Mars Express Mission
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.
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.
Figure 1: Illustration of ESA's Mars Express spacecraft (image credit: ESA)
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.
Table 1: Spacecraft mass and power budget 3)
Table 2: Spacecraft parameters
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 manuver 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).
• 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. 5)
- 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.
Figure 3: 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. 6)
Figure 4: 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)]
• 20 December 2018: The image of Figure 5 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. 7) 8)
- 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.
Figure 5: 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.
Figure 6: 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.
Figure 7: 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)
• 25 November 2018: At just before 21 hours CET (Central European Time) on 26 November, Mars will receive a new visitor: NASA's InSight lander. InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) will be the first Mars mission dedicated to studying the planet's interior, including sensing Mars quakes. Learning about the interior of the planet will inform scientists about the early formation of the rocky planets in our own Solar System, as well as the evolution of exoplanets orbiting other stars. 9)
Figure 8: Global view from Mars Express of the InSight landing site on Elysium Planitia on Mars. InSight will target a landing site centered at 4.5ºN/135.9ºE, about 600 km from Gale Crater, the region that NASA's Curiosity rover is exploring (image credit: ESA, CC BY-SA 3.0 IGO)
- Since InSight's study is focused on sensing the planet's interior, surface geology is not such an important factor in deciding the landing site as it is for other missions. Therefore, it is targeting a flat, stable surface in the Elysium Planitia region, which is captured in this wide field view from ESA's Mars Express Visual Monitoring Camera taken on 29 February 2016.
- In the image of Figure 8, Elysium Planitia is located roughly between the dark features at the bottom right (which includes Gale Crater), and the brighter arc-shaped feature above, to the right of the center of the image, which is the location of volcano Elysium Mons. The north polar ice cap is seen at the top of the image.
- ESA has already been supporting InSight's mission with its ground station network throughout the cruise to Mars, following the mission's launch in May 2018. The joint ESA-Roscosmos Trace Gas Orbiter (TGO) of the ExoMars mission, which arrived at Mars in October 2016, is ready to support data relay from InSight several times per day once it has landed safely, as required. Mars Express will also be prepared to support, on NASA's request, ad hoc relay contacts with InSight in case of emergency needs.
- TGO will also act as a data relay for the ExoMars rover mission in 2021, for which the landing site was recommended earlier this month as Oxia Planum. A region that is thought to have hosted vast volumes of water in the past, it is an ideal location to search for clues that may help reveal the presence of past life on Mars.
- NASA also just announced the landing site for its Mars 2020 rover, which is set to explore an ancient river delta in Jezero Crater. Moreover, the rover will collect rock and soil samples and store them in a cache on the planet's surface. NASA and ESA are studying future mission concepts to retrieve the samples and return them to Earth, setting the stage for the next decade of Mars exploration.
• 22 November 2018: ESA's Mars Express has imaged an intriguing part of the Red Planet's surface: a rocky, fragmented, furrowed escarpment lying at the boundary of the northern and southern hemisphere. 10)
Figure 9: This perspective view shows Nili Fossae, an escarpment sitting between the northern lowlands (lower right) and southern highlands (upper left) of Mars (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)
Legend to Figure 9: This oblique perspective was generated using data from ESA's Mars Express HRSC. This scene is part of a region imaged during Mars Express orbit 17916 on 26 February 2018, with the gathered data combined to form a detailed mosaic. The image covers a part of the martian surface centered on 78°E, 28°N. This view looks across the feature from south to north.
- This region is an impressive example of past activity on the planet and shows signs of where flowing wind, water and ice once moved material from place to place, carving out distinctive patterns and landforms as it did so.
- Mars is a planet of two halves. In places, the northern hemisphere of the planet sits a full few kilometers lower than the southern; this clear topographic split is known as the martian dichotomy, and is an especially distinctive feature on the Red Planet's surface.
Figure 10: Nili Fossae in context: Nili Fossae, an escarpment sitting between the northern lowlands and southern highlands of Mars, shown in a wider context. The region outlined by the larger white box indicates the whole area imaged during ESA's Mars Express orbit 17916, while the smaller box shows the area displayed in this image release. In this context image, north is up (image credit: NASA MGS MOLA Science Team)
- Northern Mars also displays large areas of smooth land, whereas the planet's southern regions are heavily pockmarked and scattered with craters. This is thought to be the result of past volcanic activity, which has resurfaced parts of Mars to create smooth plains in the north – and left other regions ancient and untouched.
- The star of this Mars Express image, a furrowed, rock-filled escarpment known as Nili Fossae, sits at the boundary of this north-south divide. This region is filled with rocky valleys, small hills, and clusters of flat-topped landforms (known as mesas in geological terms), with some chunks of crustal rock appearing to be depressed down into the surface creating a number of ditch-like features known as graben.
Figure 11: This color view shows the landscape around Nili Fossae, an escarpment sitting between the northern lowlands and southern highlands of Mars. It was created using data from the nadir channel of the HRSC on ESA's Mars Express orbiter, the field of view which is aligned perpendicular to the surface of Mars, and the camera's color channels. The data were acquired during spacecraft orbit 17916. The ground resolution is about 18 m/pixel and the images cover a part of the martian surface centered on 78ºE, 28ºN. North is to the right. (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)
- As with much of the surrounding environment, and despite Mars' reputation as a dry, arid world today, water is believed to have played a key role in sculpting Nili Fossae via ongoing erosion. In addition to visual cues, signs of past interaction with water have been spotted in the western (upper) part of this image – instruments such as Mars Express' OMEGA spectrometer have spotted clay minerals here, which are key indicators that water was once present.
- The elevation of Nili Fossae and surroundings, shown in the topographic view (Figure 12), is somewhat varied; regions to the left and lower left (south) sit higher than those to the other side of the frame (north), illustrating the aforementioned dichotomy. This higher-altitude terrain appears to consist mostly of rocky plateaus, while lower terrain comprises smaller rocks, mesas, hills, and more, with the two sections roughly separated by erosion channels and valleys.
- This split is thought to be the result of material moving around on Mars hundreds of millions of years ago. Similar to glaciers on Earth, flows of water and ice cut through the martian terrain and slowly sculpted and eroded it over time, also carrying material along with them. In the case of Nili Fossae, this was carried from higher areas to lower ones, with chunks of resistant rock and hardy material remaining largely intact but shifting downslope to form the mesas and landforms seen today.
Figure 12: This image shows the relative heights of the landscape in and around Nili Fossae, an escarpment sitting between the northern lowlands and southern highlands of Mars. The 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 (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)
Legend to Figure 12: The color-coded topographic 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 ESA's Mars Express during spacecraft orbit 17916. The ground resolution is about 18 m/pixel and the images cover a part of the martian surface centered on 78°E, 28°N. North is to the right.
Figure 13: This image shows Nil Fossae, an escarpment sitting between the northern lowlands and 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 channel and one stereo channel of the High Resolution Stereo Camera (HRSC) on ESA's Mars Express during spacecraft orbit 17916. It covers a part of the martian surface centered on 78ºE, 28ºN. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)
- The shapes and structures scattered throughout this image are thought to have been shaped over time by flows of not only water and ice, but also wind. Examples can be seen in this image in patches of the surface that appear to be notably dark against the ochre background, as if smudged with charcoal or ink. These are areas of darker volcanic sand, which have been transported and deposited by present-day martian winds. Wind moves sand and dust around often on Mars' surface, creating rippling dune fields across the planet and forming multi-colored, patchy terrain like Nili Fossae.
- ESA's Mars Express was launched in 2003. As well as producing striking views of the martian surface such as this, the mission has shed light on many of the planet's biggest mysteries – and helped to build the picture of Mars as a planet that was once warmer, wetter and potentially habitable.
Table 3: Extended life for ESA's science missions 11)
• 26 October 2018: The surface of Mars may appear to be perpetually still, but its many features are ever-changing – as represented in this Mars Express view of the severely eroded Greeley impact crater. 12)
- Greeley crater, named for the renowned planetary scientist Ronald Greeley, is located in one of the most ancient parts of Mars: a section of the planet's southern highlands named Noachis Terra.
- This region is thought to be some four billion years old, and is thus home to many features that formed in the very earliest days of the Solar System. Many craters have formed, changed, and eroded away in Noachis Terra, and Greeley crater is no exception.
- The subject of these Mars Express images sits between two huge, deep impact basin plains, Argyre and Hellas, and is a great example of a very old crater that has endured significant erosion over time.
- Wind, water, ice, and subsequent impacts have all played a part in wearing down the once-fresh structure of the crater. They have smoothed away and removed its walls and rims, erased any characteristic patterns in the nearby landscape that may have formed alongside the crater (such as ‘ejecta', or rays of material flung out from an impact site), and infilled and flattened out its floor.
- This floor is covered with a number of smaller impact pits and pockmarks that have occurred since Greeley crater's formation – another clear indication of the crater's immense age. With a depth of only 1.5 km Greeley crater is actually relatively shallow for a martian crater, making it somewhat difficult to pick out from the surrounding terrain.
Figure 14: The Greeley crater is a degraded impact crater in the Southern Highlands of Mars. The region outlined by the larger white box indicates the area imaged over 16 Mars Express orbits (0430, 1910, 1932, 2412, 2467, 2478, 4306, 4317, 4328, 6556, 8613, 8620, 8708, 12835, 14719, 16778). In this context image, north is up (image credit: NASA MGS MOLA Science Team) 13)
Figure 15: This image shows the landscape in and around Greeley crater, a degraded impact crater in the southern highlands of Mars. This color image was created using data from the nadir channel, the field of view which is aligned perpendicular to the surface of Mars, and the camera's color channels (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO) 14)
Legend to Figure 15: This plan view is a mosaic of data acquired by the High Resolution Stereo Camera on Mars Express over 16 of the spacecraft's orbits (0430, 1910, 1932, 2412, 2467, 2478, 4306, 4317, 4328, 6556, 8613, 8620, 8708, 12835, 14719, 16778). The ground resolution is about 100 m/pixel and the images cover a part of the martian surface ranging from 2°W to 9°E / 31.5° to 43.5°S. North is up.
- Accompanying views of the crater show it in a wider context on Mars, color-coded by topography – highlighting the relative depths of the crater, its broken-down wall, smaller superimposed craters, and other features throughout the region – and also via an oblique perspective, which looks across the crater towards the south-west. Together, these images well-characterize the crater and its environment, and offer an intriguing insight into this ancient region on our planetary neighbor.
- Greeley crater earned its moniker following a proposal by the International Astronomical Union in 2015 to name the crater after distinguished planetary scientist Ronald Greeley. Greeley passed away on 27 October 2011. Ronald Greeley's passion was Mars science. Not only was he a co-investigator for the Mars Express HRSC, but he was also involved in other Mars missions, such as Mariner, Viking, Pathfinder, Mars Global Surveyor and the Mars Exploration Rovers Spirit and Opportunity. His career in planetary research began in 1967 at NASA AMES Research Center, where he studied volcanic landforms and lava tubes on Earth and the Moon. He later worked with planetary mission data acquired by Galileo for Jupiter, Magellan for Venus, and Voyager 2 for Uranus and Neptune. He was also interested in surface features and processes that occur due to the effects of wind on other planets. In 1977 he became Professor of the School of Earth and Space Exploration at Arizona State University, where he created the Planetary Aeolian Laboratory, which is still running to this day.
- Alongside significant work in planetary science spanning not only Mars and related missions but also lunar research and missions to Venus, Jupiter, Uranus, and Neptune, Greeley was a Regents' Professor of planetary geology at Arizona State University from 1977 to 2011, and co-investigator of the Mars Express High Resolution Stereo Camera (HRSC) – the instrument that gathered the data used in these images.
Figure 16: This image shows the relative heights of the landscape in and around Greeley crater, a degraded impact crater 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, browns, and reds, as indicated on the scale to the top right. The color-coded topographic view is based on a digital terrain model of the region, from which the topography of the landscape can be derived (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO) 15)
Legend to Figure 16: This colored topographic view comprises data obtained by the High Resolution Stereo Camera on Mars Express over 16 of the spacecraft's orbits (0430, 1910, 1932, 2412, 2467, 2478, 4306, 4317, 4328, 6556, 8613, 8620, 8708, 12835, 14719, 16778). The ground resolution is about 100 m/pixel and the images cover a part of the martian surface ranging from 2°W to 9°E / 31.5° to 43.5°S. North is up.
• 25 October 2018: Since 13 September, ESA's Mars Express has been observing the evolution of an elongated cloud formation hovering in the vicinity of the 20 km-high Arsia Mons volcano, close to the planet's equator. 16)
- In spite of its location, this atmospheric feature is not linked to volcanic activity but is rather a water ice cloud driven by the influence of the volcano's leeward slope on the air flow – something that scientists call an orographic or lee cloud – and a regular phenomenon in this region.
- The cloud can be seen in this view taken on 10 October by the Visual Monitoring Camera (VMC) on Mars Express – which has imaged it hundreds of times over the past few weeks – as the white, elongated feature extending 1500 km westward of Arsia Mons. As a comparison, the cone-shaped volcano has a diameter of about 250 km.
Figure 17: Cloud formation near Arsia Mons (image credit: ESA/GCP/UPV/EHU Bilbao, CC BY-SA 3.0 IGO)
- Mars just experienced its northern hemisphere winter solstice on 16 October. In the months leading up to the solstice, most cloud activity disappears over big volcanoes like Arsia Mons; its summit is covered with clouds throughout the rest of the martian year.
- However, a seasonally recurrent water ice cloud, like the one shown in this image, is known to form along the southwest flank of this volcano – it was previously observed by Mars Express and other missions in 2009, 2012 and 2015.
- The cloud's appearance varies throughout the martian day, growing in length during local morning downwind of the volcano, almost parallel to the equator, and reaching such an impressive size that could make it visible even to telescopes on Earth.
- The formation of water ice clouds is sensitive to the amount of dust present in the atmosphere. These images, obtained after the major dust storm that engulfed the entire planet in June and July, will provide important information on the effect of dust on the cloud development and on its variability throughout the year.
- The elongated cloud hovering near Arsia Mons this year was also observed with the visible and near-infrared mapping spectrometer, OMEGA, and the High Resolution Stereo Camera (HRSC) on Mars Express, providing scientists with a variety of different data to study this phenomenon.
Figure 18: Left: Cloud on 21 September 2018 observed by HRSC (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO); Right: Cloud on 17 September 2018 observed by OMEGA (image credit: ESA/CNES/CNRS/IAS)
• 25 July 2018: Radar data collected by ESA's Mars Express point to a pond of liquid water buried under layers of ice and dust in the south polar region of Mars. 17)
- Evidence for the Red Planet's watery past is prevalent across its surface in the form of vast dried-out river valley networks and gigantic outflow channels clearly imaged by orbiting spacecraft. Orbiters, together with landers and rovers exploring the martian surface, also discovered minerals that can only form in the presence of liquid water.
- But the climate has changed significantly over the course of the planet's 4.6 billion year history and liquid water cannot exist on the surface today, so scientists are looking underground. Early results from the 15-year old Mars Express spacecraft already found that water-ice exists at the planet's poles and is also buried in layers interspersed with dust.
- The presence of liquid water at the base of the polar ice caps has long been suspected; after all, from studies on Earth, it is well known that the melting point of water decreases under the pressure of an overlying glacier. Moreover, the presence of salts on Mars could further reduce the melting point of water and keep the water liquid even at below-freezing temperatures.
- But until now evidence from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument the first radar sounder ever to orbit another planet, remained inconclusive.
- It has taken the persistence of scientists working with this subsurface-probing instrument to develop new techniques in order to collect as much high-resolution data as possible to confirm their exciting conclusion.
- Ground-penetrating radar uses the method of sending radar pulses towards the surface and timing how long it takes for them to be reflected back to the spacecraft, and with what strength. The properties of the material that lies between influences the returned signal, which can be used to map the subsurface topography.
Figure 19: ESA's Mars Express has used radar signals bounced through underground layers of ice to identify a pond of water buried below the surface. This image shows an example radar profile for one of 29 orbits over the 200 x 200 km study region in the south polar region of Mars. The bright horizontal feature at the top corresponds to the icy surface of Mars. Layers of the south polar layered deposits – layers of ice and dust – are seen to a depth of about 1.5 km. Below is a base layer that in some areas is even much brighter than the surface reflections, while in other places is rather diffuse. The brightest reflections from the base layer – close to the center of this image – are centered around 193º/81ºS in all intersecting orbits, outlining a well-defined, 20 km wide subsurface anomaly that is interpreted as a pond of liquid water (image credit:ESA/NASA/JPL/ASI/University of Rome)
- The radar investigation shows that south polar region of Mars is made of many layers of ice and dust down to a depth of about 1.5 km in the 200 km-wide area analyzed in this study. A particularly bright radar reflection underneath the layered deposits is identified within a 20 km-wide zone.
- Analyzing the properties of the reflected radar signals and considering the composition of the layered deposits and expected temperature profile below the surface, the scientists interpret the bright feature as an interface between the ice and a stable body of liquid water, which could be laden with salty, saturated sediments. For MARSIS to be able to detect such a patch of water, it would need to be at least several tens of centimeters thick.
- "This subsurface anomaly on Mars has radar properties matching water or water-rich sediments," says Roberto Orosei, principal investigator of the MARSIS experiment and lead author of the paper published in the journal Science today. "This is just one small study area; it is an exciting prospect to think there could be more of these underground pockets of water elsewhere, yet to be discovered." 18)
- "We'd seen hints of interesting subsurface features for years but we couldn't reproduce the result from orbit to orbit, because the sampling rates and resolution of our data was previously too low," adds Andrea Cicchetti, MARSIS operations manager and a co-author on the new paper. "We had to come up with a new operating mode to bypass some onboard processing and trigger a higher sampling rate and thus improve the resolution of the footprint of our dataset: now we see things that simply were not possible before."
Figure 20: Water detection under the south pole of Mars (image credit: Context map: NASA/Viking; THEMIS background: NASA/JPL-Caltech/Arizona State University; MARSIS data: ESA/NASA/JPL/ASI/Univ. Rome; R. Orosei et al 2018)
- The finding is somewhat reminiscent of Lake Vostok, discovered some 4 km below the ice in Antarctica on Earth. Some forms of microbial life are known to thrive in Earth's subglacial environments, but could underground pockets of salty, sediment-rich liquid water on Mars also provide a suitable habitat, either now or in the past? Whether life has ever existed on Mars remains an open question, and is one that Mars missions, including the current European-Russian ExoMars orbiter and future rover, will continue to explore.
- "The long duration of Mars Express, and the exhausting effort made by the radar team to overcome many analytical challenges, enabled this much-awaited result, demonstrating that the mission and its payload still have a great science potential," says Dmitri Titov, ESA's Mars Express project scientist. "This thrilling discovery is a highlight for planetary science and will contribute to our understanding of the evolution of Mars, the history of water on our neighbor planet and its habitability."
• 1 June 2018: Fifteen years ago, ESA's Mars Express was launched to investigate the Red Planet. To mark this milestone comes a striking view of Mars from horizon to horizon, showcasing one of the most intriguing parts of the martian surface. 19)
- On 2 June 2003, the Mars Express spacecraft lifted off from Baikonur, Kazakhstan, on a journey to explore our red-hued neighboring planet. In the 15 years since, it has become one of the most successful missions ever sent to Mars, as demonstrated by this image of the region known as the Tharsis province, shown here in its full glory.
- Mammoth volcanoes, sweeping canyons, fractured ground: Tharsis is one of the most geologically interesting and oft-explored parts of the planet's surface. Once an incredibly active region, displaying both volcanism and the shifting crustal plates of tectonics, it hosts most of the planet's colossal volcanoes – the largest in the Solar System.
Legend to Figure 21: It sweeps from the planet's upper horizon — marked by the faint blue haze at the top of the frame — down across a web of pale fissures named Noctis Labyrinthus (a part of Valles Marineris stretching to the upper left corner of the image), Ascraeus and Pavonis Mons (two of Tharsis' four great volcanoes at more than 20 km high), and finishes at the planet's northern polar ice cap (in this perspective, North is to the lower left).
- Sitting near Mars' equator, Tharsis covers roughly a quarter of the martian surface and is thought to have a played an important role in the planet's history. It straddles the boundary between Mars' southern highlands and northern lowlands.
- Elevation on Mars is defined relative to where the gravity is the same as the average at the martian equator. This serves as a type of ‘sea-level', even though there are no seas.
- Most of Tharsis is higher than average, at between 2 and 10 km high. The province likely formed as mushroom-shaped plumes of molten rock (magma) swelled up beneath the viscous surface over time, creating seeping flows, magma chambers, and large, rocky provinces – like Tharsis – and feeding ongoing volcanism from below.
Figure 22: This map is based on data from NASA's Viking mission. It shows the slice of Mars captured by the HRSC aboard ESA's Mars Express spacecraft to celebrate the mission's 15th anniversary: the intriguing and once-active Tharsis province. Included in this labelled view is the extensive canyon system of Valles Marineris, the web-like system of fissures comprising Noctis Labyrinthus, four volcanoes, and the northern polar cap (image credit: NASA/Viking, FU Berlin) 20)
- Tharsis is also connected to the formation of the famous Valles Marineris, which is some four times longer and deeper than the Great Canyon in Arizona, USA, and the most extensive canyon system discovered in the Solar System. This is partly visible as the dark tendrils to the upper left of the image.
- As magma swelled up beneath the crust to create the Tharsis province, the tension caused some areas to rupture and fracture. Molten rock then flooded these fractures and destabilized and separated regions of the crust yet further, resulting in both the wide, substantial troughs and fissures that comprise modern-day Valles Marineris, and the web-like Noctis Labyrinthus that sits at the canyon system's western end.
- Captured in the new view are volcanoes Pavonis Mons (top right), Ascraeus Mons (just below), Alba Mons (to the bottom left), and a small sliver of Olympus Mons (to the lower right, continuing out of frame) in caramel hues; a view of the region with labels is provided here. The location of this slice of Mars' surface is also shown in a context map of the planet and in a topographic context.
Figure 23: This context map is based on data from the MOLA (Mars Orbiter Laser Altimeter) experiment onboard NASA's MGS (Mars Global Surveyor) mission. It shows the slice of Mars captured by the HRSC aboard ESA's Mars Express spacecraft to celebrate the mission's 15th anniversary: the intriguing and once-active Tharsis province (image credit: NASA/MGS/MOLA Science Team, FU Berlin)
Legend to Figure 23: Included in this labelled view is the extensive canyon system of Valles Marineris, the web-like system of fissures comprising Noctis Labyrinthus, two out of four volcanoes, the north pole, and the so-called Martian dichotomy: the difference in altitude between the northern and southern regions of Mars. Areas at higher altitudes are shown in red-orange tones, while those at lower ones are displayed in blue-greens (as indicated by the scale to the bottom left).
• 11 April 2018: Every so often, your smartphone or tablet receives new software to improve its functionality and extend its life. Now, ESA's Mars Express is getting a fresh install, delivered across over 150 million km of space. 21)
- With nearly 15 years in orbit, Mars Express – one of the most successful interplanetary missions ever – is on track to keep gathering critical science data for many more years thanks to a fresh software installation developed by the mission teams at ESA.
- The new software is designed to fix a problem that anyone still using a five-year-old laptop knows well: after years of intense usage, some components simply start to wear out.
- The spacecraft arrived at Mars in December 2003, on what was planned to be a two-year mission. It has gone on to spend more than 14 years gathering a wealth of data from the Red Planet, taking high-resolution images of much of the surface, detecting minerals on the surface that form only in the presence of water, detecting hints of methane in the atmosphere and conducting close flybys of the enigmatic moon, Phobos.
- Today, Mars Express is in good shape, with only some minor degradation in performance, but its gyroscopes are close to failing.
- These six gyros measure how much Mars Express rotates about any of its three axes. Together with the spacecraft's two startrackers, they determine its orientation in space. This is critical for pointing its large parabolic radio antenna towards Earth and to aim its instruments – like the high-resolution stereo camera – at Mars.
- Startrackers are simple, point-and-shoot cameras that capture images of the background star field and, with some clever processing, are used to determine the craft's orientation in space every few seconds.
- The rotation information from the gyros fills in the information between these snapshots and also when the trackers lose track of the stars – which can last for minutes or even hours.
- "After looking at variations in the intensity of the gyros' internal lasers, we realized last year that, with our current usage, four of the six gyros were trending towards failure," says spacecraft operations manager James Godfrey. "Mars Express was never designed to fly without its gyros continuously available, so we could foresee a certain end to the mission sometime between January and June 2019."
- Engineers knew, however, from long experience with similar gyros on previous missions, including Rosetta and ERS-2, that it might be possible to fly the mission primarily using its startrackers, with the gyros only being switched on occasionally, to extend their lives.
- Hacking 15 year-old code: "Flying on startrackers with the gyros mostly switched off meant that a significant portion of the 15 year-old software on Mars Express would have to be rewritten, and this would be a major challenge," says operations engineer Simon Wood.
- While the spacecraft's builder provided great assistance, it was mostly up to the teams at ESA to open the code, rewrite the software, test it and prepare it for upload as soon as possible.
- "We were also helped by being able to take code flown on Rosetta and transplant it into the Mars Express guidance software," adds Simon.
- A massive, multi-month effort followed, involving teams from across the Agency working to develop the new software that would enable Mars Express to keep flying. This also meant significant changes in instrument science planning.
- "We didn't know if such a massive revision was possible – it hadn't been done before, especially as we would be in a race against time to complete it. But faced with the almost-certain end of mission, what began as wild speculation during a tea break one afternoon last summer has led to the full rewrite now being ready to send up."
- The new software was finalized earlier this year, and has undergone meticulous testing to ensure it will work as intended.
- Go/No-Go: The effort came to fruition yesterday, when the mission team met for a critical go/no-go meeting with the ESA managers to get final approval to activate the new software.
- The new code was actually uploaded to an area of spare memory on Sunday, but just like when your phone or tablet gets a software upgrade, mission controllers will have to shut Mars Express down and trigger a reboot to start running the new code, a critical step set for 16 April.
- If all goes as expected, the mission teams will then spend about two weeks testing and reconfiguring the spacecraft to ensure everything is working as it should before resuming normal science operations.
- "Similar, but much smaller fixes, have been developed in the past for other missions with old gyros, such as Rosetta, but this is certainly the most complex and extensive software rewrite we've done in recent memory," says mission manager Patrick Martin. "Thanks to the skill of ESA's teams, Mars Express will fly well into the 2020s, depending on fuel supply, and continue delivering excellent science for many years yet. I look forward to seeing continued joint science campaigns between Mars Express and other Mars missions like ESA's Trace Gas Orbiter and incoming rover missions."
- Developed by experts at the observatories of Paris Sud (OSUPS) and Lyon (OSUL), PSUP has modules for filtering, processing, and downloading data (Mars System Information, or MarsSI), and for visualizing these data in 3D–including global mineralogical maps, geomorphologic maps, and various other catalogues (Mars Visu).
Figure 24: Screenshot of the PSUP tool. This image shows a view of Mars as seen via Mars Visu. The colored layer represents Mars' surface emissivity at a wavelength of 5µm: in other words, how efficiently any given part of the planetary surface is emitting infrared radiation (yellow-red being higher and blue-purple lower, as indicated by the key in the bottom right). The data comprising this layer are from OMEGA, the Near-Infrared Mineralogical Mapping Spectrometer aboard Mars Express. Mostly obscured beneath this layer, only visible as a few slashes across the planet's face, is a background covering of Viking data provided by Mars Dataset (image credit:ESA/PSUP (OSUPS/OSUL)
• 04 November 2011: The volcanoes on Mars are true giants. As well as being home to the largest volcano in our Solar System, the 24 km high Olympus Mons, and its three neighboring shield volcanoes Arsia, Pavonis and Ascraeus, there are a number of less-frequently observed volcano complexes on the Tharsis bulge near the Martian equator that also reach impressive heights. With a base measuring 155 x 125 km, the 8000 m Tharsis Tholus may only be a ‘mid-range' volcano, but when measured against terrestrial standards, this volcano is truly gigantic. The HRSC (High Resolution Stereo Camera) operated by DLR (German Aerospace Cente) on board ESA's Mars Express spacecraft acquired images of Tharsis Tholus over the course of several orbits, which have been combined to form a mosaic image with a resolution of 14 m/pixel. The images show an area located at 13º north and 268º east. 23) 24)
Figure 25: Perspective view from the north east to the summit of Tharsis Tholus (image credit: DLR, ESA)
- Just as on Earth, volcanoes on Mars played an important role in both its climatic history and the thermal evolution of its interior. Volcanic eruptions fed ‘fresh' gases into the atmosphere, thereby affecting the density and composition of this gaseous envelope. Whether a water cycle existed on Mars or whether it once rained are some of the most exciting questions addressed by Mars exploration. Closely related to this is the question of whether conditions were ever favorable for the development of life on the now dry planet.
- A caldera two and a half kilometers deep and the size of Berlin: Tharsis Tholus differs from many of the other volcanoes on Mars in that its edifice has undergone extensive modification. The complex has not developed in the usual way, for example as a cone or a shield surrounding the volcanic center; instead, it shows signs of substantial deformation. At least two major collapses on the western and eastern flanks have taken place in the course of its four billion year history. Evidence of these events is still visible, taking the form of the steep flanks some several kilometers in height, as well as concentric and ring faults.
- The main feature of Tharsis Tholus is, however, the size of its central caldera. This slightly elongated collapse crater at the summit of the volcano, measuring roughly 32 x 34 km, extends over an area almost as large as Berlin and the base is as much as 2.7 km below the rim. The caldera may have formed when a shallow magma chamber under the volcano emptied, primarily through volcanic eruptions – during which the magma emerged at the surface in the form of lava. This emptying process caused a large cavity to form inside the volcano. As lava accumulated over this cavity, there came a point when it could no longer support the additional weight and it collapsed, forming a depression known as a ‘collapse caldera'.
Figure 26: Alternate perspective of Tharsis Tholus. The volcano towers 8 km above the surrounding terrain with a base that stretches 155 x 125 km and a central caldera measuring 32 x 34 km. The image was created using a Digital Terrain Model (DTM) obtained from the HRSC on ESA's Mars Express spacecraft. Elevation data from the DTM is color coded: purple indicates the lowest lying regions and beige the highest. The scale is in meters. In these images, the relief has been exaggerated by a factor of three (image credit: ESA/DLR/FU Berlin (G. Neukum), CC BY-SA 3.0 IGO, Ref. 24)
- But the true size of Tharsis Tholus is concealed. As the nadir image shows, the volcano is surrounded by numerous solidified lava flows, hiding the original base of the volcano. Taking into account the number and massive extent of these lava flows, it is possible that Tharsis Tholus is ‘submerged' in lava to a depth of several kilometers.
- The image data used to create the images shown here were acquired using the HRSC between 28 October and 13 November 2004 during orbits 0997, 1019, 1041 and 1052. The images were produced by the Department of Planetary Sciences and Remote Sensing in the Institute for Geological Sciences of the Freie Universität Berlin. The perspective views were computed from the HRSC stereo channels. The anaglyph was derived from one stereo channel and the nadir channel, which captures image data at the highest resolution of all the channels. The black-and-white detail image was acquired with the nadir channel. The false-color images are based on digital terrain models of the region, from which the topography of the landscape can be derived.
- November 2011 – Mars in the spotlight: Mars continues to be one of the most important targets for planetary research. On 25 November, NASA's Mars Science Laboratory, a lander carrying a rover named Curiosity, will be launched on its journey to the Red Planet. Curiosity is five times heavier than the two ‘veteran' rovers, Spirit and Opportunity, which have been exploring the Martian surface since 2004. Equipped with the most comprehensive and sophisticated suite of experiments, Curiosity will continue the quest to find evidence for the existence, past or present, of organic molecules on Mars.
- Even the Russian space program will again contribute to the exploration of Mars; on 5-8 November 2011 at 22:16 CET, the Phobos Grunt spacecraft will embark on its journey to Phobos, the larger of Mars' two moons. Once it lands in 2013, the small lander will collect samples for roughly a year. The loaded return vehicle will then blast off from Phobos and arrive back at Earth in 2014. DLR is participating in this mission by developing digital terrain models derived from HRSC image data, to support the Russians in the selection of the landing sites. Though manned missions to Mars are in the distant future, the Mars500 long-term experiment will help with preparations. In this experiment, the subjects embarked on a 520-day virtual flight to Mars inside a simulated spaceship. This journey will come to an end on 4 November, when they will ‘return to Earth'.
- The HRSC experiment on the European Space Agency's Mars Express mission is led by the Principal Investigator (PI) Prof. Dr Gerhard Neukum, who was also responsible for the technical design of the camera. The science team for the experiment consists of 40 co-investigators from 33 institutions and 10 nations. The camera was developed at DLR under the leadership of the PI and it was built in cooperation with industrial partners EADS Astrium, Lewicki Microelectronic GmbH and Jena Optronik GmbH. The instrument is operated by the DLR Institute of Planetary Research, through ESA/ESOC. The systematic processing of the HRSC image data is carried out at DLR. The scenes shown here were processed by the PI-group at the Institute for Geological Sciences of the Freie Universität Berlin.
• In 2004, a year after Europe's first mission to Mars was launched, the flight dynamics team at ESA's operations center encountered a serious problem. New computer models showed a worrying fate for the Mars Express spacecraft if mission controllers continued with their plans to deploy its giant MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) radar. 25)
Figure 27: Artist's impression of Mars Express. The background is based on an actual image of Mars taken by the spacecraft's HRSC (image credit: ESA/ATG medialab; Mars: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)
- This extremely sensitive radar instrument spans 40 meters across once fully extended, making it longer than a Space Shuttle orbiter and was built with the direct intention of finding water beneath Mars' surface. By sending out a series of chips between 1.8 and 5.0 Mhz in ‘subsurface' mode would scour the red planet for any signs of water anywhere down to a depth of a few kilometers. A secondary ‘ionosphere' mode at 0.1 to 5.4 Mhz surveyed the electrical conductivity of the Martian upper atmosphere.
- Two ‘radar booms', 20 m long hollow cylinders, 2.5 cm in diameter, and one 7 m boom, were folded up in a box like a concertina. Once the box was opened, all of the stored elastic energy from the glass fiber booms would be released, a little like a jack-in-the-box, and they would lock into a straight line.
- New and updated computer models, however, showed that these long rods would swing back and forth upon release with an even greater amplitude than previously thought, potentially coming into close contact with the delicate parts of the Mars Express body.
Deployment was postponed:
Figure 28: Artist's impression of MARSIS Boom 1 deployed (image credit: ESA)
- Plans were made to get the spacecraft in a ‘robust' mode before the deployment of each boom and while the glass fiber cylinders were extended. After each deployment the control team would conduct a full assessment of the spacecraft, taking up to a few days, before moving onto the next phase.
- The first deployment began on 4 May 2005 with one of the two 20 m ‘dipole' booms, and flight controllers at ESA's operations center quickly realized something wasn't quite right. 12 out of 13 of the boom segments had ‘snapped' into place, but one, possibly number 10, was not in position.
Deployment of the second and third booms was postponed
- Further analysis showed that prolonged storage in the cold conditions of outer space had affected the fiberglass and Kevlar material of the boom. What could be done to heat it up?
Figure 29: MARSIS boom 2 deployment begins (image credit: ESA, CC BY-SA 3.0 IGO)
- Enter the Sun: Mission teams decided to swing the 680 kg spacecraft to a position that would allow the Sun to heat the cold side of the boom. It was hoped that as the cold side expanded in the heat, the unlocked segment would be forced into place.
- One hour later, as contact was reestablished at 04:50 CET on 11 May, detailed analysis showed all segments had successfully locked in place and Boom 1 was successfully deployed!
- Following the rollercoaster rollout of the first antenna, flight controllers spent some time mulling over the events. A full investigation ensued, lessons were learnt, and plans were put in place to prevent the same irregularity from taking place in the next two deployments.
- By 14 June 2005, operators felt confident that they, and Mars Express, were ready to deploy the second boom. At 13:30 CEST the commands were sent.
- This time, Mars Express was set into a slow rotation to last 30 minutes during and after the release of the second 20 m boom. The rotation was planned so that all of the boom's hinges would be properly heated by the Sun before, during, and after deployment.
- Just three hours later and the first signs of success reached ground control, showing that Mars Express had properly re-oriented itself and was pointing towards Earth, transmitting data.
- The data confirmed that the spacecraft was working with two fully and correctly deployed booms, and their deployment had not caused any damage to the spacecraft.
- Not long after, the third boom was deployed, and the full MARSIS setup was complete on Mars Express.
Figure 30: MARSIS fully deployed (image credit: ESA, CC BY-SA 3.0 IGO)
Let the science begin
- Just four months later, and ESA was reporting on the radar's activities. MARSIS radar scientists were collecting data about a highly electrically conducting layer – surveyed in sunlight. They were also continuing the laborious analysis of data in the search for any possible signs of underground water, in a frozen or liquid state.
- Radar science is based on the detection of radio waves, reflected at the boundaries between different materials. Each material interacts with light in a different way, so as the radio wave crosses the boundary between different layers of material, an echo is generated that carries a sort of ‘fingerprint', providing information about the kind of material causing the reflection, including clues to its composition and physical state.
Figure 31: MARSIS prospecting for water (image credit: ESA)
• 17 March 2004: Thanks to ESA's Mars Express, we now know that Mars has vast fields of perennial water ice, stretching out from the south pole of the Red Planet. 26)
- Astronomers have known for years that Mars possessed polar ice caps, but early attempts at chemical analysis suggested only that the northern cap could be composed of water ice, and the southern cap was thought to be carbon dioxide ice.
- Recent space missions then suggested that the southern ice cap, existing all year round, could be a mixture of water and carbon dioxide. But only with Mars Express have scientists been able to confirm directly for the first time that water ice is present at the south pole too.
Figure 32: Map of the Mars south pole, as derived from OMEGA infrared spectral images, showing the bright polar cap, rich in carbon dioxide (light pink), surrounded by water-rich ice, free of carbon dioxide (green to blue), image credit: ESA-OMEGA
- Mars Express made observations with its OMEGA instrument to measure the amounts of sunlight and heat reflected from the Martian polar region. When planetary scientists analyzed the data, it clearly showed that, as well as carbon dioxide ice, water ice was present too.
- The results showed that hundreds of square kilometers of ‘permafrost' surround the south pole. Permafrost is water ice, mixed into the soil of Mars, and frozen to the hardness of solid rock by the low Martian temperatures. This is the reason why water ice has been hidden from detection until now - because the soil with which it is mixed cannot reflect light easily and so it appears dark.
- However, OMEGA looked at the surface with infrared eyes and, being sensitive to heat, clearly picked up the signature of water ice. The discovery hints that perhaps there are much larger quantities of water ice all over Mars than previously thought.
- Using this data, planetary scientists now know that the south polar region of Mars can be split into three separate parts. Part one is the bright polar cap itself, a mixture of 85% highly reflective carbon dioxide ice and 15% water ice.
- The second part comprises steep slopes known as ‘scarps', made almost entirely of water ice, that fall away from the polar cap to the surrounding plains. The third part was unexpected and encompasses the vast permafrost fields that stretch for tens of kilometers away from the scarps.
- The OMEGA observations were made between 18 January and 11 February this year, when it was late summer for the Martian southern hemisphere and temperatures would be at their highest. Even so, that is probably only –130 º Celsius and the ice that Mars Express has observed is a permanent feature of this location.
- During the winter months, scientists expect that carbon dioxide from the atmosphere will freeze onto the poles, making them much larger and covering some of the water ice from view.
- Mars Express and OMEGA will now continue looking for water ice and minerals across the surface of the planet. In May, another Mars Express instrument, the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS), will begin collecting data, looking for water underground.
- It will be particularly exciting when MARSIS looks at the south pole because, once planetary scientists know how deep the ice reaches, they will be able to calculate exactly how much water there is. Knowing this is very important to understand how Mars evolved and if it ever harbored life.
• 23 January 2004: Mars Express, ESA's first mission to Mars, will reach its final orbit on 28 January. It has already been producing stunning results since its first instrument was switched on, on 5 January. - The significance of the first data was emphasized by the scientists at a European press conference today at ESA's Space Operations Center, Darmstadt, Germany. 27)
- I did not expect to be able to gather together - just one month after the Mars Orbit Insertion of 25 December – so many happy scientists eager to present their first results", said Professor David Southwood, ESA Director of Science. One of the main targets of the Mars Express mission is to discover the presence of water in one of its chemical states. Through the initial mapping of the South polar cap on 18 January, OMEGA, the combined camera and infrared spectrometer, has already revealed the presence of water ice and carbon dioxide ice.
Figure 33: OMEGA image of the southern polar cap of Mars acquired on 18 January 2004,in all three bands. At right is the visible image; in the middle is carbon dioxide ice; at left is water ice. The two types of ice are mixed in some areas but distinct in others (image credit: ESA/IAS, Orsay; J-P. Bibring)
Figure 34: OMEGA observation of the South polar cap (image credit: ESA/IAS, Orsay; J-P. Bibring)
- This information was confirmed by the PFS (Planetary Fourier Spectrometer), a new high-resolution spectrometer of unprecedented accuracy. The first PFS data also show that the carbon oxide distribution is different in the northern and southern hemispheres of Mars.
Figure 35: PFS shows different CO2 distribution in the northern and southern hemispheres of Mars (image credit: ESA, CNR)
- The MaRS (Mars Radio Science Experiment) instrument, a sophisticated radio transmitter and receiver, emitted a first signal successfully on 21 January that was received on Earth through a 70- meter antenna in Australia after it was reflected and scattered from the surface of Mars. This new measurement technique allows the detection of the chemical composition of the Mars atmosphere, ionosphere and surface.
- ASPERA (Analyzer of Space Plasmas and Energetic Atoms), a plasma and energetic neutral atoms analyzer, is aiming to answer the fundamental question of whether the solar wind erosion led to the present lack of water on Mars. The preliminary results show a difference in the characteristics between the impact of the solar wind area and the measurement made in the tail of Mars. Another exciting experiment was run by the SPICAM instrument (an ultraviolet and infrared spectrometer) during the first star occultation ever made at Mars. It has simultaneously measured the distribution of the ozone and water vapor, which has never been done before, revealing that there is more water vapor where there is less ozone.
- ESA also presented astonishing pictures produced with the High Resolution Stereo Camera (HRSC). They represent the outcome of 1.87 million km2 of Martian surface coverage, and about 100 GB of processed data. This camera was also able to make the longest swath (up to 4000 km) and largest area in combination with high resolution ever taken in the exploration of the Solar System.
Figure 36: This picture was taken by the High Resolution Stereo Camera (HRSC) onboard ESA's Mars Express orbiter, in color and 3D, in orbit 18 on 15 January 2004 from a height of 273 km. The location is east of the Hellas basin at 41º South and 101º East. The area is 100 km across, with a resolution of 12 m per pixel, and shows a channel (Reull Vallis) once formed by flowing water. The landscape is seen in a vertical view, North is at the top (image credit: ESA/DLR/FU Berlin (G. Neukum), CC BY-SA 3.0 IGO)
• 19 January 2004: The image of Figure 37 shows a portion of a 1700 km long and 65 km wide swath which was taken in south-north direction across the Grand Canyon of Mars (Valles Marineris) from two perspectives. It is the first image of this size that shows the surface of Mars in high resolution (12 m/pixel), in color and in 3D. 28)
Figure 37: This HRSC image was acquired on 14 January 2004 on board ESA's Mars Express orbiter under the responsibility of the Principal Investigator Prof. Gerhard Neukum. It was processed by the Institute for Planetary Research of the German Aerospace Center (DLR), also involved in the development of the camera, and by the Institute of Geosciences of the FU (Freie Universität) Berlin. The image shows a portion of a 1700 km-long and 65 km-wide swath taken in the south-to-north direction across the huge Valles Marineris canyon (image credit: ESA/DLR/FU Berlin; G. Neukum)
Figure 38: This picture was taken by the HRSC onboard ESA's Mars Express orbiter, in color and 3D, in orbit 18 on 14 January 2004. It shows a vertical view of a mesa in the true colours of Mars. The summit plateau stands about 3 km above the surrounding terrain. The original surface was dissected by erosion, only isolated mesas remained intact. The large crater has a diameter of 7.6 km (image credit: ESA/DLR/FU Berlin (G. Neukum), CC BY-SA 3.0 IGO) 29)
Figure 39: This picture was taken by the HRSC aboard ESA's Mars Express, in color and 3D, during orbit 18 on 14 January 2004 from a height of 275 km. The location is in Valles Marineris at 5º North and 323º East. The area is 50 km across, at a resolution of 12 m/pixel, and shows mesas and cliffs as well as flow features which indicate erosion by the action of flowing water. The landscape is seen in a vertical view, with north at the bottom (image credit: ESA/DLR/FU Berlin (G. Neukum), CC BY-SA 3.0 IGO) 30)
• Following spacecraft commissioning in January 2004, most instruments began their own calibration and testing, in the process acquiring scientific data. This phase lasted until June 2004, when all the instruments but one began routine operations after the payload commissioning review. The deployment of the MARSIS radar antennas, however, was postponed. The late deployment was initially planned to maximize daylight operations of the other instruments before the pericenter naturally drifts to southern latitudes, which coincides with the nightime conditions required for subsurface sounding by MARSIS. The nominal lifetime of the orbiter is a martian year (687 days), with a potential extension by another martian year to complete global coverage and observe all seasons twice over (Ref. 1).
Sensor complement (HRSC, ASPERA, PFS, OMEGA, MARSIS, MaRS, SPICAM)
Table 4: The Mars Express scientific experiments (Ref. 1)
The lander's scientific payload totalled less than 10 kg, shared between six instruments and two dedicated tools to sample the surface and subsurface materials of Mars, plus a robotic sampling arm with 5 degrees-of-freedom. Two were mounted directly on the lander platform: the Gas Analysis Package and the Environmental Sensor Suite. The others were housed within an innovative structure called the Payload Adjustable Workbench (PAW) at the end of the robotic sampling arm: the Stereo Camera System, Microscope, X-ray Spectrometer and Mössbauer Spectrometer, together with a set of tools that included the Rock Corer Grinder, the PLanetary Underground TOol and other support equipment such as a sampling spoon, a torch and a wide-angle mirror. The PAW also carried one of the ESS sensors. The science-payload-to-landed-structure ratio is about 1/3, the highest so far of any planetary lander.
Figure 40: Mars Express with the Beagle 2 capsule still attached. 1: MARSIS. 2: HRSC. 3: OMEGA. 4: PFS. 5: SPICAM. 6: ASPERA. 7: Beagle 2. (MaRS requires no dedicated hardware), image credit: ESA
HRSC (High Resolution Stereo Camera)
The HRSC, originally developed for the Russian-led Mars-96 mission, was selected as part of the Orbiter payload for ESA's Mars Express mission. The HRSC is a pushbroom scanning instrument with nine CCD line detectors mounted in parallel in the focal plane. Its unique feature is the ability to obtain near-simultaneous imaging data of a specific site at high resolution, with along-track triple stereo, four colors and five different phase angles, thus avoiding any time-dependent variations of the observational conditions. An additional Super-Resolution Channel (SRC) – a framing device – will yield nested images in the meter-resolution range for detailed photogeologic studies. The spatial resolution from the nominal periapsis altitude of 250 km will be 10 m/pixel, with an image swath of 53 km, for the HRSC and 2.3 m/pixel for the SRC. During the mission's nominal operational lifetime of 1 martian year (2 Earth years) and assuming an average HRSC data transfer share of 40%, it will be possible to cover at least 50% of the martian surface at a spatial resolution of ≤ 15 m/ pixel. More than 70% of the surface can be observed at a spatial resolution of ≤ 30 m/pixel, while more than 1% will be imaged at better than 2.5 m/ pixel. The HRSC will thus close the gap between the medium- to low-resolution coverage and the very high-resolution images of the Mars Observer Camera on the Mars Global Surveyor mission and the in situ observations and measurements by landers. 31) 32) 33) 34)
The HRSC will make a major contribution to the study of martian geosciences, with special emphasis on the evolution of the surface in general, the evolution of volcanism, and the role of water throughout martian history. The instrument will obtain images containing morphologic and topographic information at high spatial and vertical resolution, allowing the improvement of the cartographic base down to scales of 1:50,000. The experiment will also address atmospheric phenomena and atmosphere-surface interactions, and will provide urgently needed support for current and future lander missions as well as for exobiological studies. The goals of HRSC on Mars Express will not be met by any other planned mission or instrument.
HRSC is a multi-sensor pushbroom instrument comprising multiple CCD (Charge Coupled Device) line sensors mounted in parallel for simultaneous high-resolution stereo, multicolor and multi-phase imaging of the martian surface. An additional Super Resolution Channel provides frame images embedded in the basic HRSC swath at five times greater resolution.
"The strength of HRSC is to perform high resolution digital terrain models of the martian surface in order to provide topographic context for the geoscientific evaluation of surface processes in space and time," says Ralf Jaumann, HRSC Principal Investigator from the Institute of Planetary Research, DLR, Berlin, Germany.
Figure 41: The HRSC on board ESA's Mars Express will image the entire planet in full color, 3D and with a resolution of about 10 m. Selected areas will be imaged at 2 m resolution. One of the camera's greatest strengths will be the unprecedented pointing accuracy achieved by combining images at the two different resolutions (image credit: DLR/FU Berlin/ESA 2003)
Legend to Figure 41: The camera head is the light grey unit in the middle and the top rectangular aperture. The SRC (Super Resolution Channel) is the black cylindrical aperture at lower right. The camera head and SRC together measure 515 x 300 x 260 mm. The Digital Unit is the black box at the back. The complete HRSC has a mass of 20.4 kg and consumes about 48.7 W with both camera and SRC in operation.
The Camera Unit consists of: 35)
• The Instrument Frame, which is the supporting structure for the Camera Head and the SRC and provides thermal decoupling from the spacecraft and thermal stability
• The Camera Head, comprising the objective lens with its baffle and the optical bench, which serves as the central structure and connects the objective lens with the focal plate containing the Focal Plate Modules and the Front End Electronics
• The SRC (Super Resolution Channel), mounted within the Instrument Frame.
The Digital Unit comprises a power converter supplying the digital processing electronics and the sensor electronics, a signal interface to the spacecraft, an instrument control processor and a data compression unit, all mounted in a single enclosure.
High Resolution Camera Head: The High Resolution Camera Head contains the optics, the optical bench, the spectral filters, the CCD line sensors, the sensor electronics, and a thermal control system.
The optics are of Apo-Tessar objective design with a focal length of 175 mm and f = 5.6, and are mounted in a titanium housing. The transmissivity of the optics varies from 0.37 for the blue channel (440 nm) to 0.68 in the panchromatic range.
The HRSC Camera Head contains nine CCD line sensors mounted in parallel for operation in pushbroom mode. Pushbroom or linear array sensors image a line on the planet surface perpendicular to the ground track of the spacecraft and rely on the orbital motion of the spacecraft to reposition them as they record a sequence of images known as an image swath. HRSC simultaneously provides high-resolution stereo, multicolor and multi-phase images of the Martian surface by delivering nine superimposed image swaths.
The Camera Head sensor electronics consists of three Focal Plate Modules (FPM), each of which contains three CCDs including drivers and preamplifiers, and a Front End Electronics (FEE) unit comprising four signal chains, nine focal plate controllers and one power distribution unit.
Each of the nine CCD linear arrays is made up of 5184 seven-micron square pixels, which correspond to ten meter square pixels on-ground for a spacecraft altitude of 250 km. By multiplexing the nine CCD sensors, the signal can be switched into one of the four signal chains. Signal conditioning comprises low pass filtering, correlated double sampling and programmable attenuation and gain selection. The conditioned signals undergo analogue to digital conversion before being transmitted to the instrument's Digital Unit.
Super Resolution Channel: The Super Resolution Channel (SRC) consists of a set of optics, a CCD array detector and the associated signal electronics.
The SRC optical system is a Matsukov-Cassegrain telescope with a focal length of 972 mm, positioned with its axis parallel to the optical axis of the HRSC Camera Head. The SRC sensor is a CCD array of 1024 x 1032 with 9 µm square pixels, which correspond to 2.3 m square pixels on-ground for a spacecraft altitude of 250 km.
The SRC has a mechanical interface with the HRSC Camera Head and an electrical interface with the HRSC Digital Unit. It has no direct interface to Mars Express and is treated by the spacecraft as an additional HRSC channel.
Multi Sensor Concept: The multi sensor concept of the HRSC combines stereo, multispectral and multi-phase imaging. High-resolution images from the SRC can be embedded in the pushbroom images.
Stereo imaging is performed using nadir-directed, forward looking (+18.9°), and aft-looking (-18.9°) line sensors with a spectral range of 675 ± 90 nm. Known as triple panchromatic along-track stereo, this technique permits robust stereo reconstruction by on-ground digital processing and rectification through attitude reconstruction, feature matching and bundle adjustment, followed by the generation of digital terrain models and higher-level products. In general, the nadir-looking channel delivers the highest resolution images, while the two outer stereo channel images will be transmitted at lower resolution after pixel summation.
Multispectral imaging is implemented using four additional line sensors for the blue (440 ± 45 nm), green (530 ± 45 nm), red (750 ± 20 nm) and near infrared (970 ± 45 nm) color ranges. These color images cover the same areas as the panchromatic triple stereo images and will be matched geometrically to the nadir channel panchromatic swath by a process involving digital rectification through post-facto altitude reconstruction. In general, the multispectral images will be artificially decreased in spatial resolution by on-board pixel summation for lower data rates and better signal-to-noise characteristics, giving rise to data entities referred to as macro pixels.
Two additional panchromatic line sensors having inclined forward and backward viewing directions perform multi-phase imaging. These sensors complement the information contained in the triple stereo channels and allow the determination of photometric surface characteristics. The data from these channels will normally be transmitted at lower resolution by pixel summation.
The nine line sensing channels can be allocated to one of four signal chains. Each chain can be operated independently, offering great flexibility in the processing of the data.
When the SRC is operated, one of the four signal chains is dedicated exclusively to SRC and the line sensors use the remaining three signal chains.
Pushbroom Observations and Three Line Stereo Imaging: The Camera Head with its nine CCD line sensors is operated according to the pushbroom principle:
As the spacecraft moves along its ground track, all nine CCD line sensors are exposed for a chosen exposure time and at a selected scan frequency in such a way that a contiguous image strip, having no gaps between adjacent lines, is generated. The exposure time and the scan frequency are closely connected, with the readout time between adjacent lines being negligible. Nine independent image strips are generated. The size of an image strip is defined by the number of pixels per line and the acquisition duration. The cross track dimension (swath width) changes with spacecraft altitude whereas the along track size (strip length) is limited only by spacecraft resources such as memory available to store observation data and allocated downlink capacity.
In order to guarantee square pixels the scan frequency has to be varied with changing cross-track pixel size. Thus, the scan frequency depends on the cross track pixel size, which in turn depends on the spacecraft altitude. As a result the instrument data rate changes with spacecraft altitude, reaching its maximum at pericenter. The gain of the HRSC electronics is varied with the changing scan frequency in order to compensate for the variations in the amount of light reaching the CCDs as the exposure duration changes. For a spacecraft velocity over ground at pericenter of 4.3 km s-1 the scan frequency is 425 Hz. The scan frequency decreases as the spacecraft altitude increases.
During normal HRSC imaging, nadir pointing of the spacecraft is required. Off-track pointing, where the spacecraft is turned to position the instrument line-of-sight to one side of the ground track, may be used for the acquisition of special targets.
The nine CCD line sensors of the Camera Head are located behind a single optical system and each sensor sees the planetary surface with a different viewing angle, which forms the basis for stereo imaging.
Each image point is seen from three different viewing angles, forward-looking, nadir-direction and aft- looking. Given an accurate knowledge of the position and attitude of the spacecraft at the time of image acquisition, the absolute three-dimensional position of objects in the images can be calculated.
The distance on ground spanned by the stereo viewing angle (nadir to forward-looking or nadir to aft-looking, both 18.9º) is called base length. For in-track stereo reconstruction a minimum image strip length of three base lengths is required. At pericenter, this corresponds to an imaging duration of two minutes. To perform a robust stereo reconstruction over mosaics consisting of adjacent image swaths, a sideways overlap of about 20% of the swath width is required.
Table 5: HRSC characteristics
ASPERA-3 (Analyzer of Space Plasmas and Energetic Atoms)
The ASPERA-3 instrument of Mars Express is designed to study the solar wind-Mars atmosphere interaction and to characterize the plasma and neutral gas environment in near-Mars space through energetic neutral atom (ENA) imaging and local charged-particle measurements. The studies address the fundamental question: how strongly do the interplanetary plasma and electromagnetic fields affect the martian atmosphere? This question is directly related to the problem of martian dehydration. 36) 37)
The scientific objectives of the Mars Express Orbiter mission are to study the subsurface, surface and atmosphere of Mars, as well as the interaction of the atmosphere with the interplanetary medium. ASPERA-3 will fulfil the last objective by:
- remote measurements of energetic neutral atoms (ENAs) in order to (a) investigate the interaction between the solar wind and the martian atmosphere, (b) characterize quantitatively the impact of plasma processes on atmospheric evolution, and (c) obtain the global plasma and neutral gas distributions in the near-Mars environment;
- in situ measurements of ions and electrons to (a) complement the ENA images (electrons and multi-charged ions cannot be imaged), (b) study local characteristics of the plasma (dynamics and fine structure of boundaries), (c) provide the undisturbed solar wind parameters required for interpreting ENA images.
The ASPERA-3 (Analyzer of Space Plasmas and Energetic Atoms) instrument is made up of two components:
• the Main Unit, comprising the mechanical scanner, digital processing unit (DPU), Neutral Particle Imager (NPI), Neutral Particle Detector (NPD) and Electron Spectrometer (ELS)
• the Ion Mass Analyzer (IMA), mounted separately.
Figure 42: ASPERA-3 Main Unit with the ELS protective cover removed. The NPI particle entrance and the two NPD entry ports are protected by red covers (image credit: Swedish Institute of Space Physics)
Mechanical Scanner: The mechanical scanner sweeps the three sensors mounted on it through 180 degrees to give the ASPERA-3 instrument 4π steradians (unit sphere) coverage when the spacecraft is 3-axis stabilized. The scanner is equipped with two stepper motors, which turn a worm screw. The screw drives a worm wheel, which is attached to the moving part of the scanner. The scanner payload can be turned to any arbitrary angle or perform continuous scanning. The operational rotation rates are 1.5, 3.0 and 6.0 degrees per second. The system offers an angular positioning accuracy of 0.2 degrees.
Digital Processing Unit: The Digital Processing Unit's main task is to control the sensors and the mechanical scanner. The DPU processes, compresses and stores the sensor data and forwards it (together with housekeeping data) to the spacecraft telemetry system. It also receives and implements commands sent to the ASPERA-3 instrument by the spacecraft telecommand system.
The primary design drivers for the Digital Processing Unit (DPU) are optimum use of the allocated telemetry rate and correct handling of telecommands. The ASPERA-3 instrument makes extensive use of sophisticated lossless data compression to enhance the scientific data yield. The principal compression method used is based on the Rice algorithm, an adaptive compression technique that remains efficient over a wide range of input data entropy conditions. This is achieved by employing multiple encoders, each of which is optimized to compress data in a particular entropy range. The structure of the algorithm also permits a simple interface to data packetization schemes, such as those used for space data communications, without the need to carry auxiliary information across packet boundaries.
Neutral Particle Imager: In the Neutral Particle Imager, incoming particles pass between two 150 mm diameter discs, which are separated by 3 mm and have a 5 kV potential between them. Charged particles are deflected by the electric field and captured, but neutral particles pass between the discs. The space between the discs is divided into 32 sectors by plastic spokes, forming 32 azimuth collimators with an aperture of 9 degrees by 18 degrees each. Neutrals that pass through the deflector system hit a 32-sided conical target at a grazing angle of incidence (20 degrees). The interaction between the neutral particles and the target results in production of secondary electrons and ions, and / or reflection of the primary neutrals. The particles leaving the target are detected by a Micro Channel Plate (MCP) stack with 32 anodes. The signal from the MCP gives the direction of the primary incoming neutral particle. The MCP is operated in such a way as to detect sputtered ions of the target material, ions resulting from stripping of the primary neutrals and neutrals reflected from the target surface. In order to improve the angular resolution and collimate the particles leaving the interaction surface, 32 separating walls are attached to the target, forming a star-like structure. This configuration allows the particles to experience multiple reflections and reach the MCP. The target is specially coated to prevent incoming ultraviolet photons that strike it from producing erroneous results.
The Neutral Particle Imager covers 4π steradians in one 180º sweep by the mechanical scanner and produces an image of the ENA distribution in the form of an azimuth x elevation matrix. The direction vector of 32 elements is read out once every 62.5 ms.
Neutral Particle Detector: The Neutral Particle Detector consists of two identical pinhole cameras each with a 90º Field of View (FoV) in the instrument azimuth plane and arranged to cover a FoV of 180º. Particles approaching the pinholes pass between a pair of quadrant deflector plates separated by 4.5 mm and with and 8 kV potential between them. Charged particles with energies up to 70 keV are deflected, while neutrals proceed into the camera. The deflector plates also function as a collimator in the instrument elevation direction.
The collimated Energetic Neutral Atom (ENA) beam emerging from the 4.5 x 4.5 mm pinhole hits a target at a grazing angle (20 º) and causes secondary electron emission. The secondary electrons are detected by one of two Micro Channel Plate (MCP) electron multiplier assemblies. The MCP output provides a start signal to the electronics that measures the time of flight of the ENAs over a fixed distance. The incoming ENAs are reflected from the target nearly specularly and travel to a second target. Again, secondary electrons are produced and detected by three more MCPs, which pass a stop signal to the time of flight electronics. The time of flight between the two targets gives the velocity of the incoming particle. Which of the three 'stop' MCPs detects the incoming particle determines its (instrument relative) azimuth direction.
Since secondary electron yield depends on both incident particle mass and velocity, the mass can be determined, given that the velocity is known, by analyzing the height distribution of the pulses from the MCPs.
The effects of ultraviolet radiation are suppressed by coating the targets appropriately and checking for coincidence between the start and stop signals used for the time of flight calculations.
As the mechanical scanner moves the NPD through 180º, a 2π steradians (half sphere) coverage of the incident particle field is obtained.
Electron Spectrometer: The Electron Spectrometer determines the energy spectrum of incoming electrons in each of sixteen 22.5º sectors.
The Electron Spectrometer is based around a spherical section electrostatic analyzer of 'top hat' design. The electrostatic analyzer consists of two concentric hemispherical electrodes, the outer of which has a central hole, through which electrons are admitted, covered by the 'top hat' and collimator. Electrons arriving from any azimuth angle and within the elevation field of view of the collimator pass under the 'top hat' and are deflected through the central hole in the outer hemisphere by a positive potential on the inner hemisphere. The electrostatic field between the hemispheres will deflect electrons having an energy in a particular range such that they travel between the electrodes. Electrons with energies outside the selected range will be captured.
These energy band filtered electrons exit the annular gap between the hemispheres and hit a Micro Channel Plate (MCP) electron multiplier. Beyond the MCP, the electrons strike one of sixteen anodes, each defining a 22.5º sector of incident azimuth angle.
By varying the electrostatic potential between the hemispheres of the electrostatic analyzer, the energy of the electrons selected by the filter can be changed. The voltage applied to the inner hemisphere is swept once every four seconds and the number of anode hits per sample interval is recorded to give an energy spectrum for the incoming electrons in each sector. As the ELS sensor is moved through 180º by the mechanical scanner, a complete 4π steradians (whole sphere) angular distribution of electrons is measured.
Ion Mass Analyzer: The Ion Mass Analyzer determines the mass spectrum of incoming ions in a selectable energy range. The mass range and resolution of the spectrum are also selectable.
Ions arriving at the IMA pass through an outer, grounded grid and enter the deflection system. The deflection system comprises two curved, charged plates that deflect ions arriving in the instrument elevation range from 45º above to 45º below instrument azimuth plane and from any azimuth angle into the entrance of the electrostatic analyzer.
The electrostatic analyzer consists of two concentric hemispheres with a variable electric field between them. Ions that lie within the energy pass band of the analyzer travel between the hemispheres, exit the annular space separating them, and travel on towards the magnetic mass analyzer. The electrostatic potential between the hemispheres determines the energy range of the ions that pass through the analyzer.
In the magnetic mass analyzer, the ions pass through a static, cylindrical magnetic field, which deflects light ions towards the center of the cylinder more than heavy ones. An electrostatic potential can be applied between the electrostatic analyzer and the magnet assembly to accelerate the ions. Varying this potential allows selection of the mass range to be analyzed and the mass resolution.
As the ions leave the magnetic mass analyzer they hit a Micro Channel Plate (MCP). The electrons exiting the MCP are detected by an imaging anode system. A system of 32 concentric rings measures the radial impact position, which corresponds to ion mass and 16 sector anodes measure azimuthal impact position, which corresponds to ion azimuth angle.
Figure 43: ASPERA-3 Ion Mass Analyzer with red protective cover removed to expose the particle entrance (image credit: Swedish Institute of Space Physics)
PFS (Planetary Fourier Spectrometer)
From space and Earth-based observations so far, we know that the Martian atmosphere is about one hundred times less dense than the Earth's at ground level, that it's composed almost entirely of carbon dioxide (CO2) with trace to small amounts of other gases, and that atmospheric temperatures range from 298 K (+25°C) at the equator to 140 K (-130° C) at the winter pole. We also know that the atmosphere contains a lot of dust. 38) 39)
But there are many gaps in our knowledge. We still don't know, for example, how temperature and pressure vary with altitude, what the global circulation patterns are, how the composition of the atmosphere varies with time and place, and what all the trace constituents are. We also don't know for sure how much dust there is in the atmosphere, how it's transported there, what it's made of and how it affects the Martian weather. The Planetary Fourier Spectrometer (PFS) on board Mars Express will help find the answers to some of these outstanding questions.
PFS is an infrared spectrometer optimized for atmospheric studies and covering the wavelength range 1.2 to 45 µm in two channels with a boundary at 5 µm. The spectral resolution of the instrument is better than 2 cm-1. The instrument field of view is about 2º FWHM (Full Width, Half Maximum) for the Short Wavelength (SW) channel and 4º FWHM for the Long Wavelength (LW) channel. These fields of view correspond to a spatial resolution of ten kilometers for the SW channel and 20 kilometers for the LW channel when Mars is observed from a height of 300 kilometers (the nominal height of the pericenter). 40)
PFS is equipped with a pointing device, which enables it to receive incoming radiation from the surface of Mars or to perform calibration measurements by pointing to a reference black body of known temperature or to deep space.
Figure 44: PSF interferometer: A close up of the interferometer at the heart of the PFS instrument (image credit: ESA/ASI/INAF (Formisano))
The incident radiation arrives from the pointing device and is divided into two beams by a dichroic mirror and then filtered before being directed into the two interferometers. The interferometers are of the double pendulum type and are positioned with their planes of operation one above the other so that a single motor can be used to move both pendulums. An optical reference channel controls the pendulum motion by passing light from a laser diode through the same optics as the radiation that is being analysed. The reference channel also generates the sampling signal for the analogue to digital converters that process the detector signals, triggering one sample for each 150 nm of retro-reflector motion. The interferometers are extremely sensitive to optomechanical distortions and the interferometer module must be very rigid and thermally stable to minimise these effects.
The instrument is able to perform real time Fast Fourier Transform computations in order to select the spectral range of interest for data transmission to Earth
Table 6: Summary of PFS characteristics
OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité)
The Visible and Infrared Mineralogical Mapping Spectrometer (OMEGA) builds up maps of the surface mineral composition from the visible and infrared light reflected from the planet's surface in the wavelength range of 0.5-5.2 µm. As the light reflected from the surface passes through the atmosphere before entering the instrument, OMEGA also measures the atmospheric composition.
"We want to know the iron content of the surface, the water content of the rocks and clay minerals and the abundance of non-silicate materials such as carbonates and nitrates. These measurements would allow us to reconstruct the history of the planet," says Jean-Pierre Bibring, OMEGA Principal Investigator from the Institut d'Astrophysique Spatiale, Orsay, France. 41) 42) 43)
OMEGA, derived from the Mars-96 spare model, is a visible and near-IR mapping spectrometer operating in the wavelength range 0.38-5.1 µm. It will provide global coverage of Mars by the end of the nominal mission at medium resolution (1-5 km) from orbital altitudes between 1000 km and 4000 km, and higher-resolution (a few hundred meters) snapshots of selected areas, amounting to at least a few percent of the surface. OMEGA is characterizing the composition of surface materials, studying the time and space distribution of atmospheric CO2, CO and H2O, identifying the aerosols and dust particles in the atmosphere, and monitoring the surface dust transport processes. It is contributing greatly to understanding the evolution of Mars from geological time scales to seasonal variations and is giving unique clues for understanding the H2O and CO2 cycles throughout martian evolution.
We have little more than a general knowledge of the surface composition of Mars. The north is made largely of a silica-rich volcanic rock (felsic) and the south of basaltic rock relatively poor in silica (mafic); the soil and surface layer of exposed rocks contains oxides of iron, which give the planet its reddish color. Going beyond this general description to identify particular minerals or elements at particular points on the surface, has so far proved beyond the scope of remote-sensing instruments. None has yet flown with sufficient spatial resolution for the task. That will change, however, when Mars Express carries the Infrared Mineralogical Mapping Spectrometer (OMEGA) into Martian orbit. 44)
Using the fact that different materials absorb and radiate light at different wavelengths, OMEGA will build up a map of surface composition by analyzing sunlight that has been absorbed and re-emitted by the surface. The instrument will also glean information on the composition from the wavelengths of infrared radiation given off as the surface cools. As radiation travelling from the surface to the instrument must pass through the atmosphere, OMEGA will also detect wavelengths absorbed by some atmospheric constituents, in particular dust and aerosols.
The OMEGA instrument has a mass of 29 kg.
Figure 45: The flight model of OMEGA (image credit: Institut d'Astrophysique Spatiale, OMEGA collaboration)
As Mars Express proceeds along its orbit, OMEGA will gradually build up a map of the planet in squares, or pixels, whose sides range from 4 km to 300 m in length. During the mission's lifetime, a spectral map of the entire surface will be generated at 1-4 km resolution and selected sites, making up 2-5% of the Martian surface, will be mapped at 300 m resolution. The higher resolution is available only when the spacecraft is at the lowest point in its orbit, hence its restriction to a limited area of the Martian surface. "In each channel, we make a spectrometer and an imager and couple the two. On each resolved pixel, we will have the entire spectrum from the visible to the IR," says Jean-Pierre Bibring from the IAS (Institut d'Astrophysique Spatiale), Orsay, France and Principal Investigator for OMEGA.
OMEGA's high spatial resolution is unprecedented. This and its spectral range give it the ability to pinpoint specific minerals on Mars more accurately than any instrument on a previous spacecraft. "We want to see not only that there are silicates, but to determine their class - whether they are in the form of feldspar, pyroxene, olivine, for example. We also want to know the iron content of the surface, the oxidation level of the iron, the hydration of the rocks and clay minerals and the abundance of non-silicate materials such as carbonates and nitrates. We'll be able to measure all these abundances to within a few per cent within a 100 m square," says Bibring.
Such knowledge of the composition will throw light on many of the outstanding puzzles about Mars. Our understanding of the tectonic history of the planet, for example, will deepen with more accurate knowledge of the whereabouts and composition of igneous rocks (rocks that have been molten in the interior of the planet). Accurate measurements of dust on the surface and in the atmosphere will illuminate present day wind patterns and patterns of dust transport. And climate history and present day weather patterns will be revealed in accurate measurements of the water and carbon dioxide content of the polar caps, which varies with the seasons. Some of the most interesting riddles OMEGA will help unravel, however, have to do with the history of water on the planet and the possibility that Mars was once hospitable to life.
"Two of the big questions we have about Mars are: where is the carbon dioxide? and what happened to the water? "says Bibring. "There's carbon dioxide in the atmosphere, but the pressure's very low. So either Mars has lost its carbon dioxide, or it is now in the rocks in the form of carbonates. If the carbon dioxide is in the rocks, there must have been liquid water in the past."
Figure 46: VNIR optical layout (image credit: OMEGA collaboration)
Figure 47: Illustration of the OMEGA instrument (image credit: ESA)
The instrument comprises two grating spectrographs, one working in the visible and near infrared range (VNIR channel), the other in the short wavelength infrared range (SWIR channel). 45)
The OMEGA instrument is made up of two main components:
• A Camera unit (OMEGA-CAMERA or OMEC) comprising the VNIR and SWIR spectrographs and their associated signal electronics
• A Main Electronics (OMEGA-ME or OMEM) module responsible for the control and management of the instrument
Table 7: OMEGA performance summary
VNIR (Visible and Near Infrared) channel:
The VNIR channel is based on a push-broom imaging concept with a two-dimensional silicon charge coupled device (CCD) detector and a telescope covering an 8.8° total field of view that is defined by a slit placed in the focal plane of the telescope. Target radiation leaving the slit is dispersed onto the detector by a concave holographic diffraction grating. The spectrum of a pixel area on the observed target is dispersed along a column of the detector array while spatial resolution in the direction of the spectrograph slit is obtained across the lines of the array. The spectrograph slit is oriented perpendicular to the spacecraft track so the motion of the spacecraft provides the second spatial dimension of the image. During one exposure interval, the full spectrum of every spatial pixel along the slit is obtained. Successive exposures are combined as the spacecraft moves, with the exposure times selected so that a contiguous image swath at the desired resolution is obtained.
Table 8: VNIR channel characteristics
SWIR (Short Wavelength Infrared Range) channel:
The SWIR channel of the camera has two sub-channels fed by a common telescope, slit and collimator. The collimator output beam is split into two parts by a dichroic filter, covering the 0.93 - 2.77 µm and 2.65 - 5.2 µm wavelength bands. The filtered collimator outputs are dispersed by two blazed, flat diffraction gratings operating at the first order. The dispersed slit images are re-imaged onto the detectors by two sets of optics (one per sub-channel), comprising spherical collecting mirrors, spherical field mirrors and objectives composed of four zinc selenide (ZnSe) lenses. Each sub-channel employs an indium antimonide (InSb) photovoltaic linear array detector with 128 pixels, cooled down to approximately 77 K by a Stirling cycle micro cooler.
The use of linear arrays as detectors leads to the use of a whiskbroom-imaging concept. A scanning mirror gives the cross-track field of view and the spacecraft along-track motion provides the second spatial dimension for the images.
Table 9: SWIR channel characteristics
MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding)
MARSIS is a low frequency, nadir-looking pulse limited radar sounder and altimeter with ground penetration capabilities, which uses synthetic aperture techniques and a secondary receiving antenna to isolate subsurface reflections. 46) 47) 48)
The operation altitudes for MARSIS are up to 800 km above the Martian surface for subsurface sounding and up to 1200 km for ionospheric sounding. In its standard operating mode, the instrument is capable of making measurements in 1 MHz wide bands centered at 1.8, 3.0, 4.0 and 5.0 MHz.
MARSIS functions by transmitting a linear frequency modulated chirp using a nadir-looking dipole antenna. The return signal is received on both the dipole antenna and a secondary monopole antenna oriented along the nadir axis. The secondary antenna has a null in the nadir direction and receives primarily the off-nadir surface reflections. This signal can be subtracted from the main received signal during ground processing to reduce surface clutter. Both received signals are down converted to range offset video signals before being passed to an analogue to digital converter. The resultant data are formatted by the MARSIS on-board digital processor and passed to the spacecraft for transmission to Earth.
MARSIS operates in the following modes:
• Subsurface Sounding
• Active Ionospheric Sounding
• Receive Only
MARSIS will perform Subsurface Sounding when the spacecraft is less than 800 km above the Martian surface. In the highly eccentric orbit selected for Mars Express, this corresponds to a period of about 26 minutes, allowing mapping of about 100 degrees of arc on the Martian surface per orbit. Over the nominal mission lifetime, extensive coverage at all latitudes will be possible. To achieve this global coverage MARSIS supports both dayside and nightside operations, although performance is maximized during the night (solar zenith angle above 80 degrees) when the ionosphere plasma frequency drops significantly and the lower frequency bands, which have greater ground penetration capabilities, can be used.
Active Ionospheric Sounding will be carried out during certain orbital passes when the orbiter is less than 1200 km above the surface, in order to gather scientific data on the Martian ionosphere.
Receive only mode will mainly be used to characterize, from an electromagnetic point of view, the environment in which MARSIS is working.
MARSIS will be operated in calibration mode periodically throughout the operational phase of the mission. The purpose of this mode is to acquire a limited amount of data in an unprocessed format. The unprocessed data is used to determine the characteristics of the adaptive matched filter computation that is used by the MARSIS processor to compress the dispersed echo signals from the planet surface and subsurface boundaries.
MARSIS is composed of three subsystems:
• The antennas
• The RF equipment (transmitter and receivers)
• The digital electronics
The receivers and digital electronics are housed together within the spacecraft. The transmitter electronics is housed in a separate box, also within the spacecraft.
The main transmit and receive antenna is a deployable dipole with two 20 meter elements, arranged so that its peak gain is in the spacecraft nadir direction. The clutter cancellation antenna is a seven meter long deployable monopole, arranged so that its gain null is in the spacecraft nadir direction. The clutter cancellation antenna is equipped with a low-noise preamplifier. Due to severe limitations on the available mass, the antennas are of a novel design, each consisting of a folding composite tube that supports a pair of wires constituting the conductive element of the antenna. The antennas are deployed by pyrotechnic release mechanisms.
The transmitter is connected to the primary antenna through an impedance matching network. The nominal operating frequency of the transmitter in the subsurface sounder modes is 1.3 to 5.5 MHz, with an instantaneous bandwidth of 1 MHz. For ionospheric sounding, the operating frequency varies between 0.1 and 5.4 MHz. The transmitter takes the chirp generated by the receiver/local oscillator electronics and amplifies it, delivering 5 W of RF power to the antenna.
The receiver electronics consists of the chirp generator/local oscillator and a dual channel receiver that down converts the received echoes. Each receiver channel has a selectable bandpass filter, a mixer, an amplifier chain, low-pass filtering and an analogue to digital converter. The output of the analogue to digital converters is passed to the digital electronics for processing prior to being sent to the ground station via the spacecraft's on-board data handling system.
The digital electronics is responsible for:
• Synthesis of the transmit chirp and local oscillator signals
• Control of the transmitter and receivers
• Processing of the digital data from the receivers
• Receipt and execution of telecommands from the spacecraft
• Transmission of formatted science, event and housekeeping data to the spacecraft.
MaRS (Mars Radio Science Experiment)
• Radio sounding of the neutral Martian atmosphere (occultation experiment) to derive vertical density, pressure and temperature profiles as a function of height, with a height resolution better than 100 meters.
• Radio sounding of the ionosphere (occultation experiment) to derive vertical ionospheric electron density profiles and to derive a description of the global behavior of the Martian ionosphere through its diurnal and seasonal variations depending also on solar wind conditions.
• Determination of dielectric and scattering properties of the Martian surface in specific target areas by a means of a bistatic radar experiment.
• Measurement of gravity anomalies in conjunction with simultaneous observations using the HRSC (High Resolution Stereo Camera) to construct a three-dimensional topographical model for the investigation of the structure and evolution of the Martian crust and lithosphere.
• Precise determination of the mass of the moon Phobos.
• Radio sounding of the solar corona during the superior conjunction of the planet Mars with the Sun.
The spacecraft Telemetry, Tracking and Command (TT&C) radio links between the orbiter and the Earth will be used for these investigations. A simultaneous and coherent dual-frequency downlink at X-band and S-band via the High Gain Antenna (HGA) is required to separate the contributions from the classical Doppler shift and the dispersive media effects caused by the motion of the spacecraft with respect to the Earth and the propagation of the signals through the dispersive media.
The experiment relies on the observation of the phase, amplitude, polarization and propagation times of radio signals transmitted from the spacecraft and received at ground station antennas on Earth. The radio signals are affected by the medium through which the signals propagate (atmospheres, ionospheres, interplanetary medium, solar corona), by the gravitational influence of the planet on the spacecraft and finally by the performance of the various systems involved both on the spacecraft and on ground.
Radio sounding of the atmosphere and ionosphere: As the spacecraft is entering and exiting occultation by Mars as seen from the Earth, the TT&C radio beam slices through the layers of the ionosphere and neutral atmosphere. The TT&C system is operating in the two-way mode, which means that the downlink frequencies are derived from the received uplink frequency. Changes in the received radio frequency measured with an accuracy of one part in 10-13 correspond to the detection of a change in the angle of refraction of radio rays in occultation experiments of the order of 10-8 radians.
The separation of the effects of the ionosphere and the neutral atmosphere on the radio link is feasible by using a dual-frequency downlink and due to the fortunate fact that the peak height of the ionosphere and the limit of the detectable neutral atmosphere are well separated in height.
Bistatic radar investigation of planetary surface properties: The bistatic radar configuration is distinguished from the monostatic by spatial separation of the transmitter (the spacecraft) and the receiver (ground station on Earth). It is a powerful tool for providing information about surface texture (roughness and slope) on scales comparable with the sensing wavelength (of the order of centimeters to meters). Bistatic radar may also be used to determine properties of the surface material, such as dielectric constant, through differential reflection of orthogonal polarizations. The bistatic radar geometry of an orbiting spacecraft is well suited to probing the surface of planets at a variety of latitude, longitude and incidence angles.
For a typical downlink bistatic radar experiment, the radio signal is transmitted from the spacecraft High Gain Antenna toward the planetary surface and is scattered from that surface. That part of the signal power that is reflected toward Earth is received at the ground station. Optimizing performance of the bistatic radar experiments requires accurate prediction of the orbiter trajectory for the formulation of antenna pointing strategies and prediction of signal parameters such as Doppler shift and signal amplitude. In a quasi-specular experiment the antenna is then programmed to follow a locus of points for which surface reflection would be specular if Mars were smooth. From the data recorded along these specular point paths, surface roughness can be inferred from Doppler dispersion of the echo signal; the dielectric constant of the surface material can be inferred from the echo amplitude and/or polarization properties.
In a bistatic backscatter experiment, the spacecraft antenna is aimed exactly opposite to the Earth direction, a configuration which is often much easier to implement than dynamically tracking either moving or fixed points. As the antenna beam illuminates regions on the surface, coherent backscatter enhancements will cause ice-covered areas to appear extremely bright. Repeated tracks over the polar region can be used to define the boundaries of icy polar deposits or to monitor their changes as a function of time. The spatial resolution of the measurements is approximately equal to the projection of the HGA beam on the surface. The radio echo signal is received in the open-loop mode in two orthogonal polarizations (for example, Left Circular Polarization, and Right Circular Polarization), down-converted, sampled, and stored for further processing at investigating institutions.
Determination of the mass of Phobos: The objective of this experiment is the precise determination of the mass of the Martian moon Phobos and, if feasible, also of the low degree spherical harmonics of its gravity field. The shape and volume of Phobos will be determined using observations made by the cameras carried on Mars Express, allowing the bulk density of the moon may be derived.
The method of mass determination during close encounters with small bodies is well established. The gravitational attraction of Phobos will slightly disturb the trajectory of Mars Express. The difference between predicted trajectory (without Phobos) and the actually observed trajectory will lead to the determination of the attractive forces acting on the spacecraft and from them the mass of the moon. To make these measurements, the spacecraft is operating in two-way link mode with an X-band uplink.
The observations will be performed each time the spacecraft encounters Phobos with a closest approach distance of less than 500 km.
Investigation of gravity anomalies: Gravity information can be obtained at all times when the spacecraft is using the two-way dual-frequency radio link and the spacecraft is close enough to the surface that gravity accelerations significantly affects the spacecraft velocity. Earth pointing of the HGA is required to maintain a continuous radio link. The coherent and simultaneous dual-frequency downlink allows the extraction of the dispersive effects on the downlink due to the interplanetary medium and the Earth's ionosphere. Doppler tracking data will be acquired at a rate of one sample per 10 seconds and ranging data will be collected at a rate of one point per 10 minutes.
Velocity contributions induced by attitude control movements of the spacecraft which result in a HGA motion relative to the line-of-sight to Earth may reach several mm s-1. Therefore, thruster activities and attitude control commands have to be recorded in order to reconstruct the attitude motion for later correction of derived LOS gravity accelerations.
The anticipated accuracy of an S/X-band two-way radio link is of the order of 10-5 ms-1. This translates into an accuracy of gravitational acceleration determination of the order of several mGal (one mGal = 10-5 ms-2 ~ 10-6 g), depending on the height and extent of the local topographic features.
Solar corona sounding: Solar corona sounding will be performed using a two-way radio link. A dual-frequency downlink at S-band and X-band will be used to separate the coronal dispersive effects from the classical Doppler shifts. The two-way link is a powerful tool for the derivation of electron density models from observed electron content when propagation time delay data (ranging) and dispersive Doppler shifts are compared. The two-way link can also be used as the basis for the derivation of solar wind speed by correlating uplink and downlink signals and a detector for rapidly outward propagating density enhancements originating from solar events.
Superior solar conjunctions of Mars will occur in mid-October 2004 and mid-October 2006 over the North Pole of the Sun in the plane of sky at an apparent distance from the solar disk of less than three solar radii. The dispersive effects of the solar corona on the radio link will dominate the classical and dispersive contributions from the Martian gravity field and atmosphere.
Solar coronal sounding will be performed when Mars Express is within ten degrees elongation on either side of the solar disk (40 solar radii), which will occur from mid-September to mid-November 2004 and 2006.
The Mars Express Orbiter Radio Science (MaRS) experiment makes use of the radio link between the orbiter and the ground station(s) on Earth. Frequency, amplitude and polarization information will be extracted from the radio signal received in the ground station.
The S-band uplink is received via the Low Gain Antennas (LGA) or the High gain Antenna (HGA). In coherent, two-way mode the received frequency is used to derive the downlink frequencies using the transponder frequency ratios 880/221 and 240/221 for the X-band and S-band downlinks, respectively.
The X-band uplink is received via the HGA only. In the coherent two-way mode, the received frequency is used to derive the downlink frequencies using the transponder frequency ratios 880/749 and 240/749 for X-band and S-band downlinks, respectively. An X-band uplink will enhance the performance of the experiment because X-band is less sensitive to the effect of interplanetary plasma along the propagation path.
The simultaneous and phase coherent dual-frequency downlink at X-band and S-band is transmitted via the HGA. The X-band and S-band frequencies are related by a factor of 11/3. If an uplink exists, the downlinks are also coherent with the uplink in their respective transponding ratios. The dual-frequency downlink is required in order to separate the classical Doppler shift, due to the relative motion of the spacecraft and the ground station, from the dispersive media effects, due to the propagation of the radio waves through the ionosphere and interplanetary medium. It is also required that both frequencies are transmitted via the High Gain Antenna to maximize the signal-to-noise ratio.
The ground stations will include the tracking complexes near Perth, Australia (ESA, 35 m antenna), and the Deep Space Network in California, Spain and Australia (NASA, 34 m antenna). A tracking pass consists of typically eight to ten hours of visibility. Measurements of the spacecraft range and carrier Doppler shift can be obtained whenever the spacecraft is visible. In the two-way mode the ground station transmits an uplink radio signal at S-band or X-band and receives the dual-frequency simultaneous downlink at X-band and S-band. The information about signal amplitude, received frequency and polarization is extracted and stored, together with the time of receipt.
The ground stations will employ a hydrogen maser frequency standard to achieve frequency stability of the order of one part in 10-15 over a long integration time (> 100 seconds). This stability is required for precise two-way tracking in order to achieve velocity measurement accuracies better than 10-4 ms-1 with a one second integration time.
SPICAM (SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars)
Table 10: SPICAM channels
Operational Modes: The operational modes of SPICAM are Test mode (ground use only), Star mode, Sun mode, Limb mode and Nadir mode. The operational modes are derived from the scientific objectives and the related spacecraft attitudes.
Table 11: SPICAM operating modes
In Nadir mode, the instrument will point directly at the planet and will analyse solar radiation that has travelled through the atmosphere after being reflected from the planet surface. Nadir observations allow the measurement of total column abundance of atmospheric components. In star or Sun mode, the instrument will point tangentially through the atmosphere towards a star, or the Sun, which is observed through the atmosphere as it rises or sets. The instrument then analyses the light once components of it have been absorbed by the atmosphere, allowing derivation of vertical concentration profiles for atmospheric components. In limb pointing mode, the instrument will point across the atmosphere, as during Star mode, but without a target star, and the instrument will analyse the vertical profile of aeronomic emissions.
Ultraviolet Channel: The SPICAM ultraviolet channel (SUV) is based around a holographic diffraction grating.
Table 12: Ultraviolet channel characteristics
The first optical element in the UV channel is an off-axis parabolic mirror, which collects the incident light entering through either the nadir or solar aperture and focuses it.
Table 13: Ultraviolet channel mirror characteristics
In the focal plane of the mirror, there is a slit, which can be moved in and out of the field of view by a mechanical actuator, providing two configurations:
• Slit absent, for observation of stellar occultations with a field of view of 1° x 3.16°
• Slit present, for the observation of extended sources.
The slit has two parts, with two different widths, to give different flux resolutions.
The focal plane is the entrance of the spectrometer, a holographic concave grating, which collects the incoming light and directs it to the detector block.
Table 14: Ultraviolet channel grating characteristics
The detection block consists of a CCD detector equipped with an image intensifier tube. The spectrum of a single source point in the focal plane is dispersed along the lines of the CCD. The usable spectral band is 118 to 320 nm, chosen so as to offer good resolution (~1 nm) for stellar observations and to cover the CO2 and O3 bands. The lower wavelength was selected to be just below the Lyman α wavelength and the upper wavelength was chosen to reject visible light. The quantum efficiency of the photocathode is zero beyond 320 nm and the detector is therefore solar blind. The detector has a large dynamic range - by varying the gain of the image intensifier, the spectrometer can perform individual photon counting and deal with very high input intensities.
To observe the Sun, a five-millimeter diameter mirror is positioned so as to reflect the light from the Sun via a dedicated entrance aperture onto the parabolic mirror.
Infrared Channel: The SPICAV infrared channel (SIR) is based around a scanning acousto-optical tuneable filter (AOTF)
Table 15: Infrared channel characteristics
The entrance optical system comprises a lens telescope with a diameter of 30 mm and a collimator lens, which collect the incoming radiation and direct it onto the AOTF.
The AOTF consists of a tellurium oxide (TeO2) crystal to which an acoustic wave is applied. The acoustic wave propagating in the crystal causes it to act in a similar way to a diffraction grating. A radio-frequency synthesizer drives a piezo-electric crystal attached to the TeO2 crystal to produce the wave. The frequency of excitation determines the wavelength of the acoustic waves and hence the select wavelength of the AOTF. The frequency range of the synthesizer corresponds to an AOTF passband of 1.1 - 1.7 µm.
The two output beams from the AOTF are collimated by another lens and detected by two indium gallium arsenide PIN photodiodes.
Figure 48: Flight Model Sensor Unit of SPICAM viewed from above. The IR AOTF spectrometer is at top; the UV spectrometer is at bottom. The common optical axis points to the left. For the UV spectrometer, the light enters the mechanical baffle (black), is focused by a parabolic mirror (bottom right) through a slit, then dispersed by the grating (middle left), to be refocused on the intensified CCD Detector (at center), image credit: SPICAM collaboration)
SPICAM is a low mass (4.7 kg) UV-IR spectrometer on the Mars Express orbiter, is dedicated to recovering most of the atmospheric science that was lost with Mars-96 and its set of SPICAM sensors. The new configuration of SPICAM includes a 2-channel optical sensor (3.8 kg) and an electronics block (0.9 kg).
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).