- global color and stereo high-resolution imaging with about 10 m resolution and imaging of selected areas at 2 m pix–1;

- global IR mineralogical mapping of the surface;

- radar sounding of the subsurface structure down to the permafrost;

- global atmospheric circulation and mapping of the atmospheric composition;

- interaction of the atmosphere with the surface and the interplanetary medium;

- radio science to infer critical information on the atmosphere, ionosphere, surface and interior.

The ultimate scientific objective of Beagle 2 was the detection of extinct and/or extant life on Mars, a more attainable goal being the establishment of the conditions at the landing site that were suitable for the emergence and evolution of life. In order to achieve this goal, Beagle 2 was designed to perform in situ geological, mineralogical and geochemical analysis of selected rocks and soils at the landing site. Furthermore, studies of the martian environment were planned via chemical analysis of the atmosphere, local geomorphological studies of the landing site and via the investigation of dynamic environmental processes.

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Figure 1: Illustration of ESA's Mars Express spacecraft (image credit: ESA)


Spacecraft

Mars Express is a pioneer - and not just because it is Europe's first mission to the Red Planet. It is also pioneering more economic ways of building space science missions at ESA. These new working methods have already proved effective and will be applied to future science missions in the agency’s long-term scientific program. 2)

ESA is spending just 150 million Euros (1996 prices) on Mars Express, which is about one third of the cost of previous similar missions. This sum covers for the spacecraft, the launch and the operations. Orbiter instruments and the Beagle 2 lander are provided separately. The mission was also built unusually quickly to meet its narrow launch window in June 2003.

Savings are being made by re-using existing hardware, adopting new project management practices, shortening the time from original concept to launch, and procuring the most cost-effective launcher available.

Mars Express is making maximum use of existing technology that is either 'off-the-shelf' or technology that has already been developed for Rosetta, ESA's mission to a comet. Items not – at least partly - in common with Rosetta constitute only about 35% of the spacecraft.

ESA awarded the main contract to Astrium Toulouse, France, the spacecraft prime contractor, that previously would have been done by the project team at ESTEC. In particular, Astrium is managing the technical interfaces between the spacecraft and science payload and between the spacecraft and launcher. This shift in responsibility is allowing industry to streamline procedures and ESA to reduce the size of its project team to half that of previous equivalent projects. Astrium is leading a consortium of 24 companies from 15 European countries and the US.

"This new scheme is best suited to Mars Express constraints. Industry is more responsible in terms of the interfaces, which means we can have a more efficient decision-making process," says Vincent Poinsignon, Mars Express Project Manager at Astrium.

The time from concept to awarding the design and development contract was cut from about five years to little more than one year. Astrium won the prime contract in March 1999 in competition with two other consortia. The design and development phase will take under four years, compared with up to six years for previous similar missions.

Mars Express is a 3-axis stabilized orbiter with a fixed high-gain antenna and body-mounted instruments, and is dedicated to the orbital and in situ study of the planet’s interior, subsurface, surface and atmosphere.

Spacecraft item

Mass at launch

Spacecraft bus

439 kg

Lander

71 kg

Payload

116 kg

Propellant

427 kg

Launch mass

1223 kg

Typical mean power demand

Observation

Maneuver

Communication

Spacecraft

270 W

310 W

445 W

Payload

140 W

50 W

55 W

Total

410 W

360 W

500 W

Table 1: Spacecraft mass and power budget 3)

Dimensions

Spacecraft bus dimensions

1.5 x 1.8 x 1.4 m

Thrust of main spacecraft engine

400 N

Attitude thrusters

8 at 10 N each

Propellant tank volume

2 x270 = 540 liter

Pointing accuracy

Better than 0.05º

Power source

Solar array area

11.42 m2

Lithium batteries

3 at 22.5 Amp hour each (at launch)

Thermal specification

Spacecraft bus

10-20ºC

PFS, OMEGA

-180ºC

Thermal blanket

Gold-plated AISn alloy

Table 2: Spacecraft parameters

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Figure 2: Mars Express in launch configuration at Baikonur (image credit: ESA)


Launch: The Mars Express satellite was launched on 2 June 2003 on a Soyuz-Fregat vehicle from the Baikonur Cosmodrome, Kazakhstan. 4)

Orbit: A HEO (Highly Elliptical Orbit) on Mars (quasi-polar orbit) with a periapsis of 330 km and an apoapsis of 10,530 km, period of 7hrs.

Mars Express was launched from the Fregat upper stage towards Mars with an absolute velocity of 116, 800 km/hr and a velocity relative to the Earth of 10,800 km/hr. On 19 December 2003, 5 days before orbit insertion, the Beagle-2 lander was successfully released towards the surface of the planet. However, no further contact was made with the lander and it was subsequently declared lost (Ref. 3).

In January 2015, the UK space agency announced that the lander has been identified in images from NASA's MRO (Mars Reconnaissance Orbiter). The images appeared to show the lander partially deployed on the surface.

On 25 December 2003 the orbiter underwent a successful orbit insertion maneuver and after slow orbit adjustments it reached the operational orbit.

Nominal Operational Orbit Parameters:

• Orbital inclination - 86.9°

• Apocenter - 10,530 km

• Pericenter - 330 km

• Period - 7 hr 00 m

• Observational phase at pericenter - about 1 hour

• Communications phase - 6.5-7.0 hours minimum

Operations Center: ESOC (European Space Operations Control Center) in Darmstadt communicates with the spacecraft via the ESA New Norcia ground station in Perth, Australia. The spacecraft sends housekeeping data on instrument temperatures, voltages and spacecraft orientation, for example, and science data. The ground station sends control commands to the spacecraft. Scientific data is stored onboard using the 12 Gbit solid state mass memory prior to the downlink to Earth.


The Beagle 2 descent capsule was ejected 5 days before arrival at Mars, while the orbiter was on a Mars collision course; Mars Express was then retargeted for orbit insertion. From its hyperbolic trajectory, Beagle 2 entered and descended through the atmosphere in about 5 min, intending to land at < 40 m s–1 within an error ellipse of 20 x 100 km. The fate of Beagle 2 remains unknown because no signal was ever received from the martian surface, neither by the UK’s Jodrell Bank radio telescope nor by the Mars Express and Mars Odyssey orbiters. All of them made strenuous efforts to listen for the faintest of signals for many weeks following Beagle 2’s arrival at Mars.

ESA set up a commission to investigate the potential causes of the probable accident and issued a number of recommendations for future missions. The selected landing site was in Isidis Planitia (11.6°N, 269.5°W), which is a safe area of high scientific interest – this impact basin was probably flooded by water during part of its early history, leaving layers of sedimentary rocks. The area is surrounded by geological units of a variety of ages and compositions, from densely cratered highlands to volcanic flows to younger smooth plains. The lander’s highly integrated instrument suite was expected to perform a detailed geological, mineralogical and chemical analysis of the site’s rocks and soils, provide site meteorology, and focus on finding traces of past or present biological activity. Data from this combination of instruments could have solved the issue of life on Mars. Beagle 2’s operational lifetime was planned to be up to 180 sols (about 6 months).

Figure 3: The journey of Mars Express, from drawing board through launch, to its key science highlights during ten years of operations. With its suite of seven instruments, Mars Express has studied the subsurface of the Red Planet to the upper atmosphere and beyond to the two tiny moons Phobos and Deimos, providing an in depth analysis of the planet's history and returning stunning 3D images (video credit: ESA, Published on 3 June 2013)

Figure 4: This video, based on images taken by ESA’s Mars Express, highlights Mawrth Vallis, a 600 km-long, 2 km-deep outflow channel at the boundary of the southern highlands and the northern lowlands of Mars. The video begins at the mouth of the channel in Chryse Planitia, and heads towards the apparent source region in the Arabia Terra highlands. The 4 billion year-old plateau is characterized by many impact craters, indicative of its great age. Zooming in, patches of light and dark deposits are revealed. The light-toned layered sediments are among the largest outcrops of clay minerals – phyllosilicates – on Mars. Their presence indicates the presence of liquid water in the past. The variety of water-bearing minerals and the possibility that they might contain a record of an ancient, habitable environment on Mars led scientists to propose Mawrth Vallis as a candidate landing site for the ExoMars 2020 mission. The animation is based on a color mosaic and digital terrain model derived from data collected by the high-resolution stereo camera on Mars Express and released earlier this year (video credit: ESA/DLR/FU Berlin, Published on 9 December 2016)




Mission status

• May 18, 2020: Lava mud flows on Mars. Scientists have long suspected that the 'fire-breathing' volcanoes that spread large quantities of flowing lava over Mars were not the only kind. The numerous mountain cones in the northern hemisphere of the Red Planet may be the result of mud volcanoes. However, until now, researchers have lacked knowledge about the behavior of water-rich mud on the surface of Mars. An unusual laboratory experiment involving the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) has now been able to show how mud flows at very low temperatures and under reduced atmospheric pressure. It behaves in a similar way to very specific lava flows on Earth. The results, which have now been published in the journal Nature Geoscience, add important details to the existing knowledge of Mars and its history, which has been shaped by volcanic activity. 5) 6)

- "We have long been aware that in the early history of Mars, several billion years ago, large amounts of water were released over a short period of time, eroding very large valleys in the landscape, which have long since dried up," explains Ernst Hauber of the DLR Institute of Planetary Research in Berlin-Adlershof, who was involved in the study. "Extensively eroded masses of fragmented rock were transported through these outflow channels and into the northern lowlands of the planet, where they were quickly deposited. Later, these rocky masses were covered by younger sediments and volcanic rocks." Some Mars researchers had previously suspected that these underground, water-rich sediments could have become liquefied under certain circumstances and been pushed back up to the surface under pressure. In reference to the similar rise of magma, this process, which is well documented in many sedimentary basins on Earth, is referred to as sedimentary volcanism or mud volcanism.

a) Laboratory experiments show that at very low temperatures and under very low atmospheric pressure, mud behaves similar to flowing lava on Earth.

b) Results suggest that tens of thousands of conical hills on Mars, often with a small crater at their summit, could be the result of mud volcanism.

c) Focus: Space, exploration.

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Figure 5: A mud volcano on Mars? (image credit: NASA/JPL-Caltech/University of Arizona)

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Figure 6: Active mud volcanoes on Earth (image credit: CAS/Petr Brož (CC BY-SA 4.0)

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Figure 7: Mud volcanoes on Mars? (image credit: ESA/DLR/FU Berlin CC BY-SA 3.0 IGO)

Are small volcanic cones the result of mud extrusions?

- Tens of thousands of conical hills populate the northern highlands of Mars, often with a small crater at their summit. These may be the result of mud volcanism. However, the evidence for this is not easy to acquire. This is due to the fact that little is known about the behavior of low-viscosity mud under the environmental conditions on the Martian surface. To fill this knowledge gap, a group of European scientists carried out a series of experiments in a cylindrical low-pressure vessel 90 cm in diameter and 1.8 m long, in which water-rich mud was poured over a cold sandy surface. Apart from the gravity on Mars, which could not be simulated, this experimental setup was somewhat reminiscent of building a large sandcastle under Mars-like conditions.

Figure 8: How does watery-mud move on Mars? Like pahoehoe lava! Exploration of Mars has revealed the presence of large outflow channels which have been interpreted as the products of catastrophic flood events during which a large quantity of water was released from the subsurface. The rapid burial of water-rich sediments following such flooding may have promoted an ideal setting to trigger sedimentary volcanism, in which mixtures of rock fragments and water erupt to the surface in the form of mud (video credit: Geofyzikální ústav AV ČR v. v. i)

- The aim of these unusual experiments was to find out how the changed physical parameters influence the water component of the mud and thus alter its flow behavior. The results came as a surprise. "Under the low atmospheric pressure of Mars, the mud flows behave in much the same way as 'pāhoehoe', or 'ropy', lava, which is familiar from large volcanoes on Hawaii and Iceland," says the lead author of the study, Petr Brož of the Czech Academy of Sciences. These findings were somewhat unexpected, as comparable geological processes on other bodies in the Solar System are thought to occur in a similar way to conventional volcanic processes on Earth. "Our experiments show that even a process as apparently simple as the flow of mud – something that many of us have experienced for ourselves since we were children – would be very different on Mars."

Major impact of low atmospheric pressure

- The key reason for the flow behavior of the mud is the very thin atmosphere of Mars. The pressure is 150 times lower than the pressure at sea level on Earth. This difference has a major impact. Under such conditions, liquid water on the Martian surface is not stable and begins to boil and evaporate. This process absorbs latent heat in the vapor and cools the remaining mud, which then freezes at its surface, forming a crust. In a phase transition, such as during a freezing or thawing process, latent heat is released or absorbed by a material without changing its temperature. “Of course, we already know that liquid water begins to boil sooner under low pressure – that is why, for example, it takes longer to cook pasta on a camping stove on high mountains on Earth," explains Hauber. "However, the impact of this familiar effect on mud has never been investigated in an experiment before. Once again, it turns out that different physical conditions must always be taken into account when looking at apparently simple surface features on other planets. We now know that we need to consider both mud and lava when analyzing certain flow phenomena," adds Hauber.

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Figure 9: Did water-rich sediments also reach the Martian surface? Water that flowed over the surface of Mars billions of years ago transported large quantities of sediments to the northern lowlands, where they were later covered by younger sediments and volcanic rocks. Some Mars researchers suspect that these water-rich sediments became liquefied underground and rose back to the surface under pressure – similar to this hot ‘mud spring’ at Bakhar in Azerbaijan (diameter approximately 1.5 meters). Experiments in a low-pressure chamber, in which DLR scientist Ernst Hauber was involved, have now shown that the flow behavior is similar to that of what is referred to as ‘ropy lava’ (or, according to the Hawaiian term for smooth, unbroken lava, also known as ‘ pāhoehoe’ lava), which is at a temperature of several hundred degrees Celsius. This implies that mud flows on Mars take a completely different course than those on Earth. This observation could support the assumption that many of the conical hills with central craters discovered in the north of Mars are also mud volcanoes (image credit: CAS/Peter Brosž CC BY-SA 4.0)

- The team of researchers were able to show in detail that the mud flows in the experiment behaved like pāhoehoe lava, with liquid mud spilling from ruptures in the frozen crust, and then refreezing to form a new flow lobe. When mud escapes onto the Martian surface, it is able to flow for some time before it solidifies due to the low temperatures. However, the morphology – the shape of the mud flows – is different from those found on Earth. The research work that is currently being carried out is important for investigations of other planetary bodies, because similar processes may also play a role in cryovolcanic eruptions, in which liquid water comes to the surface, instead of magma or mud, such as on icy moons in the outer Solar System.

• May 14, 2020: Nature is a powerful sculptor – as shown in this image from ESA’s Mars Express, which portrays a heavily scarred, fractured martian landscape. This terrain was formed by intense and prolonged forces that acted upon Mars’ surface for hundreds of millions of years. 7)

- Features on Mars often trick the eye. It can be difficult to tell if the ground has risen up towards you, or dropped away. This is a common phenomenon with impact craters especially, and is aptly named the ‘crater/dome illusion’; in some images, craters appear to be large domes arching up towards the viewer – but look again, and they instead become a depression in the surrounding terrain, as expected.

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Figure 10: Topographic view of Tempe Fossae on Mars. This color-coded topographic image shows part of Mars’ surface located northeast of the Tharsis volcanic province, based on data gathered by the Mars Express High Resolution Stereo Camera on 30 September 2019 during orbit 19913. This is a portion of Tempe Fossae – a series of tectonic faults that cuts across Tempe Terra in Mars’ northern highlands. This view is based on a digital terrain model (DTM) of the region, from which the topography of the landscape can be derived; lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows and reds, as indicated on the scale to the top right. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- Such a phenomenon is at play in this image from Mars Express, which shows part of Tempe Fossae, a series of faults that cuts across the region of Tempe Terra in Mars’ northern highlands.

- Upon first glance, it is difficult to tell if ground is rising up, sinking down, or a mix of both. The landscape here is scratched, scored, and wrinkled: ridges slice across the frame, interspersed with the odd impact crater, and the entire region is full of cliffs and chasms.

- The terrain here belongs to the volcanic Tharsis province, also known as Tharsis rise, which is located close to the planet equator, at the boundary between low plains in the Northern hemisphere and highlands in the South, and displays a complex geology originating from the processes involved in its formation.

- Tempe Fossae is a great example of terrain featuring two key martian features: grabens and horsts. In a way, these are opposites of one another – grabens are slices of ground that have dropped down between two roughly parallel faults, while horsts are ground that has been uplifted between faults.

- At most, the grabens seen here reach a few kilometers wide, a few hundred meters deep, and several hundred kilometers long. Both were created by volcanic and tectonic forces acting across the surface of Mars, which fractured the ground and manipulated it into new configurations. Mars Express has observed these features many times before, in regions including Claritas Fossae, Acheron Fossae, and the nearby Ascuris Planum.

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Figure 11: This image shows part of Mars’ surface, located northeast of the Tharsis volcanic province, in a wider context. This is Tempe Fossae – a series of tectonic faults that cuts across Tempe Terra in Mars’ northern highlands. The area outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera on 30 September 2019 during orbit 19913 (image credit: NASA MGS MOLA Science Team)

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Figure 12: Located just northeast of the colossal Tharsis volcanic-tectonic province on Mars, the landscape shown in this image from ESA’s Mars Express is a mix of faults, elevated ground, deep valleys, and largely parallel ridges, extending both down into the surface and up above the martian crust. This is a portion of Tempe Fossae – a series of tectonic faults that cuts across Tempe Terra in Mars’ northern highlands. This region is a great example of terrain featuring two key martian features: grabens and horsts. In a way, these are opposites of one another – grabens are slices of ground that have dropped down between two roughly parallel faults, while horsts are ground that has been uplifted between faults. Both were created by tremendous volcanic and tectonic forces acting across the surface of Mars, which fractured the ground and manipulated it into new configurations. The surface to the right of the frame is smoother, created as lava flooded the region before cooling and solidifying, and some perpendicular slices across the predominantly parallel ridges can be seen to the left of the frame. As the nearby Tharsis province grew larger, it stretched and stressed the surrounding crust – and these features are evidence of a change in the direction of stress. This image comprises data gathered on 30 September 2019 during orbit 19913. The ground resolution is approximately 15 m/pixel and the images are centered at about 279°E/36°N. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- Despite any initial visual confusion, this landscape is a mix of faults, elevated ground, deep valleys, and largely parallel ridges, extending both down into the surface and up above the martian crust. The crater/dome illusion is actually just a trick of the light caused by our eyes incorrectly interpreting shadows. Comparing this image to the aforementioned image of Ascuris Planum, a similar terrain, highlights this nicely, demonstrating the importance of lighting conditions in photography.

- Our Earth-bound eyes are accustomed to seeing images lit from above, but this is not the default orientation for spacecraft, which can gather data at all angles of sunlight.

- Mars Express has a peculiar orbit that is not Sun-synchronous. Sun-synchronous orbits pass over the same spot on a planetary surface at roughly the same local time of day on every orbit – for instance, Earth orbiters passing over a certain city at around noon every day. Mars Express, however, does not do this, and can therefore gather data at a wide range of local times on Mars. As a result, it experiences a range of different illumination conditions as it observes the Red Planet, and produces a varied array of observations and snapshots of our planetary neighbor.

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Figure 13: This image shows part of Mars’ surface located northeast of the Tharsis volcanic province in 3D when viewed using red-green or red-blue glasses. This is a portion of Tempe Fossae – a series of tectonic faults that cuts across Tempe Terra in Mars’ northern highlands. This anaglyph was derived from data obtained by the nadir and stereo channels of the High Resolution Stereo Camera (HRSC) on ESA’s Mars Express during spacecraft orbit 19913. It covers a part of the martian surface centered at 279ºE/36ºN. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- To the right of the frame (pointing to the planet’s north), the surface becomes significantly smoother, with grabens and horsts almost nowhere to be seen. This smoother profile is a result of lava flooding these features before cooling and solidifying, in-filling and resurfacing this part of Mars.

- While most of the ridges seen here run parallel to one another from the upper left to lower right, there are also a few scratches cutting across in a perpendicular direction. This is an effect of location, as this patch of terrain is just northeast of the well-known Tharsis province, a past hotspot on Mars for substantial volcanic and tectonic activity.

- Tharsis is sizeable. The province measures several thousand kilometers across and five kilometers high on average relative to martian ‘sea level’ – a level that, given the planet’s lack of seas, is arbitrarily defined on Mars based on elevation and atmospheric pressure. It hosts the largest volcanoes in the entire Solar System, ranging from 15 to over 20 km in height.

- As the province grew larger and larger over several hundreds of millions of years, it stretched and stressed the surrounding crust, causing it to fracture and tear in different directions. The perpendicular slices seen in this image are evidence of a change in the direction of stress.

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Figure 14: This image shows a part of Mars’ surface located northeast of the Tharsis volcanic province. This is a portion of Tempe Fossae – a series of tectonic faults that cuts across Tempe Terra in Mars’ northern highlands. It comprises data gathered on 30 September 2019 during orbit 19913. The ground resolution is approximately 15 m/pixel and the images are centered at about 279ºE/36ºN. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. This HRSC stereo imaging was then used to derive a digital elevation model, upon which this oblique view is based (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- While the formation of Tharsis caused tectonic activity locally, as shown by these slices, it also influenced Mars’ crust on a much larger scale and is thought to have had a major influence in forming Valles Marineris, the largest canyon in the Solar System. Widespread erosion has occurred in Valles Marineris since its formation, shaping and sculpting the landscape into the canyon system we see today.

- Exploring the geology of Mars is a key objective of Mars Express. Launched in 2003, the spacecraft has been orbiting the Red Planet for over a decade and a half; it has since been joined by the ESA-Roscosmos ExoMars Trace Gas Orbiter (TGO), which arrived in 2016, while the ExoMars Rosalind Franklin rover and its accompanying surface science platform are scheduled for launch in 2022.

- The fleet of spacecraft currently at Mars, operated by several space agencies, are able to image the planet’s surface at scales from the global (with a spatial resolution of around ten meters) to the local (spatial resolution of around one meter). This combination allows scientists to characterize geological processes at global, regional, and local scales, enabling them to work towards a fuller understanding of Mars and its intriguing history.

• March 5, 2020: Known for its wide swathes of rippling, textured, gently sloping dunes, the Terra Sabaea region on Mars is home to many fascinating geological features – including the prominent Moreux crater, the star of a new image from ESA’s Mars Express. 8)

- The crater is roughly three kilometers deep, and spans 135 kilometers from edge to edge. While the surrounding material is visible in hues of butterscotch and caramel, the crater’s walls are dark, resembling a smudged ring of charcoal, and dark brown-black dunes cover the crater floor. This darkness is thought to be a result of the dunes comprising sandy material rich in pyroxene and olivine, minerals with a typically dark appearance.

- The dunes and flow features in Moreux crater are intriguing. Many of the features surrounding the central peak and southern region of the crater (to the left of the image) appear to have been formed by substantial and episodic glacial activity over the past few million years. Many other features, most notably the sickle-shaped dunes covering the crater floor, show signs of being eroded or formed by wind-related processes.

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Figure 15: This image comprises data gathered on 30 October 2019 during orbit 20014. The ground resolution is approximately 16 m/pixel and the images are centered at about 44ºE/42ºN. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- The Moreux crater on Mars showcases numerous intriguing geological processes and features. It sits at the northern edge of Terra Sabaea, a large area of the Red Planet that is speckled with impact craters and covered in glacial flows, dunes, fretted terrain and intricate ridge networks.

- When compared to other impact craters on both Mars and Earth, Moreux crater appears a little misshapen and messy – the result of ongoing erosion over martian history. Its egg-shaped rim is broken up, its dark walls are ridged, rippled and mottled, and its center features a prominent clustered ‘peak’, created as material from the crater floor rebounded and rose upwards following the initial impact.

- It is difficult to get a sense of scale when viewing this peak from orbit, but Moreux crater’s central peak is sizeable, reaching around two kilometers in height. The crater itself is roughly three km deep, and spans 135 km from edge to edge.

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Figure 16: This color-coded topographic image shows a feature on Mars’ surface named Moreux crater, based on data gathered by the Mars Express High Resolution Stereo Camera on 30 October 2019 during orbit 20014. This view is based on a digital terrain model (DTM) of the region, from which the topography of the landscape can be derived; lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows and reds, as indicated on the scale to the bottom left. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- The range of colors featured in images like this one, taken by the High Resolution Stereo Camera on Mars Express, reveals much about the composition of a particular region, material or feature.

- In the case of Moreux crater, the color differences are stark: while the surrounding material is visible in hues of butterscotch and caramel, the crater’s walls are dark, resembling a smudged ring of ash or charcoal. Dark brown and black dunes cover the crater floor, while the peak remains a pale yellow-orange. Dark, prominent ejecta, comprising material flung outwards during the crater-forming collision, spread outwards from the crater rim, discoloring and encroaching upon the lighter surrounding terrain.

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Figure 17: Perspective view of the Moreux crater on Mars observed on 30 October 2019 during orbit 20014 (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

• February 13, 2020: Mars is very much a world of two halves, as this new image from ESA’s Mars Express highlights, showing where these dramatically different regions come together as one. 9)

- The morphology and characteristics of the martian surface differ significantly depending on location. The northern hemisphere of Mars is flat, smooth and, in places, sits a few kilometers lower than the southern. The southern hemisphere, meanwhile, is heavily cratered, and peppered with pockets of past volcanic activity.

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Figure 18: Where north meets south: fragmented terrain on Mars. This color-coded topographic image shows a region of Mars’ surface named Nilosyrtis Mensae, based on data gathered by the Mars Express High Resolution Stereo Camera on 29 September 2019 during orbit 19908. This view is based on a digital terrain model (DTM) of the region, from which the topography of the landscape can be derived; lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows and reds, as indicated on the scale to the top right. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- A transition zone known as ‘dichotomy boundary’ separates the northern lowlands and southern highlands. Large parts of this region are filled with something scientists call fretted terrain: blocky, broken-up, fragmented swathes of terrain where the rough, pockmarked martian south gives way to the smoother north.

- This new image from the Mars Express High Resolution Stereo Camera (HRSC) shows exactly that: a region of fretted terrain named Nilosyrtis Mensae.

- Nilosyrtis Mensae has a labyrinthian appearance, with numerous channels and valleys carving through the terrain. Water, wind and ice been strongly affecting this region, dissecting and eroding the terrain, along with changes in martian geology: valleys have formed over time and sliced across the region, and once-defined impact craters have slowly degraded, their walls and features gradually wearing away.

- The large crater to the right of Figure 19 is an example of this degradation: it has a smooth, rounded appearance, with gently sloping walls, softened edges, and a flat bottom that has been widened and filled by sedimentary material over time. This worn-away morphology reflects both the crater’s advanced age, and the levels of erosion it has undergone since it formed.

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Figure 19: This image shows shows a region of Mars’ surface named Nilosyrtis Mensae in 3D when viewed using red-green or red-blue glasses. This anaglyph was derived from data obtained by the nadir and stereo channels of the HRSC on ESA’s Mars Express during spacecraft orbit 19908. It covers a part of the martian surface centered at about 69ºE/31ºN. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- Such erosion processes also created rounded hills and isolated flat-topped hills, or ‘mesas’, that are visible within the crater and across the region more widely. These stand apart from their surroundings as isolated features, and contribute to the blocky, fractured appearance of fretted terrain.

- Scientists are interested in Nilosyrtis Mensae not only for its location in this intriguing transition zone between north and south, but also for the secrets it could hold about the history of water on Mars.

- Observations of this region by missions such as Mars Express have revealed ridges, grooves and other surface textures indicative of flowing material – most likely ice.

- The climate and atmosphere of ancient Mars allowed ice and snow to accumulate and move around across the planet’s surface.

- Ice is thought to have flowed through the various valleys and across the plateaus in this region, in the form of slow-moving glaciers that swept up debris as they travelled. Such features would be similar to rock glaciers here on Earth: either icy flows covered in layers of mud and sediment, or flowing mixtures of ice, mud, snow and rock interspersed with larger rocks and boulders.

- Studying and characterizing the various processes at play across the surface of Mars is a key aim of Mars Express. Launched in 2003, the spacecraft has now been orbiting the Red Planet for over a decade and a half. Meanwhile, the ESA-Roscosmos ExoMars Trace Gas Orbiter (TGO) joined in 2016, soon to be joined by the ExoMars Rosalind Franklin rover and its accompanying surface science platform, scheduled for launch in July.

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Figure 20: This image shows a region of Mars’ surface named Nilosyrtis Mensae. It comprises data gathered on 29 September 2019 during orbit 19908. The ground resolution is approximately 15 m/pixel and the images are centered at about 69ºE/31ºN. This image was created using data from the nadir and color channels of the HRSC. The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. This perspective looks over the region from north to south (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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Figure 21: This image shows shows a region of Mars’ surface named Nilosyrtis Mensae in wider context. The area outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera on 29 September 2019 during orbit 19908 (image credit: NASA MGS MOLA Science Team)

• January 13, 2020: ESA’s Mars Express has captured beautiful images of the icy cap sitting at Mars’ north pole, complete with bright swathes of ice, dark troughs and depressions, and signs of strong winds and stormy activity. 10)

- The poles of Mars are covered in stacked layers of ice that subtly shift in extent and composition throughout the year.

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Figure 22: This image shows part of the ice cap sitting at Mars’ north pole, complete with bright swathes of ice, dark troughs and depressions, and signs of strong winds and stormy activity. The landscape here is a rippled mix of color. Dark red and ochre-hued troughs appear to cut through the icy white of the polar cap; these form part of a wider system of depressions that spiral outwards from the very center of the pole. Visible to the left of the frame are a few extended streams of clouds, aligned perpendicularly to a couple of the troughs. These are thought to be caused by small local storms that kick up dust into the martian atmosphere, eroding scarps and slopes as they do so and slowly changing the appearance of the troughs over time. - This image comprises data gathered on 16 November 2006 during orbit 3670. The ground resolution is approximately 15 m/pixel and the images are centered at about 244ºE/85ºN. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is to the upper right (image credit: ESA/DLR/FU Berlin , CC BY-SA 3.0 IGO)

- During summer, the pole is permanently covered by thick layers of mostly water ice; during winter, temperatures plummet below -125º Celsius and carbon dioxide begins to precipitate and build up as ice, creating a thinner additional layer a couple of meters thick. Winter also brings carbon dioxide clouds, which can obscure the polar features below and make it difficult to see clearly from orbit.

- This view from Mars Express’ High Resolution Stereo Camera (HRSC) suffers from very little such cloud cover, and shows the northern polar cap during the summer of 2006.

- The landscape is a rippled mix of color, from the bright whites of water ice to the dark reds and browns of martian dust, and displays a number of interesting phenomena.

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Figure 23: Mars' north pole in context. This image shows shows the ice cap at Mars' north pole. The area outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera on 16 November 2006 during orbit 3670 (image credit: Topography data by MOLA Science team. Reference body for elevations: Mars sphere map compilation by Freie Universität Berlin)

- Dark red and ochre-hued troughs appear to cut through the ice cap. These form part of a wider system of depressions that spiral outwards from the very center of the pole. When viewed on a larger scale, as in the context map, this pattern becomes evident: the rippling troughs curve and bend and slice outwards in an anti-clockwise orientation, wrapping around the pole and creating a pattern akin to zebra stripes.

- These spiralling features are thought to have formed via a mix of processes, the most significant one being wind erosion. It is thought that winds circle radially away from the center of the north pole, moving outwards cyclically to create the spiral pattern we see.

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Figure 24: This image shows part of the ice cap at Mars' north pole in 3D when viewed using red-green or red-blue glasses. This anaglyph was derived from data obtained by the nadir and stereo channels of the HRSC on ESA’s Mars Express during spacecraft orbit 3670. It covers a part of the martian surface centered at about 244ºE/85ºN. North is to the upper right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- These winds, known as katabatic winds, move cold, dry air downslope under the force of gravity, often originating in areas of higher elevation (such as glaciers or snow-covered plateaus) and flowing down into lower, warmer areas such as valleys and depressions. They are acted upon by the Coriolis force as they move, which causes them to deviate from a straight path and form the aforementioned spiral pattern we see.

- Visible to the left of the frame are a few extended streams of clouds, aligned perpendicularly to a couple of the troughs. These are thought to be caused by small local storms that kick up dust into the martian atmosphere, eroding scarps and slopes as they do so and slowly changing the appearance of the troughs over time.

- The poles, and any active processes taking place in these regions, are particularly interesting areas of Mars. These layers of ice hold information about Mars’ past, particularly concerning how its climate has evolved and changed in the last few millions of years: ice mixes with layers of surface dust and settles at the north and south poles, capturing a snapshot of the planet’s characteristics during that period of history.

- A key goal of HRSC is to explore the various phenomena occurring in the martian atmosphere, such as winds and storms, and the many intriguing geological processes that take place across – and below – the Red Planet’s surface.

- The camera has been returning impressively detailed views of Mars for many years. Mars Express arrived at the Red Planet in late 2003, and has revealed much about the planet and its history – including mapping its surface at resolutions of 10 m/pixel or greater, exploring how wet and humid early Mars may have been, characterizing its amazing volcanoes and bizarre surface features and geography, and probing deeper to determine the structure and components of its sub-surface.

- This aim of characterizing the entirety of the planet and its history will be continued and furthered by the ESA-Roscosmos ExoMars Trace Gas Orbiter, which arrived at Mars in 2016, and the ExoMars Rosalind Franklin rover and its accompanying surface science platform, which will arrive next year.

- This image is published to coincide with the Seventh International Conference on Mars Polar Science and Exploration, which is taking place in Argentina from 13 to 17 January 2020. This is the latest in a series of international and interdisciplinary conferences to share knowledge about the intriguing polar regions of the Red Planet.

• December 12, 2019: ESA’s Mars Express has captured detailed views of the small, scarred and irregularly shaped moon Phobos from different angles during a unique flyby. 11)

- Mars has two moons: Phobos and the smaller and more distant Deimos, named after the Greek mythological personifications of fear (Phobos – hence ‘phobia’) and terror (Deimos).

- Mars Express has explored this duo since it began observing the Red Planet in 2004: it has viewed Phobos with the beautiful rings of Saturn in the background, skimmed past the moon at a distance of just 45 km, used its High Resolution Stereo Camera to take incredibly detailed 360-degree images of Phobos and its intriguingly marked surface, and approached Deimos to produce an array of images and pin down the moon's location and motions.

- A new image sequence from Mars Express now captures Phobos’ motions and surface in detail. The movie comprises 41 images taken on 17 November 2019, when Phobos passed Mars Express at a distance of 2400 km. Mars Express is currently the only spacecraft capable of close encounters with Phobos.

Figure 25: ESA’s Mars Express recently tracked Phobos as the moon passed in front of the spacecraft's camera, capturing detailed views of the small, irregularly shaped body at different angles and stages of the flyby. This sequence comprises 41 images taken by the HRSC on 17 November 2019 during orbit 20,076, when Phobos passed Mars Express at a distance of roughly 2400 km. The images have a resolution of 21 m/pixel. This opportunity allowed the spacecraft to capture many features across the moon’s surface; alongside a number of impact craters (including the large and prominent Stickney crater), one can see a number of linear marks and furrows (video credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

Figure 26: Phobos phase angles explained. This image sequence nicely demonstrates the concept of ‘phase angle’: the angle between a light source (in this case, the Sun) and the observer (Mars Express’ HRSC), as viewed from the target object itself (Phobos). The initial phase angle is 17 degrees, drops to almost 0 degrees mid-way through (when Phobos is at its brightest), and then rises to 15 degrees by the end of the animation. - Phase angle, and how it corresponds to what we see, is represented in the animation to the right of the image sequence (video credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- These data nicely illustrate the concept of ‘phase angle’: the angle between a light source (in this case, the Sun) and the observer (Mars Express), as viewed from the target object itself (Phobos). The initial phase angle is 17 degrees, drops to almost 0 degrees mid-way through (when Phobos is at its brightest), and then rises to 15 degrees by the end of the animation.

- To gain a mental image of this trajectory, one can imagine Mars Express observing Phobos from one side, slowly moving across to draw level with it, and then moving away to the other side, drawing an arc in the sky between Phobos and the Sun.

- Images acquired across a range of phase angles, as shown here, are incredibly useful for scientists. Different shadows are cast as the Sun’s position changes relative to the target object: this illuminates and highlights the surface features and enables calculations of feature height, depth and relief, and reveals much about the roughness, porosity and reflectivity of the surface material itself.

- A phase angle of zero degrees occurs when the Sun is directly behind the observer. In this alignment, all of the light illuminating Phobos hits the surface vertically and is thus largely reflected back into space, causing the target object to brighten up noticeably, as seen in the animation, and shadows to disappear. The lowest phase angle in this animation is not precisely zero, but 0.92 degrees.

- This arrangement – of the Sun, Mars Express and Phobos where the latter is observed at a phase angle of near zero – is very rare, and happens only three times a year at most. Other chances to achieve a phase angle of under one will not occur until April and September 2020 (in the latter case, Mars Express may achieve a phase angle of precisely zero).

- As such, Mars Express takes every opportunity to view this small and intriguing moon from this angle, to shed light on its properties, behavior, possible origin, orbital characteristics and location in space – and to probe its potential as a mission destination.

- Phobos may be an unfamiliar world, but the phenomenon shown in the movie is familiar to anyone who has seen a full Moon. To create a full Moon, the Sun, Earth and Moon align in a roughly straight line (although, due to orbital inclinations, an exact line-up is rare, and results in a lunar eclipse). Here the phase angle – the angle between the light source (the Sun) and observer on the surface of the Earth, as viewed from the Moon – is zero, just as in the movie of Phobos. Today, 12 December, marks the last full Moon of 2019. So look up, and think of Mars’ tiny moon Phobos!

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Figure 27: This schematic accompanies a new sequence of Phobos images, created as the small martian moon passed in front of ESA’s Mars Express. The images were captured at different phase angles. The phase angle (marked as 'ϕ' in the graphic) is the angle between a light source (in this case, the Sun) and the observer (Mars Express), as viewed from the target object itself (Phobos). In the movie of Phobos, the initial phase angle is 17 degrees (A), drops to almost zero degrees mid-way through (when Phobos is at its brightest, B), and then rises to 15 degrees by the end of the animation (B), image credit: DLR

• November 21, 2019: Where the two hemispheres of Mars meet, the planet is covered in broken-up terrain: a sign that slow-but-steady flows of icy material once forged their way through the landscape, carving out a fractured web of valleys, cliffs and isolated mounds of rock. 12)

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Figure 28: This image shows a region of Mars named Deuteronilus Mensae. The area outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera on 25 February 2018 during orbit 17913 (image credit: NASA MGS MOLA Science Team)

- Mars is a planet of two halves. Its hemispheres are drastically different; the smooth northern lowlands sit up to three kilometers below the rugged southern highlands, and the surface in Mars’ northern regions appears to be far younger than the ancient swaths of the south.

- Where these regions meet, they sometimes form a transition area filled with a wide range of intriguing geological features, patterns and processes: a type of landscape unique to Mars known as fretted terrain. Fretted terrain is found in a couple of key areas on Mars, and an especially good example, named Deuteronilus Mensae, can be seen in these images from Mars Express’ High Resolution Stereo Camera (HRSC).

- This landscape shows clear and widespread signs of significant, lasting erosion. As is common with fretted terrain, it contains a mix of cliffs, canyons, scarps, steep-sided and flat-topped mounds (mesa), furrows, fractured ridges and more, a selection of which can be seen dotted across the frame.

- These features were created as flowing material dissected the area, cutting through the existing landscape and carving out a web of winding channels. In the case of Deuteronilus Mensae, flowing ice is the most likely culprit. Scientists believe that this terrain has experienced extensive past glacial activity across numerous martian epochs.

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Figure 29: This color-coded topographic view shows a region of Mars named Deuteronilus Mensae. Lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows and reds, as indicated on the scale to the top right. This view is based on a digital terrain model of the region, from which the topography of the landscape can be derived. It comprises data gathered on 25 February 2018 during orbit 17913. The ground resolution is approximately 13 m/pixel and the images are centered at about 25.5ºE/44ºN. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- It is thought that glaciers slowly but surely ate away at the plains and plateaus that once covered this region, leaving only a scattering of steep, flat, isolated mounds of rock in their wake.

- Smooth deposits cover the floor itself, some marked with flow patterns from material slowly moving downhill – a mix of ice and accumulated debris that came together to form and feed viscous, moving flows of mass somewhat akin to a landslide or mudflow here on Earth.

- Studies of this region by NASA’s Mars Reconnaissance Orbiter have shown that most of the features seen here do indeed contain high levels of water ice. Estimates place the ice content of some glacial features in the region at up to 90%. This suggests that, rather than hosting individual or occasional icy pockets and glaciers, Deuteronilus Mensae may actually represent the remnants of an old regional ice sheet. This ice sheet may once have covered the entire area, lying atop the plateaus and plains. As the martian climate changed this ice began to shift around and disappear, slowly revealing the rock beneath.

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Figure 30: Perspective view of Deuteronilus Mensae. This image from ESA’s Mars Express shows a region of Mars named Deuteronilus Mensae. This oblique perspective view was generated using a digital terrain model and Mars Express data gathered on 25 February 2018 during orbit 17913. The ground resolution is approximately 13 m/pixel and the images are centered at about 25.5ºE/44ºN. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface.

- Overall, the features seen in these Mars Express images are reminiscent of the rock- and debris-covered glaciers found in cold regions of Earth. Glaciers may actually be relatively common on both past and present-day Mars; recent studies suggest that the planet may have belts of glacial activity above and below its equator, containing huge amounts of ice covered in thick protective layers of dust, and many other areas show signs of having hosted glaciers in the past – just like Deuteronilus Mensae.

- Mars Express has been orbiting the Red Planet since 2003. Using the HRSC, which obtained these new images, the mission has continually mapped the martian surface and characterized various key properties of and phenomena on the planet – from the presence of a planet-wide groundwater system to intricate old river systems, various intriguing surface deposits, giant regional dust storms, spikes of tell-tale gases in the planet’s atmosphere, and much more.

- The mission will continue to explore the Red Planet in collaboration with the ESA-Roscosmos ExoMars Trace Gas Orbiter, which arrived at Mars in 2016, and the ExoMars Rosalind Franklin rover and its accompanying surface science platform, which will arrive in 2021.

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Figure 31: This image from ESA’s Mars Express shows a region of Mars named Deuteronilus Mensae. This oblique perspective view was generated using a digital terrain model and Mars Express data gathered on 25 February 2018 during orbit 17913. The ground resolution is approximately 13 m/pixel and the images are centered at about 25.5ºE/44ºN. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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Figure 32: This image shows shows a region of Mars named Deuteronilus Mensae in 3D when viewed using red-green or red-blue glasses. This anaglyph was derived from data obtained by the nadir and stereo channels of the High Resolution Stereo Camera (HRSC) on ESA’s Mars Express during spacecraft orbit 17913. It covers a part of the martian surface centered at about 25.5ºE/44N. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

• November 13, 2019: In June, NASA’s Curiosity rover reported the highest burst of methane recorded yet, but neither ESA’s Mars Express nor the ExoMars Trace Gas Orbiter recorded any signs of the illusive gas, despite flying over the same location at a similar time. 13)

- Methane is of such fascination because on Earth a large proportion is generated by living things. It is known that methane has a lifetime of several hundred years before it is broken down by the Sun’s radiation, so the fact that it is detected on Mars suggests it has been released into the atmosphere recently – even if the gas itself was generated billions of years ago.

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Figure 33: Key methane measurements at Mars. This graphic summarizes significant measurement attempts of methane at Mars. Reports of methane have been made by Earth-based telescopes, ESA’s Mars Express from orbit around Mars, and NASA’s Curiosity located on the surface at Gale Crater; they have also reported measurement attempts with no or very little methane detected. More recently, the ESA-Roscosmos ExoMars Trace Gas Orbiter reported an absence of methane, and provided a very low upper limit. -In order to reconcile the range of results, which show variations in both time and location, scientists have to understand better the different processes acting to create and destroy methane (image credit: ESA)

- The methane mystery on Mars has had many twists and turns in recent years with unexpected detections and non-detections alike. Earlier this year it was reported that ESA’s Mars Express had detected a signature that matched one of Curiosity’s detections from within Gale Crater.

- A recent spike by Curiosity, measured on 19 June 2019, and the highest yet at 21 ppbv, adds to the mystery because preliminary analysis suggest that Mars Express did not detect any on this occasion. (For comparison, the concentration of methane in Earth’s atmosphere is around 1800 ppbv, meaning that for every billion molecules in a given volume, 1800 are methane.)

- The Mars Express measurements were taken in the martian daytime about five hours after Curiosity’s nighttime measurements; data collected by Mars Express over one day before also did not reveal any signatures. Meanwhile Curiosity’s readings had returned to background levels when further measurements were taken in the following days.

- The Mars Express measurement technique allowing data to be inferred right down to the martian surface with its limit of detection around 2 ppbv.

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Figure 34: How methane is created and destroyed on Mars is an important question in understanding the various detections and non-detections of methane at Mars, with differences in both time and location. Although making up a very small amount of the overall atmospheric inventory, methane in particular holds key clues to the planet’s current state of activity. - This graphic depicts some of the possible ways methane might be added or removed from the atmosphere. One exciting possibility is that methane is generated by microbes. If buried underground, this gas could be stored in lattice-structured ice formations known as clathrates, and released to the atmosphere at a much later time. - Methane can also be generated by reactions between carbon dioxide and hydrogen (which, in turn, can be produced by reaction of water and olivine-rich rocks), by deep magmatic degassing or by thermal degradation of ancient organic matter. Again, this could be stored underground and outgassed through cracks in the surface. Methane can also become trapped in pockets of shallow ice, such as seasonal permafrost. -Ultraviolet radiation can both generate methane – through reactions with other molecules or organic material already on the surface, such as comet dust falling onto Mars – and break it down. Ultraviolet reactions in the upper atmosphere (above 60 km) and oxidation reactions in the lower atmosphere (below 60 km) acts to transform methane into carbon dioxide, hydrogen and water vapor, and leads to a lifetime of the molecule of about 300 years. - Methane can also be quickly distributed around the planet by atmospheric circulation, diluting its signal and making it challenging to identify individual sources. Because of the lifetime of the molecule when considering atmospheric processes, any detections today imply it has been released relatively recently. - Continued exploration at Mars – from orbit and the surface alike – along with laboratory experiments and simulations, will help scientists to better understand the different processes involved in generating and destroying methane (image credit: ESA)

- The ESA-Roscosmos Trace Gas Orbiter (TGO), the most sensitive detector for trace gases at Mars, also did not detect any methane while flying nearby within a few days before and after Curiosity’s detection.

- In general, TGO is capable of measuring at parts per trillion levels and accessing down to about 3 km altitude, but this can depend on how dusty the atmosphere is. When measurements were taken at low latitudes on 21 June 2019, the atmosphere was dusty and cloudy, resulting in measurements accessing 20-15 km above the surface with an upper limit of 0.07 ppbv.

- The global lack of methane recorded by TGO is adding to the overall mystery, and corroborating the results of the different instruments is keeping all teams busy.

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Figure 35: The story of methane on Mars is a subject of intense debate. On Earth, methane is mainly created by living organisms, but also through natural geological processes. It has a relatively short lifetime of around 400 years – because it is broken down by ultraviolet light – so detecting it on another planet raises exciting questions as to how it is produced. Previous observations of Mars, by both Earth-based telescopes and ESA’s Mars Express, hint at seasonal variations in methane abundance, with concentrations varying with location and time. NASA’s Curiosity rover has also reported methane ‘spikes’, with one corresponding to a detection by Mars Express. Curiously, the ExoMars Trace Gas Orbiter, the most sensitive atmosphere analyzer at Mars, has not yet detected any. In order to reconcile the range of results, which show variations in both time and location, scientists have to understand better the different processes acting to create and destroy methane. - This set of infographics highlight’s ESA’s contribution to Mars exploration as we ramp up to the launch of our second ExoMars mission, and look beyond to completing a Mars Sample Return mission (image credit: ESA, S. Poletti)

• 28 October 2019: Mars Express, ESA's first planetary mission, is a true marathon runner among spacecraft. Launched on 2 June 2003, the spacecraft arrived at Mars during the night of 25 December that same year. On 26 October 2019, this spacecraft completed its twenty-thousandth orbit around Mars. Mars Express is in good company in Martian orbit: NASA’s Mars 2001 Odyssey and Mars Reconnaissance Orbiter have also been studying the Red Planet for more than 10 years. Odyssey has been in orbit since 2001 and the Reconnaissance Orbiter since March 2006. 14)

- The HRSC (High Resolution Stereo Camera) developed and built by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) in collaboration with German industry, has been photographing the planet's surface on board Mars Express since January 2004 – at resolutions of up to 10 m per pixel, in color and in three dimensions. This is the first global topographic collection of image data for a planet other than Earth. In total, the resistant stereo scanner has recorded 363 GB (GigaByte) of raw data that have been pre-processed on Earth to produce 5.5 GB of scientifically useful image data. The HRSC has recorded 75 percent of the planet’s approximately 150 million km2 surface at image resolutions of 10 to 20 m/pixel.

- The topographic image maps generated using the HRSC are of great scientific benefit. Digital HRSC terrain models are also used when selecting landing sites, such as for NASA's InSight geophysical observatory or the ExoMars (ESA, due to launch in 2020), Curiosity and Mars 2020 (NASA) rovers.

- The orbit of Mars Express is highly elliptical, passing from pole to pole and taking the spacecraft to distances between 240 km to over 10,000 km from the Martian surface. The 'anniversary' of the twenty-thousandth orbit gave researchers in the HRSC experiment team, led by Ralf Jaumann at the DLR Institute of Planetary Research in Berlin, the opportunity to visit Mars during a simulated overflight of various 'chaotic areas', outflow channels and craters to the east of the Valles Marineris canyon, just north of the equator.

• 10 October 2019: Mars may seem to be an alien world, but many of its features look eerily familiar – such as this ancient, dried-up river system that stretches out for nearly 700 km across the surface, making it one of the longest valley networks on the planet. 15)

- The area of Mars shown in these new images from ESA’s Mars Express spacecraft lies just south of the planet’s equator, and is known to have been shaped by a mix of flowing water and impacts: events where rocks sped inwards from space to collide with the martian surface.

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Figure 36: This image from ESA’s Mars Express shows a dried-up river valley on Mars named Nirgal Vallis. It comprises data gathered on 16 November 2018 during Mars Express Orbit 18818. The ground resolution is approximately 14 m/pixel and the images are centered at about 315ºE/27ºS. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera. The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- Both of these mechanisms are visible here: a number of impact craters, some large and some small, can be seen speckled across the ochre, caramel-hued surface, and a tree-like, forked channel cuts prominently through the center of the frame.

- This ancient valley system is named Nirgal Vallis, and was once filled with running water that spread across Mars. By exploring the characteristics of the surrounding craters, scientists estimate the system’s age to be between 3.5 and 4 billion years old.

- The part of Nirgal Vallis captured in these images lies towards the western end of the river system, where it is slowly spreading out and dissipating; the eastern end is far less branched and more clearly defined as a single valley, and opens out into the large Uzboi Vallis – the suspected location of a large, ancient lake that has long since dried up.

- Nirgal Vallis is a typical example of a feature known as an amphitheatre-headed valley. As the name suggests, rather than ending bluntly or sharply, the ends of these tributaries have the characteristic semi-circular, rounded shape of an Ancient Greek amphitheatre. Such valleys also typically have steep walls, smooth floors, and, if sliced through at a cross-section, adopt a ‘U’ shape. The valleys pictured here are about 200 m deep and 2 km wide, and their floors are covered in sandy dunes; the appearance of these dunes indicates that martian winds tend to blow roughly parallel to the valley walls.

- We see valleys like this often on Earth, including valleys found in the Chilean Atacama Desert, the Colorado Plateau, and on the islands of Hawaii. Mars also hosts a few of them, with Nanedi Valles and Echus Chasma joining Nirgal Vallis as clear examples of this intriguing feature. Both of these features also resemble terrestrial drainage systems, where meandering, steep-sided valleys – thought to have been formed by free-flowing water – have carved their way through hundreds of kilometers of martian rock, forging through old volcanic plains, lava flows, and material deposited by strong martian winds over time

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Figure 37: Nirgal Vallis in context. This image shows a dried-up river valley on Mars named Nirgal Vallis. The area outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera on 16 November 2018 during orbit 18818 (image credit: NASA MGS MOLA Science Team)

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Figure 38: Topographic view of Nirgal Vallis. This color-coded topographic view shows a dried-up river valley on Mars named Nirgal Vallis. Lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows, and reds, as indicated on the scale to the top right. This view is based on a digital terrain model of the region, from which the topography of the landscape can be derived. It comprises data obtained by the High Resolution Stereo Camera on Mars Express on 16 November 2018 during orbit 18818. The ground resolution is approximately 14 m/pixel and the images are centered at about 315ºE/27ºS. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- Valleys such as Nirgal Vallis are ubiquitous in the low-latitude regions surrounding the martian equator, indicating that these areas once experienced a far milder and more Earth-like climate. Despite the arid, hostile world we see today, Mars is thought to have once been a far warmer and wetter planet – and we see signs of this in the diverse mix of features and minerals found across its surface.

- Scientists believe that Nirgal Vallis formed in a similar way to morphologically similar valleys we see on Earth. As there appear to be no branching, tree-like tributaries feeding into the main valley of Nirgal Vallis, it is likely that water was replenished on ancient Mars by a mix of precipitation and overland flow from the surrounding terrain.

- The system may also have its roots in a process known as groundwater sapping: when water struggles to travel vertically through a medium, and so instead continually seeps laterally through material in layers beneath the surface. We see this kind of mechanism on Earth in environments where surface material is very fine and loose and thus difficult for water to penetrate – largely silty, sandy, unconsolidated, and fine-grained environments, where lower layers of the surface are permeable and friendlier to water than those above.

- The spacecraft captured these observations using its HRSC, an instrument that is mapping the whole surface Mars in full color and at high resolution. Its aim – of characterizing and understanding the Red Planet in its entirety – will be supported and continued by the ESA-Roscosmos ExoMars Trace Gas Orbiter, which arrived at Mars in 2016, and the ExoMars Rosalind Franklin rover and its accompanying surface science platform, which will arrive next year. Together, this ground-breaking fleet will help unlock the mysteries of Mars.

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Figure 39: Perspective view of Nirgal Vallis. This image from ESA’s Mars Express shows a dried-up river valley on Mars named Nirgal Vallis. This oblique perspective view was generated using a digital terrain model and Mars Express data gathered on 16 November 2018 during Mars Express orbit 18818. The ground resolution is approximately 14 m/pixel and the images are centered at about 315°E/27°S. This image was created using data from the nadir and color channels of the HRSC (High Resolution Stereo Camera). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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Figure 40: Nirgal Vallis in 3D. This image shows Nirgal Vallis, a dried-up river valley on Mars, in 3D when viewed using red-green or red-blue glasses. This anaglyph was derived from data obtained by the nadir and stereo channels of the HRSC on ESA’s Mars Express during spacecraft orbit 18818. It covers a part of the martian surface centered at about 315ºE/27ºS. North is to the right (image credit: ESA/DLR/FU Berlin, , CC BY-SA 3.0 IGO)

• 19 September 2019: This beautiful view from ESA’s Mars Express stretches from the bright, cloud-covered north pole of Mars to the contrasting hues of the northern hemisphere and the cratered terrain in the south. 16)

- Mars Express has been orbiting Mars since 2003. The spacecraft has sent back myriad breathtaking images of our planetary neighbor in the past decade and a half, captured by the probe’s on-board HRSC (High Resolution Stereo Camera) – and this image is no different.

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Figure 41: This image from ESA’s Mars Express shows a beautiful slice of the Red Planet from the northern polar cap downwards, and highlights cratered, pockmarked swathes of the Terra Sabaea and Arabia Terra regions. It comprises data gathered on 17 June 2019 during orbit 19550. The ground resolution at the center of the image is approximately 1 km/pixel and the images are centered at about 44ºE/26ºN. This image was created using data from the nadir and color channels of the HRSC. The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is up (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- he spacecraft imaged this slice across the planet’s surface in June 2019, when the camera took several global views. Visible at the top of the frame is Mars’ ethereal north pole: this is permanently covered by a cap of frozen water and carbon dioxide, which thickens in the northern martian winter and thins in the summer.

- The northern polar cap is seen here encircled by bright, eye-catching clouds, tendrils of which snake downwards from the polar region to obscure some of the planet’s northern hemisphere. As this image shows, this patch of Mars is a mix of different tones and colors – a reflection of the different chemical and physical characteristics of the material that makes up the surface. Mars’ two hemispheres are very different in a number of ways.

- Most notably, the northern hemisphere sits several kilometers lower than the southern, and the two are separated by a distinctive, rugged boundary formed of canyons, cliffs and scarps, fractures, valleys, flat-topped mounds known as mesas, and many other features. The northern hemisphere is also characterized by low-lying plains that are largely unmarked by impact craters and thus thought to be relatively young, while the southern hemisphere is ancient, showing signs of intense cratering.

- This separation can be seen here, and is shown especially clearly in the accompanying topographic context map.

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Figure 42: Topographic context. This color-coded topographic image shows a slice of the Red Planet from the northern polar cap downwards, and highlights cratered, pockmarked swathes of the Terra Sabaea and Arabia Terra regions. The area outlined in the center of the image indicates the area imaged by the Mars Express High Resolution Stereo Camera on 17 June 2019 during orbit 19550. This context map is based on data gathered by NASA’s Viking and Mars Global Surveyor missions; lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows, and reds, as indicated on the scale to the bottom left (image credit: NASA/MGS/MOLA Science Team, FU Berlin)

- The dark and dusty young plains of the northern hemisphere sit just below the white northern cap; these meet and merge with a prominent escarpment that slices across the planet, creating a dark scar on the tan-colored surface. Below this, in tones ranging from rusty orange to pale butterscotch, are the southern highlands, featuring more craters than it is possible to count.

- Two main regions are shown here: Arabia Terra (towards the upper left) and Terra Sabaea (to the middle and lower right, forming the main bulk of the highlands visible in this slice).

- The light region stretching out of frame to the bottom right is Hellas Planitia, a plain that is home to the Hellas basin: one of the largest basins identified on Mars – and, in fact, in the Solar System – at 2300 km across.

- The split between Mars’ two hemispheres is known as the martian dichotomy, and remains one of the greatest mysteries about the planet.

- Was it formed due to geological processes within Mars’ mantle? Did the planet’s crust once comprise various moving tectonic plates, as we see on Earth, that pushed against one another to form the dichotomy? Could it have been created by one or more colossal past impacts – or by another process entirely?

- Observations of the boundary zone between the two hemispheres show that this region has been altered over time by wind and water, including by glaciers. Mars is thought to have seen various bursts of glacial activity over the years, where deposits of ice – sometimes hidden beneath layers of soil or dust – form viscous flows that slowly move across the surface, altering it as they go.

- Mars Express was recently joined at Mars by the ESA-Roscosmos ExoMars Trace Gas Orbiter (TGO), which arrived in 2016 and has since been analyzing the martian atmosphere and mapping the planet’s surface. Mars Express and the TGO will soon welcome the ExoMars Rosalind Franklin rover and its accompanying surface science platform, which are scheduled for launch in July of 2020.

- This growing fleet will continue ESA’s long-standing presence at Mars, and further our understanding of the planet and its many remaining scientific mysteries – including the martian dichotomy.

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Figure 43: A slice of Mars in context: Terra Sabaea and Arabia Terra. This image shows a slice of the Red Planet from the northern polar cap downwards, and highlights cratered, pockmarked swathes of the Terra Sabaea and Arabia Terra regions. The area outlined in the center of the image indicates the area imaged by the Mars Express High Resolution Stereo Camera on 17 June 2019 during orbit 19550. This context map is based on data gathered by NASA’s Viking and Mars Global Surveyor missions (image credit: NASA/Viking, FU Berlin)

• 08 August 2019: ESA’s Mars Express has captured the cosmic contrast of Terra Cimmeria, a region in the southern highlands of Mars marked by impact craters, water-carved valleys, and sand and dust in numerous chocolate and caramel hues. 17)

- Mars is often referred to as the Red Planet, due to the characteristic hue of its orb in the sky. Up close, however, the planet is actually covered in all manner of colors – from bright whites and dark blacks to yellows, reds, greens, and the cappuccino tones seen here.

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Figure 44: This image from ESA’s Mars Express shows Terra Cimmeria, a region found in the southern highlands of Mars. It comprises data gathered on 11 December 2018 during Mars Express orbit 18904. The ground resolution is approximately 13 meters per pixel and the images are centered at about 171º East and 40º South. This image was created using data from the nadir and color channels of the HRSC. The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- These differences in color are visible from telescopes on Earth. They are undeniably visually striking, but also reveal a significant amount about the composition and properties of the surface material itself.

- These views, based on Mars Express data are a great example of the diversity found on the martian surface: the darker regions towards the right (north) in the image at the top of this page are rich in minerals of volcanic origin, the most common of which found on Mars is basalt. The lighter patches to the left (south) are instead largely covered in fine silicate dust.

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Figure 45: Perspective view of Terra Cimmeria. This image from ESA’s Mars Express shows Terra Cimmeria, a region found in the southern highlands of Mars. This oblique perspective view was generated using a digital terrain model and Mars Express data gathered on 11 December 2018 during Mars Express Orbit18904. The ground resolution is approximately 13 meters per pixel and the images are centered at about 171º East and 40º South. This image was created using data from the nadir and color channels of the HRSC. The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- Mars is thought to have once seen significant volcanic activity. The planet hosts some of the largest volcanoes in the Solar System, including the very biggest, Olympus Mons, and has several notable volcanic provinces (two of which are Tharsis and Elysium). The volcanoes within these regions once released ash and dust that covered and coated the surface of Mars, forming dark basaltic sands that were swept around and covered up by other material over time.

- The largest crater in the image measures 25 km across and is 300 m deep – this relatively shallow depth is likely due to the crater being filled up with other material since its formation. Surrounding this crater are various plains, valleys, and ‘mesas’ – steep-sided mounds rising up from the martian surface.

- Some of these features are the remnants of a former water-filled valley system, seen most clearly to the upper right of the frame. These valleys spread across Terra Cimmeria, once moving water and material throughout the area.

- This water was locked up within surface ice and snow, but recent research points towards several episodes of melting that unlocked the water from glaciers and sent it flowing across Mars in liquid form.

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Figure 46: Perspective view of Terra Cimmeria. This image from ESA’s Mars Express shows Terra Cimmeria, a region found in the southern highlands of Mars. This oblique perspective view was generated using a digital terrain model and Mars Express data gathered on 11 December 2018 during Mars Express Orbit18904. The ground resolution is approximately 13 meters per pixel and the images are centered at about 171º East and 40º South. This image was created using data from the nadir and color channels of the HRSC. The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- To the left of the frame, thin dark trails can be seen snaking and sweeping across Terra Cimmeria – a tell-tale sign that ‘dust devils’ were once present here. Dust devils form as eddies of wind that displace the top layer of dust from the martian surface, sending it swirling up into the air. This in turn reveals a deeper layer of material that is different in color, creating a sharp visible contrast.

- Another group of dark, but larger, wind-formed features known as ‘wind streaks’ can be seen near the center-left of the image at the top of the page – shown below also as an anaglyph. These form in a similar way to dust devil tracks, except that they are caused not by eddies, but by local winds being forced over topographic features such as craters or cliffs.

- Because of this, streaks can appear to emanate from these features. Wind streaks are useful indicators in atmospheric studies; for instance, the wind that formed the streaks in this image was blowing in a roughly south-easterly direction (given that north is to the right).

- Whether altered by water, wind, impact or other means, the surface of Mars is a dynamic environment – and ESA’s Mars Express, in orbit around Mars since 2003, has managed to capture all manner of phenomena on the Red Planet in the past 16 years.

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Figure 47: Terra Cimmeria in context. This image shows Terra Cimmeria, a region found in the southern highlands of Mars. The area outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera on 11 December 2018 during orbit 18904 (image credit: NASA MGS MOLA Science Team)

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Figure 48: Terra Cimmeria in 3D. This image shows Terra Cimmeria, a region found in the southern highlands of Mars, in 3D when viewed using red-green or red-blue glasses. This anaglyph was derived from data obtained by the nadir and stereo channels of the HRSC on ESA’s Mars Express during spacecraft orbit 18904. It covers a part of the martian surface centered at about 171º East and 40º South. North is to the right(image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- Using instruments including its High Resolution Stereo Camera, responsible for these new images, the spacecraft has watched as giant dust storms kicked up material into the air to obscure wide regions of the surface from view; spotted signs of ancient sub-surface water systems that hint at the planet’s wetter past; and probed the martian atmosphere for signs of molecules we know to be linked to life on Earth.

- It has found signs of tectonic activity at far more recent timescales than previously thought; watched strange clouds form and dissipate with the seasons; explored the patches of ice found at Mars’ northern and southern poles; and characterized the planet’s two small, mysterious moons, Phobos and Deimos.

- ESA’s fleet at Mars grew with the arrival of the ESA-Roscosmos ExoMars Trace Gas Orbiter in 2016, which has been making a detailed analysis of the planet’s atmosphere and mapping its surface. Next year, the ExoMars Rosalind Franklin rover and its accompanying surface science platform will be launched to further our understanding of Mars from the planet’s intriguing surface.

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Figure 49: Topographic view of Terra Cimmeria. This color-coded topographic view shows Terra Cimmeria, a region found in the southern highlands of Mars. Lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows, and reds, as indicated on the scale to the top right. This view is based on a digital terrain model of the region, from which the topography of the landscape can be derived. It comprises data obtained by the HRSC on Mars Express on 11 December 2018 during orbit 18904. The ground resolution is approximately 13 m/pixel and the images are centered at about 171º East and 40º South. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

• 04 July 2019: ESA’s Mars Express has been keeping an eye on local and regional dust storms brewing at the north pole of the Red Planet over the last month, watching as they disperse towards the equator. 18)

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Figure 50: Spiral dust storm on Mars. This image was taken by the High Resolution Stereo Camera on 26 May and covers an area of about two thousand by five thousand kilometers (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- Local and regional storms lasting for a few days or weeks and confined to a small area are common place on Mars, but at their most severe can engulf the entire planet, as experienced last year in a global storm that circled the planet for many months.

Figure 51: Mars dust storm in motion. This series of images captured by the Visual Monitoring Camera onboard ESA’s Mars Express covers about 70 minutes of motion as a dust storm moves along the north polar ice cap of Mars on 29 May 2019. The storm moved with an approximate speed of 20 m/s. The polar ice cap covers much of the left of the image while the storm is seen on the right. - Mars Express was moving along its orbit but the images have been re-projected as if the observer was stationary, to make the motion of the storm clearer. The illumination angle of the Sun changes between image frames, highlighting the structures in the dust clouds. The black margins arise from the variable distance of Mars Express to the planet along its orbit: closer to the planet it cannot always image the same parts of the surface in consecutive images (image credit: ESA/GCP/UPV/EHU Bilbao)

- It is currently spring in the northern hemisphere of Mars, and water-ice clouds and small dust-lifting events are frequently observed along the edge of the seasonally retreating ice cap.

- Many of the spacecraft at Mars return daily weather reports from orbit or from the surface, providing global and local impressions of the changing atmospheric conditions. ESA’s Mars Express observed at least eight different storms at the edge of the ice cap between 22 May and 10 June, which formed and dissipated very quickly, between one and three days.

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Figure 52: Dust storm season on Mars. Between late May and early June, several different irregular- and spiral-shaped dust storms were seen to be building up at the north polar ice cap of Mars. The images shown here were taken by the HRSC onboard ESA’s Mars Express when the spacecraft was at an altitude of about ten thousand kilometers. The long image strips cover an area of about two thousand by five thousand kilometers, extending from the north pole equatorward to the large volcanoes Olympus Mons and Elysium Mons. - The montage of images shows three different storms developing on 22 May, 26 May, and 6 June. In the latter case, the cameras watched the storm evolve until 10 June, as it moved southward towards the volcanoes. Wispy patches of light-colored clouds can be seen at the outer margin of the polar cap and also several thousand kilometers away, close to the Elysium volcanoes. At the same time, wispy clouds can be seen along the edge of the ice cap, and also further south (left) around the large volcanoes. The dark patches are the result of dust blown volcanic material on the surface (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- The two cameras onboard the spacecraft, the High Resolution Stereo Camera (HRSC) and the Visual Monitoring Camera (VMC), have been monitoring the storms over the last weeks. The image of Figure 50, taken by HRSC on 26 May, captures a spiral-shaped dust storm, its brown color contrasting against the white ice of the north polar ice cap below.

- Meanwhile the animated sequence (Figure 51) was compiled from images of a different storm captured by the VMC over a period of 70 minutes on 29 May. This particular storm started on 28 May and continued to around 1 June, moving towards the equator during that time.

- The montage of images (Figure 52) shows three different storms developing on 22 May, on 26 May, and between 6 and 10 June. In the latter case, the cameras watched the storm evolve for several days as it moved in an equatorward direction.

- At the same time, wispy patches of light-colored clouds can be seen at the outer margin of the polar cap and also several thousand kilometers away, close to the volcanoes Elysium Mons and Olympus Mons.

- Together with the MARCI camera onboard NASA’s Mars Reconnaissance Orbiter, Mars Express observed that when the dust storms reached the large volcanoes, orographic clouds – water ice clouds driven by the influence of the volcano’s leeward slope on the air flow – that had previously been developing started to evaporate as a result of the air mass being heated by the influx of dust.

- These regional dust storms only last a few days; the elevated dust is transported and spread out by global circulation into a thin haze in the lower atmosphere, around 20–40 km altitude. Some traces of dust and clouds remained in the volcanic province into mid-June.

• 27 June 2019: The cracked, uneven, jumbled landscape seen in this image from ESA’s Mars Express forms an intriguing type of terrain that cannot be found on Earth: chaotic terrain. 19)

- The feature visible here, Aurorae Chaos, is located in the ancient and equatorial Margaritifer Terra region of Mars. The terrain here is heavily cratered, and shows signs of myriad fascinating features – many of which are thought to be linked to past water activity.

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Figure 53: Plan view of Aurorae Chaos. This image from ESA’s Mars Express shows Aurorae Chaos, a large area of chaotic terrain located in the Margaritifer Terra region on Mars, and comprises data gathered on 31 October 2018 during orbit 18765. The ground resolution is approximately 14 m/pixel and the images are centered at about 327ºE/11ºS. This image was created using data from the nadir and color channels of the HRSC (High Resolution Stereo Camera). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- These images show the southern part of Aurorae Chaos in detail, highlighting various swathes of fractured rock, mismatched peaks, flat-topped mounds (mesas), scarps, jumbled cliffs, and eroded craters.

- These characteristic features sweep across the surface, and connect to a number of small outflow channels that spread into this image from beyond the top of the frame in the main color image. These channels form the eastern end of one of Mars’ most famous features – a giant valley system called Valles Marineris, which cuts deep into the surface and spans thousands of kilometers.

- This canyon is colossal: about 10 times as long, 20 times as wide, and 4.5 times as deep as Arizona’s Grand Canyon here on Earth. The Grand Canyon was carved out by running water, and is thus an excellent example of fluvial erosion – although this kind of erosion is different to that which formed Aurorae Chaos. At its eastern end, the martian canyon runs into a large network of steep-sided depressions that sit roughly four kilometers below the surrounding plains and host numerous chaotic terrains.

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Figure 54: Aurorae Chaos in context. This image shows Aurorae Chaos, a large area of chaotic terrain located in the Margaritifer Terra region on Mars. The region outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera on 31 October 2018 during orbit 18765 (image credit: NASA MGS MOLA Science Team)

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Figure 55: This color-coded topographic view shows Aurorae Chaos, a large area of chaotic terrain located in the Margaritifer Terra region on Mars. Lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows, and reds, as indicated on the scale to the top right. This view is based on a digital terrain model of the region, from which the topography of the landscape can be derived. It comprises data obtained by the High Resolution Stereo Camera on Mars Express on 31 October 2018 during orbit 18765. The ground resolution is approximately 14 m/pixel and the images are centered at about 327ºE/11ºS. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- These differences in height are well illustrated in the accompanying topographic, perspective, and 3D views of this region, while the position of Aurorae Chaos with respect to surrounding valleys and chaotic terrain can be seen in the contextual view.

- The division between the chaotic terrain and plains can also be seen clearly in these images. The left (south) side of the image is notably smoother and more featureless than the jumbled right (north) side, and the two regions are split by a prominent line carving diagonally across the frame. The transition area around this scarp is especially broken and fractured; this is thought to be caused as the martian crust stretched and moved.

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Figure 56: Perspective view of Aurorae Chaos. This image from ESA’s Mars Express shows Aurorae Chaos, a large area of chaotic terrain located in the Margaritifer Terra region on Mars. This oblique perspective view was generated using a digital terrain model and Mars Express data gathered on 31 October 2018 during orbit 18765. The ground resolution is approximately 14 m/pixel and the images are centered at about 327ºE/11ºS. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera. The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- The ancient chaotic terrain we see on Mars holds information about how water once permeated and interacted with the planetary surface, including how it was transported, stored, and released.

- Chaotic terrain is thought to have formed as chunks of the martian surface collapsed in dramatic events triggered by the heating of material containing ice or water-bearing minerals – possibly due to climatic or volcanic heat sources, or an impact from an asteroid or comet. This released large amounts of water, causing the terrain above to subside. The water then drained away quickly, leaving behind the messy, broken patterns seen in regions such as Aurorae Chaos, which is thought to have formed some 3.5 billion years ago.

- However, it is not just visual evidence that suggests that water had a large role to play here. The wider region of Margaritifer Terra has been found to contain various sulphates and ancient clay deposits, indicating the past presence of evaporative processes and water-related outflows; some clays are even thought to require standing water in order to form, suggesting that large pools of liquid water may once have existed in this region.

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Figure 57: This image shows Aurorae Chaos, a large area of chaotic terrain located in the Margaritifer Terra region on Mars, in 3D when viewed using red-green or red-blue glasses. This anaglyph was derived from data obtained by the nadir and stereo channels of the High Resolution Stereo Camera (HRSC) on ESA’s Mars Express during spacecraft orbit 18765. It covers a part of the martian surface centered at about 327ºE/11ºS. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- Over the past 15 years Mars Express has imaged various chaos terrains on Mars, including Iani Chaos and Ariadnes Colles, using its High Resolution Stereo Camera, and continues to study the martian surface from orbit today. Our ability to explore Mars will be aided by the arrival of the ESA-Roscosmos ExoMars rover, named Rosalind Franklin, and an accompanying surface science platform in 2021. Together with the ExoMars Trace Gas Orbiter, which entered Mars orbit in 2016, they will continue our quest to explore the secrets of the Red Planet from orbit and from the ground.

• 16 May 2019: Mars was once believed to be crisscrossed by a system of irrigation canals – dark troughs that sliced across the planet’s surface, excavated by an intelligent society of thirsty martians. The astronomer who promoted this idea lends his name to the crater shown in this image from ESA’s Mars Express: Lowell crater. 20) 21)

- American astronomer and author Percival Lowell is perhaps best known for popularizing this canal theory in the late 1800s and early 1900s; the idea was initially proposed by Italian scientist Giovanni Schiaparelli, who noted the presence of dark lines on Mars in observations from the 1870s. Schiaparelli described these features as canali, later translated not as ‘channels’ or ‘gullies’, but as ‘canals’ – a phrase that hinted at a somewhat more artificial origin.

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Figure 58: This image shows Lowell crater on Mars. The region outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera during orbits 2640, 2662, 2684, 16895, 18910, 18977, and 18984 (the latter three in December 2018 and January 2019). The ground resolution is approximately 50 m/pixel and the images cover a region from 274.5º to 283º East and 49º to 54.5º South (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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Figure 59: This image from ESA’s Mars Express shows Lowell crater on Mars. This oblique perspective view was generated using a digital terrain model and Mars Express data gathered during orbits 2640, 2662, 2684, 16895, 18910, 18977, and 18984 by the spacecraft’s High Resolution Stereo Camera (HRSC). The ground resolution is approximately 50 m/pixel and the images cover a region from 274.5º to 283º East and 49º to 54.5º South. This image was created using data from the nadir and color channels of the HRSC. The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- Lowell crater is roughly 200 km in diameter and located in a region of Mars known as Aonia Terra, within the planet’s ancient southern highlands. The impact that created it is thought to have taken place between 3.7 and 3.9 billion years ago; it has endured erosion and infilling since. Its crater floor has become covered and flattened by various layers of sediment, and its outer rim is marked by small dunes and gullies.

- The image also highlights a ring of mountains rising up from the crater floor and spanning 90 km across. This so-called ‘peak ring’ is thought to have formed along with the crater. The immense energy of a large impact event causes material to surge upwards before collapsing down again, forming the kind of complex morphology seen here, with an irregular mountain range encircling the crater’s center, inside the main crater rim.

- Such features are also seen in craters here on Earth, and on Venus, Mercury, and the Moon. One notable terrestrial crater that displays this kind of peak ring is Chicxulub crater – famous for its role in the extinction of the dinosaurs some 66 million years ago. Studies and simulations of Chicxulub, which is around the same size as Lowell crater, have shown a peak ring that formed as a huge, unstable central peak later collapsed.

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Figure 60: This color-coded topographic view shows Lowell crater on Mars. Lower parts of the surface are shown in blue and purple hues, while higher altitude regions show up in whites, yellows, and reds, as indicated on the scale to the top right. This view is based on a digital terrain model of the region, from which the topography of the landscape can be derived. It comprises data obtained by the Mars Express High Resolution Stereo Camera during orbits 2640, 2662, 2684, 16895, 18910, 18977, and 18984. The ground resolution is approximately 50 m/pixel and the images cover a region from 274.5º to 283º East and 49º to 54.5º South. North is up (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

• 13 May 2019: One has a thick poisonous atmosphere, one has hardly any atmosphere at all, and one is just right for life to flourish – but it wasn’t always that way. The atmospheres of our two neighbors Venus and Mars can teach us a lot about the past and future scenarios for our own planet. 22)

- Rewind 4.6 billion years from the present day to the planetary construction yard, and we see that all the planets share a common history: they were all born from the same swirling cloud of gas and dust, with the newborn Sun ignited at the center. Slowly but surely, with the help of gravity, dust accumulated into boulders, eventually snowballing into planet-sized entities.

- Rocky material could withstand the heat closest to the Sun, while gassy, icy material could only survive further away, giving rise to the innermost terrestrial planets and the outermost gas and ice giants, respectively. The leftovers made asteroids and comets.

- The atmospheres of the rocky planets were formed as part of the very energetic building process, mostly by outgassing as they cooled down, with some small contributions from volcanic eruptions and minor delivery of water, gases and other ingredients by comets and asteroids. Over time the atmospheres underwent a strong evolution thanks to an intricate combination of factors that ultimately led to the current status, with Earth being the only known planet to support life, and the only one with liquid water on its surface today.

- We know from space missions such as ESA’s Venus Express, which observed Venus from orbit between 2006 and 2014, and Mars Express, investigating the Red Planet since 2003, that liquid water once flowed on our sister planets, too. While the water on Venus has long since boiled away, on Mars it is either buried underground or locked up in ice caps. Intimately linked to the story of water – and ultimately to the big question of whether life could have arisen beyond Earth – is the state of a planet’s atmosphere. And connected to that, the interplay and exchange of material between the atmosphere, oceans and the planet’s rocky interior.

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Figure 61: A comparison of terrestrial planets. The four terrestrial (meaning 'Earth-like') planets of our inner Solar System: Mercury, Venus, Earth and Mars. These images were taken by the Mariner 10, Apollo 17 and Viking missions (image credit: ESA)

Planetary recycling

- Back at our newly formed planets, from a ball of molten rock with a mantle surrounding a dense core, they stated to cool down. Earth, Venus and Mars all experienced outgassing activity in these early days, which formed the first young, hot and dense atmospheres. As these atmospheres also cooled, the first oceans rained down from the skies.

- At some stage, though, the characteristics of the geological activity of the three planets diverged. Earth’s solid lid cracked into plates, in some places diving below another plate in subduction zones, and in other places colliding to create vast mountain ranges or pulling apart to create giant rifts or new crust. Earth’s tectonic plates are still moving today, giving rise to volcanic eruptions or earthquakes at their boundaries.

- Venus, which is only slightly smaller than Earth, may still have volcanic activity today, and its surface seems to have been resurfaced with lavas as recently as half a billion years ago. Today it has no discernible plate tectonics system; its volcanoes were likely powered by thermal plumes rising through the mantle – created in a process that can be likened to a ‘lava lamp’ but on a gigantic scale.

- Mars, being a lot smaller, cooled off more quickly than Earth and Venus, and when its volcanoes became extinct it lost a key means of replenishing its atmosphere. But it still boasts the largest volcano in the entire Solar System, the 25 km high Olympus Mons, likely too the result of continuous vertical building of the crust from plumes rising from below. Even though there is evidence for tectonic activity within the last 10 million years, and even the occasional marsquake in present times, the planet is not believed to have an Earth-like tectonics system either.

- It is not just global plate tectonics alone that make Earth special, but the unique combination with oceans. Today our oceans, which cover about two-thirds of Earth’s surface, absorb and store much of our planet’s heat, transporting it along currents around the globe. As a tectonic plate is dragged down into the mantle, it warms up and releases water and gases trapped in the rocks, which in turn percolate through hydrothermal vents on the ocean floor.

- Extremely hardy lifeforms have been found in such environments at the bottom of Earth’s oceans, providing clues as to how early life may have begun, and giving scientists pointers on where to look elsewhere in the Solar System: Jupiter’s moon Europa, or Saturn’s icy moon Enceladus for example, which conceal oceans of liquid water beneath their icy crusts, with evidence from space missions like Cassini suggesting hydrothermal activity may be present.

- Moreover, plate tectonics helps to modulate our atmosphere, regulating the amount of carbon dioxide on our planet over long timescales. When atmospheric carbon dioxide combines with water, carbonic acid is formed, which in turn dissolves rocks. Rain brings the carbonic acid and calcium to the oceans – carbon dioxide is also dissolved directly in oceans – where it is cycled back into the ocean floor. For almost half of Earth’s history the atmosphere contained very little oxygen. Oceanic cyanobacteria were the first to use the Sun’s energy to convert carbon dioxide into oxygen, a turning point in providing the atmosphere that much further down the line allowed complex life to flourish. Without the planetary recycling and regulation between the mantle, oceans and atmosphere, Earth may have ended up more like Venus.

Extreme greenhouse effect

- Venus is sometimes referred to as Earth’s evil twin on account of it being almost the same size but plagued with a thick noxious atmosphere and a sweltering 470ºC surface. Its high pressure and temperature is hot enough to melt lead – and destroy the spacecraft that dare to land on it. Thanks to its dense atmosphere, it is even hotter than planet Mercury, which orbits closer to the Sun. Its dramatic deviation from an Earth-like environment is often used as an example of what happens in a runaway greenhouse effect.

- The main source of heat in the Solar System is the Sun’s energy, which warms a planet’s surface up, and then the planet radiates energy back into space. An atmosphere traps some of the outgoing energy, retaining heat – the so-called greenhouse effect. It is a natural phenomenon that helps regulate a planet’s temperature. If it weren’t for greenhouse gases like water vapor, carbon dioxide, methane and ozone, Earth’s surface temperature would be about 30 degrees cooler than its present +15ºC average.

- Over the past centuries, humans have altered this natural balance on Earth, strengthening the greenhouse effect since the dawn of industrial activity by contributing additional carbon dioxide along with nitrogen oxides, sulphates and other trace gases and dust and smoke particles into the air. The long-term effects on our planet include global warming, acid rain and the depletion of the ozone layer. The consequences of a warming climate are far-reaching, potentially affecting freshwater resources, global food production and sea level, and triggering an increase in extreme-weather events.

- There is no human activity on Venus, but studying its atmosphere provides a natural laboratory to better understand a runaway greenhouse effect. At some point in its history, Venus began trapping too much heat. It was once thought to host oceans like Earth, but the added heat turned water into steam, and in turn, additional water vapor in the atmosphere trapped more and more heat until entire oceans completely evaporated. Venus Express even showed that water vapor is still escaping from Venus’ atmosphere and into space today.

- Venus Express also discovered a mysterious layer of high-altitude sulphur dioxide in the planet’s atmosphere. Sulphur dioxide is expected from the emission of volcanoes – over the mission’s duration Venus Express recorded large changes in the sulphur dioxide content of the atmosphere. This leads to sulfuric acid clouds and droplets at altitudes of about 50-70 km – any remaining sulphur dioxide should be destroyed by intense solar radiation. So it was a surprise for Venus Express to discover a layer of the gas at around 100 km. It was determined that evaporating sulfuric acid droplets free gaseous sulfuric acid that is then broken apart by sunlight, releasing the sulphur dioxide gas.

- The observation adds to the discussion what might happen if large quantities of sulphur dioxide are injected into Earth’s atmosphere – a proposal made for how to mitigate the effects of the changing climate on Earth. The concept was demonstrated from the 1991 volcanic eruption of Mount Pinatubo in the Philippines, when sulphur dioxide ejected from the eruption created small droplets of concentrated sulfuric acid – like those found in Venus’ clouds – at about 20 km altitude. This generated a haze layer and cooled our planet globally by about 0.5ºC for several years. Because this haze reflects heat it has been proposed that one way to reduce global temperatures would be to inject artificially large quantities of sulphur dioxide into our atmosphere. However, the natural effects of Mt Pinatubo only offered a temporary cooling effect. Studying the enormous layer of sulfuric acid cloud droplets at Venus offers a natural way to study the longer term effects; an initially protective haze at higher altitude would eventually be converted back into gaseous sulfuric acid, which is transparent and allows all the Sun’s rays through. Not to mention the side-effect of acid rain, which on Earth can cause harmful effects on soils, plant life and water.

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Figure 62: Appearances can be deceiving. This thick, cloud-rich atmosphere rains sulfuric acid and below lie not oceans but a baked and barren lava-strewn surface. Welcome to Venus. The second planet from the Sun is often coined Earth’s ‘evil twin’ on account of it being almost the same size but instead plagued with a poisonous atmosphere of carbon dioxide and a sweltering 470ºC surface. Its high pressure and temperature is hot enough to melt lead and destroy the spacecraft that dare to land on it. Thanks to its dense atmosphere, it is even hotter than planet Mercury, which orbits closer to the Sun (image credit: ESA/MPS/DLR-PF/IDA) 23)

Global freezing

- Our other neighbor, Mars, lies at another extreme: although its atmosphere is also predominantly carbon dioxide, today it hardly has any at all, with a total atmospheric volume less than 1% of Earth’s.

- Mars’ existing atmosphere is so thin that although carbon dioxide condenses into clouds, it cannot retain sufficient energy from the Sun to maintain surface water – it vaporises instantly at the surface. But with its low pressure and relatively balmy temperatures of -55ºC (ranging from -133ºC at the winter pole to +27ºC during summer), spacecraft don’t melt on its surface, allowing us greater access to uncover its secrets. Furthermore, thanks to the lack of recycling plate tectonics on the planet, four billion year old rocks are directly accessible to our landers and rovers exploring its surface. Meanwhile our orbiters, including Mars Express, which has been surveying the planet for more than 15 years, are constantly finding evidence for its once flowing waters, oceans and lakes, giving a tantalizing hope that it might have once supported life.

- The Red Planet too would have started out with a thicker atmosphere thanks to the delivery of volatiles from asteroids and comets, and volcanic outgassing from the planet as its rocky interior cooled down. It simply couldn’t hold on to its atmosphere most likely because of its smaller mass and lower gravity. In addition, its initial higher temperature would have given more energy to gas molecules in the atmosphere, allowing them to escape more easily. And, having also lost its global magnetic field early in its history, the remaining atmosphere was subsequently exposed to the solar wind – a continuous flow of charged particles from the Sun – that, just as on Venus, continues to strip away the atmosphere even today.

- With a decreased atmosphere, the surface water moved underground, released as vast flash-floods only when impacts heated the ground and released the subsurface water and ice. It is also locked up in the polar ice caps. Mars Express also recently detected a pool of liquid water buried within two kilometers of the surface. Could evidence of life also be underground? This question is at the heart of Europe’s ExoMars rover, scheduled to launch in 2020 and land in 2021 to drill up to two meters below the surface to retrieve and analyze samples in search for biomarkers.

- Mars is thought to be currently coming out of an ice age. Like Earth, Mars is sensitive to changes in factors such as the tilt of its rotational axis as it orbits the Sun; it is thought that the stability of water at the surface has varied over thousands to millions of years as the axial tilt of the planet and its distance from the Sun undergo cyclical changes. The ExoMars Trace Gas Orbiter, currently investigating the Red Planet from orbit, recently detected hydrated material in equatorial regions that could represent former locations of the planet’s poles in the past.

- The Trace Gas Orbiter’s primary mission is to conduct a precise inventory of the planet’s atmosphere, in particular the trace gases which make up less than 1% of the planet’s total volume of atmosphere. Of particular interest is methane, which on Earth is produced largely by biological activity, and also by natural and geological processes. Hints of methane have previously been reported by Mars Express, and later by NASA’s Curiosity rover on the surface of the planet, but the Trace Gas Orbiter’s highly sensitive instruments have so far reported a general absence of the gas, deepening the mystery. In order to corroborate the different results, scientists are not only investigating how methane might be created, but also how it might be destroyed close to the surface. Not all lifeforms generate methane, however, and the rover with its underground drill will hopefully be able to tell us more. Certainly the continued exploration of the Red Planet will help us understand how and why Mars’ habitability potential has changed over time.

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Figure 63: This image from ESA’s Mars Express shows a network of dried-up valleys on Mars, and comprises data gathered on 19 November 2018 during Mars Express orbit 18831. The ground resolution is approximately 14 m/pixel and the images are centered at 66°E/17°S. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

Exploring farther

- Despite starting with the same ingredients, Earth’s neighbors suffered devastating climate catastrophes and could not hold on to their water for long. Venus became too hot and Mars too cold; only Earth became the ‘Goldilocks’ planet with the just-right conditions. Did we come close to becoming Mars-like in a previous ice age? How close are we to the runaway greenhouse effect that plagues Venus? Understanding the evolution of these planets and the role of their atmospheres is tremendously important for understanding climatic changes on our own planet as ultimately the same laws of physics govern all. The data returned from our orbiting spacecraft provide natural reminders that climate stability is not something to be taken for granted.

- In any case, in the very long term – billions of years into the future – a greenhouse Earth is an inevitable outcome at the hands of the aging Sun. Our once life-giving star will eventually swell and brighten, injecting enough heat into Earth’s delicate system to boil our oceans, sending it down the same pathway as its evil twin.

- We are changing our natural world faster than at any other time in history. Understanding the intricacies of how Earth works as a system and the impact that human activity is having on natural processes are huge environmental challenges. Satellites are vital for taking the pulse of our planet, delivering the information we need to understand and monitor our precious world, and for making decisions to safeguard our future. Earth observation data is also key to a myriad of practical applications to improve everyday life and to boost economies. This week we focus on the world’s biggest conference on Earth observation where thousands of scientists and data users discuss the latest results and look to the future of Earth observation(Ref. 22) .

• 08 April 2019: Bacterial resistance to antibiotics is one of humankind’s major long-term health challenges. Now research into helping humans live on Mars could help address this looming problem. 24)

- Dennis Claessen, associate professor at the Institute of Biology at Leiden University, the Netherlands works in synthetic biology, in which bacteria are engineered to solve problems that cannot be tackled – or are not tackled well – by ‘wild’ bacteria. A team of his students entered the iGEM (International Genetically Engineered Machine) competition with a solution to the problem of growing non-toxic plants on Mars, but needed ‘Martian’ gravity to test their ideas.

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Figure 64: This perspective view shows the central part of Nicholson Crater, at approximately 0.0º South and 195.5º East, looking west. Nicholson Crater, measuring approximately 100 km wide, is located at the southern edge of Amazonis Planitia, north-west of a region called Medusae Fossae. The HRSC on board ESA's Mars Express obtained this image during orbit 1104 around Mars with a ground resolution of approximately 15.3 m/pixel. The image was released on 14 July 2000 [image credit: ESA/DLR/FU Berlin (G. Neukum), CC BY-SA 3.0 IGO)]

- “The soil on Mars has perchlorate chemical compounds in it, which can be toxic for humans,” explains Prof. Claessen. High doses of perchlorate can inhibit the thyroid gland’s uptake of iodine and interfere with foetal development. - Our students started ‘building’ a bacterium that would degrade the perchlorate to chlorine and oxygen, but they needed to know whether that bacterium would behave the same way in the partial gravity of Mars as it would on Earth.”

Figure 65: The RPM (Random Positioning Machine) was developed by the Dutch office of Airbus for ESA to do experiments in zero or reduced gravity here on Earth without going into space. The RPM rotates any enclosed experiment ‘randomly’ to minimize the influence of Earth’s gravity, thereby simulating what would be experienced in space. The original models could successfully simulate zero gravity, typically referred to as microgravity. The newer RPM 2.0 can additionally simulate partial gravity, which is between 0g and 1g (video credit: Airbus, Released on 5 April 2019)

- The challenge was to find a way to reproduce Mars gravity on Earth, and the students solved it using the RPM.

- The first recorded experiment on living systems using machines to manipulate gravity was done in 1806 using a rotating waterwheel. Two hundred years later the RPM, designed by the Netherlands-based Airbus team for ESA, is the latest instrument developed to experiment in zero or reduced gravity without going into space.

- As its name suggests, the RPM continually changes its orientation at random, so that items placed within it have no opportunity to adjust to a steady gravity direction. The original design could successfully simulate zero gravity while the newer RPM 2.0 can additionally simulate partial gravity, the stages between normal Earth gravity and the weightless environment.

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Figure 66: Streptomyces colony at Leiden University (image credit: Institute of Biology at Leiden University, The Netherlands)

- “The RPM machines offer a great alternative to organizations looking to do experiments in zero and partial gravity,” says Derk Schneemann at Verhaert Netherlands. Derk is the Dutch broker in the network of ESA Innovation Partners that facilitates the re-use of space technology in other sectors and which is part of ESA’s Business Applications and Space Solutions Program.

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Figure 67: iGEM team from the Institute of Biology at Leiden University, The Netherlands (image credit: Institute of Biology at Leiden University)

- “During their experiments they noticed that when bacteria grew in partial gravity, they became stressed as they accumulated waste around them that they couldn’t get rid of. This holds great potential because when microbes belonging to the Streptomyces family become stressed, they usually start making antibiotics.

- “Seventy percent of all the antibiotics humans use are derived from Streptomyces bacteria and we know they have the potential to produce even more. Using the RPM to stress them in new ways may help us to find ones we’ve never seen before.”

Detoxifying soil on Mars and Earth

- Prof. Claessen is now building a Dutch consortium to investigate soil detoxification on a larger scale. This will have applications on Earth. For example, there is a lot of perchlorate in Chile’s Atacama Desert, where the soil is believed to resemble that on Mars. Atacama soil was previously used as fertilizer in the US, but later it was found that perchlorate had been washed from this fertilizer into groundwater used for drinking.

- Once the Dutch team is in place and has funding, research into Streptomyces microbes (which are naturally found in Earth’s soil, where they play a vital role in breaking down organic matter) and antibiotics could also be done with the RPM.

- “To find that they hold the potential for discovering new antibiotics as well is even more exciting, as antibiotic resistance is something we need to tackle urgently,” adds Derk Schneemann.

Space for health

- The United Nations World Health Organization marks World Health Day on 7 April every year. The third Sustainable Development Goal underlines the right to health: Ensure healthy lives and promote well-being for all at all ages. On-orbit research, space technology and space applications can help improve health on Earth by monitoring our environment, helping track disease, improving diagnostics, and working on new medicines among other things. The UN is also focusing particularly this year on universal health coverage.

• 01 April 2019: A reanalysis of data collected by ESA’s Mars Express during the first 20 months of NASA’s Curiosity mission found one case of correlated methane detection, the first time an in-situ measurement has been independently confirmed from orbit. 25)

- Reports of methane in the martian atmosphere have been intensely debated, with Mars Express contributing one of the first measurements from orbit in 2004, shortly after its arrival at the Red Planet.

- The molecule attracts such attention because on Earth methane is generated by living organisms, as well as geological processes. Because it can be destroyed quickly by atmospheric processes, any detection of the molecule in the martian atmosphere means it must have been released relatively recently – even if the methane itself was produced millions or billions of years ago and lay trapped in underground reservoirs until now.

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Figure 68: Data collected by the Planetary Fourier Spectrometer onboard ESA’s Mars Express during the first 20 months of NASA’s Curiosity mission found one case of correlated methane detection, the first time an in-situ measurement has been independently confirmed from orbit. Ten other observations in the Mars Express study period reported no detections at the limit of the spectrometer’s sensitivity; all of these correspond to a period of low measurements reported by Curiosity (image credit: ESA/Giuranna et al (2019)

Legend to Figure 68: The details of the methane spike are illustrated in this graphic – the Mars Express detection was made one day after the elevated reading recorded by Curiosity, which is exploring Gale Crater, just south of the martian equator. Taken together, the two results can be used to examine the possible source region for the methane.

Two independent analyses were made, by examining a wide region around Gale Crater. The region was divided into grids of about 250 x 250 km2, and in one study, computer simulations were used to predict the probability of methane emission from each square (indicated by the numbers in each square). The simulations took into account the measured data, expected atmospheric circulation patterns, and methane release intensity and duration based on the geological phenomenon of ‘gas seepage’. In the other study, geologists scrutinized the region for features where gas seepage is expected – these are the kind of features that might be associated with methane release.

The geological analysis pointed to one of the regions that the computer simulations predicted would be the most likely region for methane release. The area marked by the black dots is thought to contain shallow ice that could easily trap subsurface methane, and tectonic faults in the grid square between this region and Gale Crater are thought to extend below the surface and break through the ice, causing the episodic release of methane.

- While spacecraft and telescopic observations from Earth have in general reported no or very low detections of methane, or measurements right at the limit of the instruments’ capabilities, a handful of spurious spikes, along with Curiosity’s reported seasonal variation at its location in Gale Crater, raise the exciting question of how it is being generated and destroyed in present times.

- Now, for the first time, a strong signal measured by the Curiosity rover on 15 June 2013 is backed up by an independent observation by the Planetary Fourier Spectrometer (PFS) onboard Mars Express the next day, as the spacecraft flew over Gale Crater.

- The study exploited a new observation technique, allowing the collection of several hundred measurements in one area over a short period of time. The teams also developed a refined analysis technique to get the best out of their data.

- “In general we did not detect any methane, aside from one definite detection of about 15 parts per billion by volume of methane in the atmosphere, which turned out to be a day after Curiosity reported a spike of about six parts per billion,” says Marco Giuranna from the Institute for Space Astrophysics and Planetology in Rome, Italy, the principal investigator for the PFS experiment, and lead author of the paper reporting the results in Nature Geoscience today. 26)

- “Although parts per billion in general means a relatively small amount, it is quite remarkable for Mars – our measurement corresponds to an average of about 46 tons of methane that was present in the area of 49,000 km2 observed from our orbit.”

- Ten other observations in the Mars Express study period that reported no detections at the limit of the spectrometer’s sensitivity corresponded to a period of low measurements reported by Curiosity.

Pinpointing the source

- At the time of the Curiosity detection, it was speculated that the methane originated north of the rover, because the prevailing winds were southward, and that the release likely occurred inside the crater.

- “Our new Mars Express data, taken one day after Curiosity’s recording, change the interpretation of where the methane originated from, especially when considering global atmospheric circulation patterns together with the local geology,” adds Marco. “Based on geological evidence and the amount of methane that we measured, we think that the source is unlikely to be located within the crater.”

- Marco and his colleagues made two independent analyses to home in on potential source regions of the methane, dividing up a wide region around Gale Crater into grids of about 250 x 250 km2.

- In one study, collaborators from the Royal Belgian Institute for Space Aeronomy in Brussels applied computer simulations to create one million emission scenarios for each square, in order to predict the probability of methane emission for each of those locations. The simulations took into account the measured data, expected atmospheric circulation patterns, and methane release intensity and duration based on the geological phenomenon of ‘gas seepage’.

- In the other parallel study, geologists from the National Institute of Geophysics and Volcanology in Rome, Italy and the Planetary Science Institute in Tucson, Arizona, scrutinized the region around Gale Crater for features where gas seepage is expected – these are the kind of features that might be associated with methane release.

- This process is well known on Earth to occur along tectonic faults and from natural gas fields, with a variety of release intensities. For example, on Earth, gas emission from active mud volcanoes is typically continuous with background variations, but also with sudden strong bursts, while other seeps might release gas intermittently.

- Episodic gas release, that is, generally long quiescence with no emission in between short-duration bursts, is typical of the expulsion of gas from small or ‘dying’ seeps or due to seismic events. On Mars, episodic gas expulsions could also be created during a meteorite impact, liberating gas trapped below the surface.

- “We identified tectonic faults that might extend below a region proposed to contain shallow ice. Since permafrost is an excellent seal for methane, it is possible that the ice here could trap subsurface methane and release it episodically along the faults that break through this ice,” says co-author Giuseppe Etiope from the National Institute of Geophysics and Volcanology in Rome.

- “Remarkably, we saw that the atmospheric simulation and geological assessment, performed independently of each other, suggested the same region of provenance of the methane.”

- “Our results support the idea that methane release on Mars might be characterized by small, transient geological events rather than a constantly replenishing global presence, but we also need to understand better how methane is removed from the atmosphere, and how to reconcile the Mars Express data with results from other missions,” adds co-author Frank Daerden from the Royal Belgian Institute for Space Aeronomy in Brussels.

- “We will re-analyze more of the data collected by our instrument in the past, while continuing our ongoing monitoring efforts, including coordinating some observations with the ExoMars Trace Gas Orbiter,” concludes Marco.

- The ESA-Roscosmos ExoMars Trace Gas Orbiter, which is designed to make the most detailed inventory of the martian atmosphere yet, began its science observations in April 2018.

- “Mars Express was the first to report a significant detection of methane from orbit around Mars, and now, fifteen years later, we can announce the first simultaneous and co-located detection of methane with a rover on the surface,” says Dmitri Titov, ESA’s Mars Express project scientist.

- “With the spacecraft and its payload still operative, Mars Express is one of the most successful space missions to be sent to Earth’s planetary neighbor. We expect more exciting science from joint efforts by both ESA orbiters at Mars.”

- The Mars Express detections were made by the Planetary Fourier Spectrometer (PFS). The Curiosity measurements were made by the Tunable Laser Spectrometer – Sample Analysis at Mars instrument (TLS-SAM).

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Figure 69: How methane is created and destroyed on Mars is an important question in understanding the various detections and non-detections of methane at Mars, with differences in both time and location. Although making up a very small amount of the overall atmospheric inventory, methane in particular holds key clues to the planet’s current state of activity. This graphic depicts some of the possible ways methane might be added or removed from the atmosphere. One exciting possibility is that methane is generated by microbes. If buried underground, this gas could be stored in lattice-structured ice formations known as clathrates, and released to the atmosphere at a much later time (image credit: ESA)

Legend to Figure 69: Methane can also be generated by reactions between carbon dioxide and hydrogen (which, in turn, can be produced by reaction of water and olivine-rich rocks), by deep magmatic degassing or by thermal degradation of ancient organic matter. Again, this could be stored underground and outgassed through cracks in the surface. Methane can also become trapped in pockets of shallow ice, such as seasonal permafrost.

Ultraviolet radiation can both generate methane – through reactions with other molecules or organic material already on the surface, such as comet dust falling onto Mars – and break it down. Ultraviolet reactions in the upper atmosphere (above 60 km) and oxidation reactions in the lower atmosphere (below 60 km) acts to transform methane into carbon dioxide, hydrogen and water vapor, and leads to a lifetime of the molecule of about 300 years.

Methane can also be quickly distributed around the planet by atmospheric circulation, diluting its signal and making it challenging to identify individual sources. Because of the lifetime of the molecule when considering atmospheric processes, any detections today imply it has been released relatively recently.

But other generation and destruction methods have been proposed which explain more localized detections and also allow a faster removal of methane from the atmosphere, closer to the surface of the planet. Dust is abundant in the lower atmosphere below 10 km and may play a role, along with interactions directly with the surface. For example, one idea is that methane diffuses or ‘seeps’ through the surface in localized regions, and is adsorbed back into the surface regolith. Another idea is that strong winds eroding the planet’s surface allows methane to react quickly with dust grains, removing the signature of methane. Seasonal dust storms and dust devils could also accelerate this process.

Continued exploration at Mars – from orbit and the surface alike – along with laboratory experiments and simulations, will help scientists to better understand the different processes involved in generating and destroying methane.

• 28 March 2019: The winds of Mars are responsible for myriad features across the planet’s surface – including the dark dunes and wispy, filament-like streaks seen in this image from ESA’s Mars Express. 27)

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Figure 70: This image from ESA’s Mars Express shows Chalcoporos Rupes, a region on Mars that shows signs of dust and wind activity. It comprises data gathered on 3 January 2019 during Mars Express Orbit 18983. The ground resolution is approximately 13 m/pixel and the images are centered at about 23º East and 53º South. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- The intriguing features shown here are ‘dust devil’ tracks: as the Sun heats up the martian ground during the day, vortices form that lift warm air from near the surface, whipping up dust as they do so, shaping and sculpting it into swirling, column-shaped, tornado-like whirlwinds (click here for videos of dust devils made by NASA's Mars rover Spirit).

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Figure 71: This image shows part of the Noachis quadrangle on Mars: Chalcoporos Rupes. The region outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera on 3 January 2019 during Mars Express Orbit 18983. The ground resolution is approximately 13 m/pixel and the images are centered at about 23º East and 53º South (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- These dust devils range across the entire planet, lifting the top, brighter layer of dust from the surface, and leaving darker paths in their wake. They are most often seen in the martian spring and summer, lasting for a few months at most before their tracks become obscured by dust that has been buffeted around by storms and winds.

- These Mars Express images show a curving, looping, crisscrossing web of dust devil tracks in the southern hemisphere of the planet, around an escarpment feature known as Chalcoporos Rupes. This area is covered in a thick layer of dust and is not infrequently home to wind-related activity.

- Areas of Mars that most regularly see dust devils include Amazonis Planitia, Argyre Planitia, Hellas Basin, and two impact craters that lie close to the region shown here: Proctor and Russell.

- Proctor, Russell, and Chalcoproros Rupes are based in Mars’ Noachis quadrangle, an area so thickly pockmarked with impact craters that it is thought to be one of the oldest parts of the planet.

- Both the craters visible in this frame boast dense, dark, eye-catching patches of rippling sand dunes, while the surrounding terrain is decorated with a broad web of dunes and signs of past dust devil activity.

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Figure 72: This color-coded topographic view shows Chalcoporos Rupes, a region on Mars that shows signs of dust and wind activity. Lower parts of the surface are shown in blue and purple hues, while higher altitude regions show up in whites, yellows, and reds, as indicated on the scale to the top right. This view is based on a digital terrain model of the region, from which the topography of the landscape can be derived. It comprises data obtained by the High Resolution Stereo Camera on Mars Express on 3 January 2019 during Mars Express Orbit 18983. The ground resolution is approximately 13 m/pixel and the images are centered at about 23º East and 53º South. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- Martian dust devils are similar to those seen on Earth in especially dry, arid, desert landscapes – but they are far larger. They can tower up to eight kilometers high on the Red Planet, creating paths that are hundreds of meters wide and stretch out for a few kilometers.

- Their colossal size makes them highly effective at carrying dust high up into Mars’ atmosphere – in fact, these devils may lift as much material as a martian global dust storm does at its peak.

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Figure 73: This image from ESA’s Mars Express shows Chalcoporos Rupes, a region on Mars that shows signs of dust and wind activity. This oblique perspective view was generated using a digital terrain model and Mars Express data gathered on 3 January 2019 during Mars Express Orbit 18983. The ground resolution is approximately 13 m/pixel and the images are centered at about 23º East and 53º South. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface (image credit: ESA)

- Such dust storms are immense and impressive. Mars Express captured signs of a burgeoning storm near Mars’ north pole in April of last year, highlighting an intense boundary between the planet’s usual, calm, ochre-hued surface and an incoming wall of dust clouds – and this was a somewhat modest dust storm compared to those that blanket the entirety of Mars and rage on for months.

- Dust devils have been seen often on Mars, both by Mars Express and other missions – including the ESA-Roscosmos ExoMars Trace Gas Orbiter, which recently imaged an impressive pattern of dust devil tracks in the Terra Sabaea region of Mars that may be the result of hundreds or even thousands of small martian tornadoes coming together and leaving their mark on the planet’s surface.

- The Trace Gas Orbiter will be joined by a rover – recently named Rosalind Franklin – and a surface science platform, due to launch in 2020. These will allow the ExoMars mission to explore the Red Planet in even greater detail in coming years.

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Figure 74: This 3D image, best viewed using red-green or red-blue glasses, shows Chalcoporos Rupes, a region on Mars that shows signs of dust and wind activity. This anaglyph was derived from data obtained by the nadir and stereo channels of the High Resolution Stereo Camera (HRSC) on ESA’s Mars Express during spacecraft orbit 18983. It covers a part of the martian surface centered at about 23º East and 53º South. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

• 28 February 2019: Mars Express has revealed the first geological evidence of a system of ancient interconnected lakes that once lay deep beneath the Red Planet’s surface, five of which may contain minerals crucial to life. 28)

- Mars appears to be an arid world, but its surface shows compelling signs that large amounts of water once existed across the planet. We see features that would have needed water to form – branching flow channels and valleys, for example – and just last year Mars Express detected a pool of liquid water beneath the planet’s south pole.

- A new study now reveals the extent of underground water on ancient Mars that was previously only predicted by models.

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Figure 75: Example of features identified in a deep basin on Mars that show it was influenced by groundwater billions of years ago (image credit: NASA/JPL-Caltech/MSSS)

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Figure 76: This image shows the distribution of a number of deep craters (marked as dots) recently explored as part of a study into groundwater on Mars. The background image is shown in colors representing topography: reds and oranges are lower elevations, and blues and greens are higher ones. The study found that the floors of the basins, which sit over 4000 m deep, show signs of past water – the first geological evidence that the Red Planet once had a system of interconnected groundwater-fed lakes that spanned the entire planet [image credit: Topography: NASA/MGS/MOLA; Crater distribution: F. Salese et al (2019)]

- “Early Mars was a watery world, but as the planet’s climate changed this water retreated below the surface to form pools and ‘groundwater’,” says lead author Francesco Salese of Utrecht University, the Netherlands. “We traced this water in our study, as its scale and role is a matter of debate, and we found the first geological evidence of a planet-wide groundwater system on Mars.” 29)

- Salese and colleagues explored 24 deep, enclosed craters in the northern hemisphere of Mars, with floors lying roughly 4000 m below martian ‘sea level’ (a level that, given the planet’s lack of seas, is arbitrarily defined on Mars based on elevation and atmospheric pressure).

- They found features on the floors of these craters that could only have formed in the presence of water. Many craters contain multiple features, all at depths of 4000 to 4500 m – indicating that these craters once contained pools and flows of water that changed and receded over time.

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Figure 77: This diagram shows a model of how crater basins on Mars evolved over time and how they once held water. This model forms the basis of a new study into groundwater on Mars, which found that a number of deep basins – with floors sitting over 4000 km deep – show signs of having once contained pools of water. Images (from the context camera onboard NASA’s Mars Reconnaissance Orbiter) show examples of the different features observed in the basins. — There are three main stages: in the first (top), the crater basin is flooded with water and water-related features – deltas, sapping valleys, channels, shorelines, and so on – form within. In the second stage (middle), the planet-wide water level drops and new landforms emerge as a result. In the final stage (bottom), the crater dries out and becomes eroded, and features formed over the previous few billions of years are revealed [image credit: Images: NASA/JPL-Caltech/MSSS; Diagram adapted from F. Salese et al. (2019)]

- Features include channels etched into crater walls, valleys carved out by sapping groundwater, dark, curved deltas thought to have formed as water levels rose and fell, ridged terraces within crater walls formed by standing water, and fan-shaped deposits of sediment associated with flowing water.

- The water level aligns with the proposed shorelines of a putative martian ocean thought to have existed on Mars between three and four billion years ago.

- “We think that this ocean may have connected to a system of underground lakes that spread across the entire planet,” adds co-author Gian Gabriele Ori, director of the Universita D’Annunzio’s International Research School of Planetary Sciences, Italy.

- “These lakes would have existed around 3.5 billion years ago, so may have been contemporaries of a martian ocean.”

- The history of water on Mars is a complex one, and is intricately linked to understanding whether or not life ever arose there – and, if so, where, when, and how it did so.

- The team also spotted signs of minerals within five of the craters that are linked to the emergence of life on Earth: various clays, carbonates, and silicates. The finding adds weight to the idea that these basins on Mars may once have had the ingredients to host life. Moreover, they were the only basins deep enough to intersect with the water-saturated part of Mars’ crust for long periods of time, with evidence perhaps still buried in the sediments today.

- Exploring sites like these may thus reveal the conditions suitable for past life, and are therefore highly relevant to astrobiological missions such as ExoMars – a joint ESA and Roscosmos endeavor. While the ExoMars Trace Gas Orbiter is already studying Mars from above, the next mission will launch next year. It comprises a rover – recently named after Rosalind Franklin – and a surface science platform, and will target and explore martian sites thought to be key in the hunt for signs of life on Mars.

- “Findings like this are hugely important; they help us to identify the regions of Mars that are the most promising for finding signs of past life,” says Dmitri Titov, ESA’s Mars Express project scientist.

- “It is especially exciting that a mission that has been so fruitful at the Red Planet, Mars Express, is now instrumental in helping future missions such as ExoMars explore the planet in a different way. It’s a great example of missions working together with great success.”

• 21 February 2019: These images from ESA’s Mars Express satellite show a branching, desiccated system of trenches and valleys, signs of ancient water flow that hint at a warmer, wetter past for the Red Planet. 30)

- We see Mars as a cold, dry world, but plenty of evidence suggests that this was not always the case. Research in past years instead increasingly indicates that the planet once had a thicker, denser atmosphere that was able to lock in far greater amounts of warmth, and therefore facilitate and support the flow of liquid water on the surface below.

- While this is no longer the case, we see clear signs of past water activity tracing across the martian surface. This image shows one such region: a system of valleys in the southern highlands of Mars, located east of a large, well-known impact crater called Huygens and north of Hellas, the largest impact basin on the planet. At 3.5 to four billion years old, the southern highlands are some of the oldest and most heavily cratered parts of Mars, with many signs of ancient water flow observed here.

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Figure 78: This image from ESA’s Mars Express shows a valley network on Mars. This oblique perspective view was generated using a digital terrain model and Mars Express data gathered on 19 November 2018 during Mars Express orbit 18831. The ground resolution is approximately 14 m/pixel and the images are centered at 66ºE/17ºS. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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Figure 79: This image shows the landscape in and around a network of dried-up valleys on Mars. The region outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera on 19 November 2018 during orbit 18831. The different colors across the frame represent the elevation of the terrain, as indicated by the bar at the bottom (image credit: Topography: NASA MGS MOLA Science Team; Map compilation: Freie Universitat Berlin)

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Figure 80: This color-coded topographic view shows the relative heights of the terrain in and around a network of dried-up valleys on Mars. Lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows, and reds, as indicated on the scale to the top right. This view is based on a digital terrain model of the region, from which the topography of the landscape can be derived. It comprises data obtained by the High Resolution Stereo Camera on Mars Express on 19 November 2018 during Mars Express orbit 18831. The ground resolution is approximately 14 m/pixel and the images are centered at 66°E/17°S. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- The topography of this region suggests that water flowed downhill from the north (right in the main color, topography and 3D images) to the south (left), carving out valleys up to two kilometers across and 200 meters deep as it did so. We see these valleys as they stand today, having undergone significant and heavy erosion since they were formed. This erosion is visible in the form of broken down, smoothed, fragmented and dissected valley rims, especially in the valleys cutting from east to west.

- Overall, the valley system appears to branch out significantly, forming a pattern a little like tree branches stemming from a central trunk. This kind of morphology is known as ‘dendritic’ – the term is derived from the Greek word for tree (dendron), and it is easy to see why. Various channels split off from the central valley, forming little tributaries that often split again on their journey outwards.

- This kind of dendritic structure is also seen in drainage systems on Earth. A particularly good example is that of the Yarlung Tsangpo river, which snakes its way from its source in western Tibet down through China, India, and Bangladesh. In the case of this image of Mars, these branching channels were likely formed by surface water runoff from a once-strong river flow, combined with extensive rainfall. This flow is thought to have cut through existing terrain on Mars, forging new paths and carving a new landscape.

- While it is unclear where all of this water came from originally – precipitation, groundwater, melting glaciers? – all of this required a far warmer and more watery past for Mars than the planet we see today.

- A tantalizing question raised by this warmer and wetter climate is whether conditions would have been suitable for life – a topic at the heart of Mars exploration. Next year, ESA and Roscosmos will launch the ExoMars mission comprising a rover – recently named Rosalind Franklin – and a surface science platform. The rover will drive to interesting locations to drill below the surface in search for signs of life – the first mission of its kind. Meanwhile, the ExoMars Trace Gas Orbiter continues to analyze the atmosphere in greater detail than ever, with a particular interest for gases potentially related to biological or geological activity, and to identify subsurface locations where water-ice or hydrated minerals are present.

- This succession of spacecraft at Mars – both in orbit and on the surface – ensures ESA’s long-term presence in Mars science and exploration. The next step that ESA together with international partners are considering is returning a sample of Mars to Earth – an ambitious task that will provide scientific treasures for generations to come.

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Figure 81: This image from ESA’s Mars Express shows a network of dried-up valleys on Mars, and comprises data gathered on 19 November 2018 during Mars Express orbit 18831. The ground resolution is approximately 14 m/pixel and the images are centered at 66ºE/17ºS. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

• 11 January 2019: Fifteen years ago, early on the evening of Saturday 10 January 2004, over a dozen scientists crammed into a tiny, somewhat austere room at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) research center in Berlin Adlershof to stare intently at two monitors. They were awaiting the first images from 'their' experiment, the High Resolution Stereo Camera (HRSC). Just over two weeks earlier, the Mars Express spacecraft, launched by the European Space Agency (ESA), had reached its destination and maneuvered into a stable, elliptical orbit over the poles of the planet. As planned, the experiments were now set to begin. Despite an initial unsatisfactory test, HRSC showed its full potential during the tenth orbit. Mars Express transmitted razor-sharp high-definition image data with a perfect brightness distribution via ESA's ground stations to the ESOC (European Space Operations Center) in Darmstadt. In Berlin the images were greeted with boundless enthusiasm. This was the beginning of a success story that has lasted one-and-a-half decades. 31)

- HRSC is a German camera experiment developed by DLR and carried on board ESA's Mars Express orbiter; it is operated by the DLR Institute of Planetary Research. The spacecraft has been orbiting Mars since Christmas 2003. This week, on 8 January 2019, the orbiter, dubbed 'MEX' by the scientists and engineers involved in the project, completed its 19,000th orbit of Mars. ESA's first planetary mission is a true 'marathon runner'. In total, the probe has travelled approximately 950 million km around Mars, in addition to its almost 500-million-kilometer journey from Earth to Mars. The total distance travelled is roughly the same as the distance from the Sun to Saturn.

- The high-resolution image data, in stereo and color, form the basis for the global mapping of Mars and the creation of digital terrain models that reveal the planet's topography. HRSC has acquired image data during more than 5000 orbits around Mars and thus provided more data than any other German experiment investigating objects within the Solar System. To date, approximately 360 GB of compressed raw data have been acquired and transmitted to Earth. After 'unpacking' the data packets and putting them through the first stage of processing, the researchers have 5400 GB of data, which form the basis for further processing into image data suitable for cartography.

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Figure 82: The striking landscape of Hydraotes Chaos on Mars acquired by HRSC (image credit: ESA/DLR/FU Berlin)

- The images have resulted in an ever-growing collection of spectacular views of the diverse Martian landscape – from image and terrain model mosaics to animations that can be derived from the digital landscape models. The camera has provided coverage of 80 percent of the Martian surface at high resolution (better than 20 m/pixel). Originally, Mars Express was intended to last only one Martian year, which equates to two Earth years. Due to its success, ESA has already extended the mission seven times, and it is now set to run until the end of 2022. Ralf Jaumann from the DLR Institute of Planetary Research in Berlin-Adlershof is the Principal Investigator for the experiment; the science and engineering team includes 51 co-investigators. Hundreds of scientists worldwide are now working with the data from the experiment. The systematic processing of the camera data is performed at the DLR Institute of Planetary Research. Staff in the Department of Planetary Science and Remote Sensing at the Freie Universität Berlin create the image products that are published monthly.

The German space industry implements a DLR concept for mapping Mars

- HRSC, which was developed by DLR in conjunction with German industry, represented, at the time, a completely new camera concept that had never previously been used for planetary mapping. Its unique optics – an Apo-Tessar telescope built by Jena-Optronik GmbH – illuminate nine line sensors positioned transverse to the direction of flight that, due to the forward motion of the orbiter, image the same strip of the Martian surface – like a scanner – working line-by-line, one after the other. In doing so, each sensor images the same area on the surface from a different angle. Back on Earth, the four stereo image strips and the nadir channel, which is oriented perpendicular to Mars and provides the highest image resolution at 10 to 12 m/pixel, are used to create 3D models of the planet's surface. The remaining four of the nine line sensors are equipped with special color filters for acquiring multispectral data. The focal plane with the nine sensors forms the heart of the camera and was developed by DLR. Lewicki Microelectronic GmbH built the camera's electronics. Dornier (later Astrium and now Airbus Defence and Space) in Friedrichshafen assembled the complete instrument. The camera system was originally developed for use on the Russian Mars 96 mission, but it was lost shortly after launch on 17 November 1996. The flight spare camera, which is of identical construction, was then used on Mars Express instead – after the incorporation of a number of improvements.

• 10 January 2019: ESA’s Mars Express entered orbit around the Red Planet on 25 December 2003. The spacecraft began returning the first images from orbit using its HRSC (High Resolution Stereo Camera) just a couple of weeks later, and over the course of its fifteen year history has captured thousands of images covering the globe. 32)

Figure 83: This video compilation highlights some of the stunning scenes revealed by this long-lived mission. From breathtaking horizon-to-horizon views to the close-up details of ice- and dune-filled craters, and from the polar ice caps and water-carved valleys to ancient volcanoes and plunging canyons, Mars Express has traced billions of years of geological history and evolution [video credit: ESA/DLR/FU Berlin (CC BY-SA 3.0 IGO), Published on 10 January 2019]

• 20 December 2018: The image of Figure 84 shows what appears to be a large patch of fresh, untrodden snow – a dream for any lover of the holiday season. However, it’s a little too distant for a last-minute winter getaway: this feature, known as Korolev crater, is found on Mars, and is shown here in beautiful detail as seen by Mars Express. 33) 34)

- ESA’s Mars Express mission launched on 2 June 2003, and reached Mars six months later. The satellite fired its main engine and entered orbit around the Red Planet on 25 December, making this month the 15-year anniversary of the spacecraft’s orbit insertion and the beginning of its science program.

- These images are an excellent celebration of such a milestone. Taken by the Mars Express HRSC (High Resolution Stereo Camera), this view of Korolev crater comprises five different ‘strips’ that have been combined to form a single image, with each strip gathered over a different orbit. The crater is also shown in perspective, context, and topographic views, all of which offer a more complete view of the terrain in and around the crater.

- The Korolev crater is 82 km across and found in the northern lowlands of Mars, just south of a large patch of dune-filled terrain that encircles part of the planet’s northern polar cap (known as Olympia Undae). It is an especially well-preserved example of a martian crater and is filled not by snow but ice, with its center hosting a mound of water ice some 1.8 km thick all year round.

- This ever-icy presence is due to an interesting phenomenon known as a ‘cold trap’, which occurs as the name suggests. The crater’s floor is deep, lying some two km vertically beneath its rim.

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Figure 84: Korolev crater in context: This image shows the landscape in and around Korolev crater, an 82 km across feature found in the northern lowlands of Mars. The region outlined by the bold white box indicates the area imaged by the Mars Express HRSC over orbits 18042 (captured on 4 April 2018), 5726, 5692, 5654, and 1412. The other white boxes indicate the data gathered by Mars Express over each individual orbit. The blue hues across the frame represent the elevation of the terrain, as indicated by the bar at the bottom (image credit: NASA MGS MOLA Science Team)

- The very deepest parts of Korolev crater, those containing ice, act as a natural cold trap: the air moving over the deposit of ice cools down and sinks, creating a layer of cold air that sits directly above the ice itself.

- Behaving as a shield, this layer helps the ice remain stable and stops it from heating up and disappearing. Air is a poor conductor of heat, exacerbating this effect and keeping Korolev crater permanently icy.

- The crater is named after chief rocket engineer and spacecraft designer Sergei Korolev (1907-1966), dubbed the father of Soviet space technology. Korolev worked on a number of well-known missions including the Sputnik program – the first artificial satellites ever sent into orbit around the Earth, in 1957 and the years following, the Vostok and Vokshod programs of human space exploration (Vostok being the spacecraft that carried the first ever human, Yuri Gagarin, into space in 1961) as well as the first interplanetary missions to the Moon, Mars, and Venus. He also worked on a number of rockets that were the precursors to the successful Soyuz launcher – still the workhorses of the Russian space program, and used for both crewed and robotic flights.

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Figure 85: Plan view of Korolev crater. This plan mosaic comprises five different observational strips that have been combined to form a single image, gathered over orbits 18042 (captured on 4 April 2018), 5726, 5692, 5654, and 1412. It covers a region centered at 165º E, 73º N, and has a resolution of ~21 m/pixel. This image was created using data from the nadir and color channels of the HRSC. The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- The region of Mars has also been of interest to other missions, including ESA’s ExoMars program, which aims to establish if life ever existed on Mars. The CaSSIS (Color and Stereo Surface Imaging System) instrument aboard the ExoMars Trace Gas Orbiter, which began operating at Mars on 28 April 2018, also snapped a beautiful view of part of Korolev crater – this was one of the very first images the spacecraft sent back to Earth after arriving at our neighboring planet. CaSSIS imaged a 40 km long chunk of the crater’s northern rim, neatly showcasing its intriguing shape and structure, and its bright icy deposits.

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Figure 86: Topography of the Korolev crater. This color-coded topographic view shows the relative heights of the terrain in and around the Korolev crater, an ice-filled crater in the northern lowlands of Mars. Lower parts of the surface are shown in blues and purples, while higher-altitude regions show up in whites, browns, and reds, as indicated on the scale to the top right. The crater’s thick deposit of ice can be seen at the center of the frame. This view is based on a digital terrain model of the region, from which the topography of the landscape can be derived. It comprises data obtained by the HRSC on Mars Express over orbits 18042 (captured on 4 April 2018), 5726, 5692, 5654, and 1412.It covers a region centered at 165º E, 73º N, and has a resolution of ~ 21 m/pixel (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)