ExoMars (Exobiology on Mars)
Background: Establishing whether life ever existed, or is still active on Mars today, is one of the outstanding scientific questions of our time. The ExoMars Program seeks to timely address this and other important scientific goals, and to demonstrate key flight and in situ enabling technologies underpinning European and Russian ambitions for future exploration missions. The ExoMars Program is a cooperative undertaking between ESA (European Space Agency) and the Russian federal space agency, Roscosmos. 1)
Within ESA, ExoMars is an element of the Aurora Exploration Program, an optional program executed under the supervision of the Program Board for Human Spaceflight, Microgravity and Exploration (PBHME). However, the ESA Science Program also participates to ExoMars. The objective of the Aurora Program is to explore Solar System objects having a high potential for the emergence of life. Aurora aims to develop technologies and address scientific questions in a step-wise fashion, seeking to advance the level of technical and scientific readiness with each successive mission.
Within Roscosmos, ExoMars is part of the Russian federal space program and is supported by RAS (Russian Academy of Sciences).
To prepare for future exploration missions and to support the Program's scientific objectives, ExoMars will achieve the following technology objectives:
• EDL (Entry, Descent, and Landing) of a payload on the surface of Mars
• Surface mobility with a Rover
• Access to the subsurface to acquire samples
• Sample acquisition, preparation, distribution, and analysis.
In addition to these technology objectives already agreed in the Aurora Declaration, the following new technology objectives result from the cooperation with Roscosmos:
• Qualification of Russian ground-based means for deep-space communications in cooperation with ESA's ESTRACK
• Adaptation of Russian on-board computer for deep space missions and ExoMars landed operations
• Development and qualification of throttleable braking engines for prospective planetary landing missions.
The scientific objectives of ExoMars are:
• To search for signs of past and present life on Mars
• To investigate the water/geochemical environment as a function of depth in the shallow subsurface
• To study martian atmospheric trace gases and their sources.
In addition to these science objectives already agreed in the Aurora Declaration, the following new scientific objective results from the cooperation with Roscosmos:
• To characterise the surface environment.
The ExoMars Program consists of two missions, in 2016 and 2018. ESA and Roscosmos have agreed a well-balanced sharing of responsibilities for the various mission elements. 2)
Table 1: Second ExoMars mission moves to next launch opportunity in 2020 3)
The ExoMars 2016 mission will be launched on a Roscosmos-provided Proton rocket. It includes the TGO (Trace Gas Orbiter) and the EDM (Entry, descent and landing Demonstrator Module), both contributed by ESA. The TGO will carry European and Russian scientific instruments for remote observations, while the EDM will have a European payload for in situ measurements during descent and on the martian surface.
In November 2013, ESA named the EDM Schiaparelli in honor the 19th century Italian astronomer Giovanni Schiaparelli (1835-1910). He observed bright and dark straight-line surface features on Mars which he called ‘canali'. This term was mistakenly translated into English as ‘canal' instead of ‘channel', conjuring up images of vast irrigation networks constructed by intelligent beings living on Mars. The controversy ended in the early 20th century, thanks to better telescopes offering a clearer view of the planet. — The name was suggested by a group of Italian scientists to the president of the Italian space agency, ASI, who then proposed it to ESA. Italy is the largest European contributor to the ExoMars program. 4)
The ExoMars 2018 mission will land a Rover, provided by ESA, making use of a DM (Descent Module) contributed by Roscosmos. The DM will travel to Mars on an ESA-provided CM (Carrier Module). Roscosmos will launch the spacecraft composite on a Proton rocket. The Rover will be equipped with a European and Russian suite of instruments, and with Russian RHUs (Radioisotope Heating Units). The Rover will also include a 2 m drill for subsurface sampling and a SPDS (Sample Preparation and Distribution System), supporting the suite of geology and life seeking experiments in the Rover's ALD (Analytical Laboratory Drawer). The Russian SP (Surface Platform) will contain a further suite of instruments, mainly concentrating on environmental and geophysical investigations. 5)
NASA will also deliver important elements to ExoMars: The Electra UHF (Ultra-High Frequency) radio package on TGO for Mars surface proximity link communications with landed assets (such as the Rover and Surface Platform); engineering support to EDM; and a major part of MOMA (Mars Organic Molecule Analyzer), the organic molecule characterization instrument on the Rover.
Table 2: An overview of some Mars parameters 6)
Spacecraft of ExoMars 2016 mission:
The ExoMars mission is the first ESA-led robotic mission of the Aurora Program and combines technology development with investigations of major scientific interest. The main objectives of this mission are to search for evidence of methane and other trace atmospheric gases that could be signatures of active biological or geological processes and to test key technologies in preparation for ESA's contribution to subsequent missions to Mars. 7)
The ExoMars 2016 mission spacecraft includes the following elements, all developed under the leadership of the prime contractor TAS-I (Thales Alenia Space-Italia). 8)
• TGO (Trace Gas Orbiter), developed by Thales Alenia Space, France
• EDM, developed directly by TAS-I
• MSA (Main Separation Assembly), developed by RUAG.
The TGO accommodates scientific instrumentation for the detection of atmospheric trace gases and the study of their temporal and spatial evolution. In addition, it will provide telecommunications support for the 2016 mission, for the 2018 mission and possible other assets until 2022.
The objectives of the ExoMars 2016 mission are to:
1) Validate landing on the planet Mars with a demonstration capsule weighing about 600 kg, using a control system based on a radar altimeter, and with a carbon fiber shock absorber to attenuate the hard contact with the surface.
2) Gather as much information as possible during entry into the Martian atmosphere.
3) Carry out scientific sampling on the surface for a short period.
4) Observe the Martian atmosphere and surface for two years from the orbiter at an altitude of 400 km.
5) Provide the telecommunication support needed by the rover for the 2018 mission.
The EDM is mainly conceived to demonstrate EDL (Entry Descent and Landing) technologies for future planetary exploration missions. The following technologies are foreseen to be demonstrated:
• TPS (Thermal Protection System)
• PAS (Supersonic Parachute System)
• Radar technologies for ground relative altitude and velocity measurements
• Propulsion technologies for attitude control and braked landing
• Crushable material for impact load attenuation.
Figure 1: Artist's rendition of the deployed ExoMars 2016 Trace Gas Orbiter (TGO) and Schiaparelli – the entry, descent and landing demonstrator module (image credit: ESA, ATG medialag)
TGO (Trace Gas Orbiter)
The technical team behind the ExoMars spacecraft involves companies across more than 20 countries. The prime contractor, Thales Alenia Space Italia, is leading the industrial team building the spacecraft (Ref. 6). As a part of the European industrial team, OHB System AG was responsible for developing the core module of the TGO, which comprises the structure as well as the thermal and propulsion system for the 2016 mission. OHB as member of the core industrial team, is responsible for the major German contribution to ExoMars.
Table 3: Main technical characteristics of the ExoMars Trace Gas Orbiter
NASA's participation in the 2016 ExoMars Trace Gas Orbiter includes two "Electra" telecommunication radios. Used successfully on NASA's Mars Reconnaissance Orbiter, Electra acts as a communications relay and navigation aid for Mars spacecraft. Electra's UHF radios support navigation, command, and data-return needs. 9)
TGO's Electra radios use a design from NASA/JPL with special features for relaying data from a rover or stationary lander to an orbiter passing overhead. Relay of information from Mars-surface craft to Mars orbiters, then from Mars orbit to Earth, enables receiving much more data from the surface missions than would otherwise be possible.
As an example of Electra capabilities, during a relay session between an Electra on the surface and one on an orbiter, the radios can maximize data volume by actively adjusting the data rate to be slower when the orbiter is near the horizon from the surface robot's perspective, faster when it is overhead.
Figure 2: This image shows a step in installation and testing of the first of the orbiter's Electra radios, inside a clean room at Thales Alenia Space, in Cannes, France, in June 2014 (image credit: NASA/JPL-Caltech/ESA/TAS) 10)
RCS (Reaction Control System): TGO requires a challenging propulsion subsystem. The TGO RCS will provide the thrust to the spacecraft for all initial trajectory corrections, DSMs (Deep Space Maneuvers) during the cruise phase to Mars and also the high thrust necessary for the final MOI (Mars Orbit Insertion) maneuver. Subsequently, it shall perform 3-axis attitude control of the TGO once in orbit around Mars for the remainder of its seven year lifetime. 11)
The selected RCS is a helium-pressurized bi-propellant propulsion system utilizing MMH (Monomethylhydrazine) as the fuel and mixed oxides of nitrogen (MON-1) as the oxidizer. The architecture is derived from previous flight proven European applications, however the detailed layout is unique and driven by the specific configuration of the TGO spacecraft and the redundancy needs of the ExoMars 2016 mission.
All RCS architecture and engineering activities have been performed by OHB-System (including all subsystem analyses), while Airbus Defence and Space has responsibility for the mechanical configuration, procurement and manufacturing of equipment, integration and acceptance test to ensure that the system requirements defined by TAS-F are satisfied. The subsystem test program has been defined by OHB-System and performed by Airbus DS at the Airbus DS, OHB-System and TAS facilities. Out of the 92 components comprising the flight RCS, 67 are manufactured by Airbus DS including all tanks and thrusters.
Figure 3: The ExoMars Trace Gas Orbiter and Schiaparelli (top) during vibration testing in 2015, the high-gain antenna is on the right (image credit: ESA, S. Corvaja)
Schiaparelli / EDM (Entry, Descent and Landing Demonstrator Module)
Landing on Mars: Despite a number of prominent US successes since the 1970s, landing on Mars remains a significant challenge. As part of the ExoMars program, a range of technologies has been developed to enable a controlled landing. These include a special material for thermal protection, a parachute system, a radar altimeter system, and a final braking system controlled by liquid-propellant retrorockets. Schiaparelli is designed to test and demonstrate these technologies, in preparation for future missions (Ref. 6).
Three days before reaching Mars, Schiaparelli will separate from TGO and coast towards the planet in hibernation mode, to reduce its power consumption. It will be activated a few hours before entering the atmosphere at an altitude of 122.5 km and at a speed of 21 000 km/h. An aerodynamic heatshield will slow the lander down such that at an altitude of about 11 km, when the parachute is deployed, it will be travelling at around 1650 km/h.
Schiaparelli will release its front heat shield at an altitude of about 7 km and turn on its radar altimeter, which can measure the distance to the ground and its velocity across the surface. This information is used to activate and command the liquid propulsion system once the rear heatshield and parachute has been jettisoned 1.3 km above the surface. At this point, Schiaparelli will still be travelling at nearly 270 km/h, but the engines will slow it to less than 2 km/h by the time it is 2 m above the surface. At that moment, the engines will be switched off and Schiaparelli will freefall to the ground, where the final impact, at just under 11 km/h, will be cushioned by a crushable structure on the base of the lander.
Although Schiaparelli will target the plain known as Meridiani Planum in a controlled landing, it is not guided, and the module has no obstacle-avoidance capability. It has, however, been designed to cope with landing on a terrain with rocks as tall as 40 cm and slopes as steep as 12.5º.
Because Schiaparelli is primarily demonstrating technologies needed for landing, it does not have a long scientific mission lifetime: it is intended to survive on the surface for just a few days by using the excess energy capacity of its batteries. However, a set of scientific sensors will analyse the local environment during descent and after landing, including performing the first measurements of atmospheric particle charging effects, to help understand how global dust storms get started on Mars. A communication link with TGO will provide realtime transmission of the most important operational data measured by Schiaparelli during its descent. Shortly after Schiaparelli lands, TGO will start a main engine burn and will return over the landing site only four sols later. In the meantime, the remainder of the entry, descent and landing data, along with some of the science instrument data, will be sent to Earth via ESA's Mars Express and NASA satellites already at Mars.
Schiaparelli builds on a heritage of designs that have been evaluated and tested by ESA during earlier ExoMars studies. The module accommodates a series of sensors that will monitor the behaviour of all key technologies during the mission. These technologies include a special material for thermal protection, a parachute system, a radar Doppler altimeter system, and a braking system controlled by liquid propulsion. The data will be sent back to Earth for post-flight reconstruction in support of future European missions to Mars.
Table 4: Main technical characteristics of Schiaparelli – the ExoMars EDM (Entry, Descent and Landing Demonstrator Module)
Figure 4: Photo of the Schiaparelli/EDM structural model which is being lowered onto the Multishaker at ESA/ESTEC (image credit: ESA, A. Le Floc'h, Ref. 4)
Launch preparations: In late December 2015, the ExoMars 2016 Trace Gas Orbiter and Schiaparelli (the entry, descent and landing demonstrator module) travelled aboard two Antonov 124 cargo jets from Turin, Italy, to the Baikonur Cosmodrome in Kazakhstan to be readied for launch in March. 12)
Since then, engineering teams, totaling about 65 people, from Thales Alenia Space (Italy and France), the ExoMars project team, instrument teams, and specialists from the Baikonur Cosmodrome have been steadily working through an intensive and painstaking program of final testing and preparation of the two spacecraft, which at 4.300 kg will be the heaviest spacecraft composite ever to be sent to Mars.
All this has to be completed in time for a launch scheduled for 14 March at the beginning of the 12-day launch window for this mission.
Central to the launch campaign activities is the cleanroom. Almost everything in the cleanroom has been transported from Europe for this launch campaign – hence the need for a third Antonov flight. In addition to the specialist lifting equipment and the ground support trolleys needed to move the two spacecraft, the teams have also had to prepare a dedicated ISO 7 environment cleanroom tent, within the "normal" ISO 8 cleanroom environment, for handling Schiaparelli which, being a Mars lander, must be regularly sampled to check that it satisfies the planetary protection regulations. For analysis of these samples a dedicated microbiological laboratory was brought from Turin and installed close to the cleanroom area.
The readying of the TGO (Trace Gas Orbiter) has included a series of system health checks, such as checking that signals could be sent to all spacecraft units and that they responded. The health of the payload – the four science instruments, ACS, CaSSIS, FREND and NOMAD – was checked in a similar manner by verifying that commands could be sent to them and that these commands were carried out. The flight model of FREND was swapped for the flight spare model.
Another important test that has been completed was with the Trace Gas Orbiter and the launch vehicle adapter. Mechanical fit checks and separation tests had already been done in Cannes last year. Here in Baikonur, the team checked the mechanical connections, and also verified that all electrical circuits were completed.
In parallel, Schiaparelli is also being prepared for launch and is subject to tests similar to those performed on the orbiter. The instruments, sensors (DREAMS and COMARS+) and systems have all been thoroughly checked. A leak test has been carried out. Engineers have uploaded the final software and charged the batteries - since Schiaparelli has no solar panels the fully charged batteries are essential for the surface operations. 13)
The mating of the TGO (Trace Gas Orbiter) and Schiaparelli began on 12 February, 2016 with the two spacecraft having been transferred into the fuelling area, where a mounting platform surrounding the orbiter facilitates the activities that need to be done about 4 m off the ground.
TGO and Schiaparelli are mechanically linked with the MSA (Main Separation Assembly), which attaches to TGO with 27 screws. The MSA holds onto Schiaparelli with three separation mechanisms comprising compressed and angled springs that are held by NEAs (Non-Explosive Actuators). When the NEAs are released on 16 October, as the spacecraft approaches Mars, Schiaparelli will be gently pushed away from TGO, at the same time being imparted with a rotation that will serve to stabilize its atmospheric entry.
Legend to Figure 5: This spacecraft has one fuel tank and one oxidizer tank, each with a capacity of 1207 liter. When fuelling is complete, the tanks will contain about 1.5 ton of MON (mixed oxides of nitrogen) and 1 ton of MMH (monomethylhydrazine). The propellant is needed for the main engine and the 10 thrusters (plus 10 backup thrusters) that are used for fine targeting and critical maneuvers.
Legend to Figure 6: On March 2, 2016, the Breeze upper stage and spacecraft were encapsulated together within the two fairing halves. Prior to the encapsulation, they were tilted horizontally and the first fairing half was rolled underneath the spacecraft and Breeze, on a track inside the cleanroom. The second fairing half was then lowered into place by means of an overhead crane, encapsulating the payload.
Figure 7: Proton-M rocket with ExoMars 2016 Trace Gas Orbiter and Schiaparelli module at the launch pad in Baikonur, Kazakhstan (image credit: ESA, B. Bethge) 17)
Launch: The European-Russian ExoMars (TGO and the EDM Schiaparelli lander) satellite was launched on March 14, 2016 (09:31 GMT) on a Proton-M/Briz vehicle from the Baikonur Cosmodrome, Kazakhstan. The launch provider was ILS (International Launch Services) KhSC (Khrunichev State Research and Production Space Center). 18) 19) 20)
Orbit: The SCC (ExoMars Spacecraft Composite) will be inserted into a T-2 transfer trajectory to Mars. The arrival on Mars is planned on Oct. 19, 2016 after a 9-month cruise phase.
During the cruise phase, the TGO will support all necessary operations and communications with Earth, and will provide the EDM (Schiaparelli) with the required power/energy. During this period, the EDM will be mostly in hibernation mode to minimize the TGO energy consumption, and nominally will be switched on only for three checkouts: the EDM commissioning checkout few days after the launch, the mid-cruise checkout to verify the EDM health status after the DSM (Deep Space Maneuver), and the preseparation checkout few hours before the separation from the TGO.
The EDM will be released by the TGO three days before the arrival at Mars (i.e. on Oct. 16th, 2016) by means of a 3-points spin-up separation mechanism (MSA). The separation provides a relative velocity higher than 0.3 m/s and a spin rate of 2.5 rpm. The spin rate will allow the EDM for maintaining the attitude needed to reach the Mars atmosphere EIP (Entry Interface Point) with a null angle of attack. The duration of the EDM coast phase (3 days), driven by the TGO need to have enough time to correct its orbit after the EDM separation and prepare the critical MOI ()Mars Orbit Injection) maneuver, is challenging for the EDM as the dispersions, coming from the navigation and from the separation mechanism, will propagate for quite a long time, by increasing the trajectory dispersions at Mars EIP.
During the coast phase, the EDM will be mainly in hibernation mode, to minimize the energy consumption from its batteries. Shortly before the arrival at the Mars EIP, the EDM will wake up from the hibernation to prepare the EDL phase.
The ExoMars Orbiter, TGO, will be inserted into an elliptical orbit around Mars and then sweep through the atmosphere to finally settle into a circular, approximately 400 km altitude orbit ready to conduct its scientific mission; inclination = 74º, period of ~2 hours.
TGO will also serve as a data relay for the second ExoMars mission, comprising a rover and a surface science platform, planned for launch in 2018. It will also provide data relay for NASA rovers.
Figure 8: 2016 has been an eventful and promising year for ESA's ExoMars mission. After successfully placing the Trace Gas Orbiter into Mars' orbit on 19 October, the orbiter has sent back its first images, tested its instruments and performed in orbit calibration measurements and health checks (video credit: ESA, published on 13 December 2016)
Legend to Figure 8: The Schiaparelli lander collected almost all of its expected data before its unexpected crash landing on the Martian surface. Crucial lessons will be learnt from this for the recently approved 2020 ExoMars mission, which will put Europe's first rover on Mars.
The precise cause of the lander loss is still being investigated but preliminary technical investigations have found that the atmospheric entry and slowing down in the early phases went exactly as planned.
In all, since its launch in March 2016, the ExoMars mission has been a mixture of successes and one unexpected set back. Looking ahead, the Trace Gas Orbiter will start aerobraking in March 2017 to gradually slow down over the following months. By the end of 2017, the orbiter will be in a lower, near circular orbit of 400 km and ExoMars' primary science mission can begin.
• July 20,2021: The ESA-Roscosmos Trace Gas Orbiter has set new upper limits on how much methane, ethane, ethylene and phosphine is in the martian atmosphere – four so-called ‘biomarker' gases that are potential signs of life. 21)
- Searching for biomarkers on Mars is a primary goal of the ExoMars Trace Gas Orbiter. A key biomarker of interest is methane, as much of the methane found on Earth is produced by living things or geological activity – and so the same may be true for Mars.
Figure 9: This graphic summarizes significant measurement attempts of methane at Mars. Reports of methane have been made by Earth-based telescopes, ESA's Mars Express from orbit around Mars, and NASA's Curiosity located on the surface at Gale Crater; they have also reported measurement attempts with no or very little methane detected. More recently, the ESA-Roscosmos ExoMars Trace Gas Orbiter reported an absence of methane, and provided a very low upper limit (image credit: ESA)
- The ‘methane mystery' on Mars has been ongoing for many years, with contradictory findings from missions including ESA's Mars Express and NASA's Curiosity rover capturing sporadic spikes and bursts of the gas in Mars' atmosphere, fluctuations both in orbit and at the planet's surface, signs of the gas varying with the seasons, or not observing any methane at all.
- Previous estimates from Mars and ground-based missions range from 0.2 up to 30 parts per billion by volume (ppbv), indicating up to 30 molecules of methane per billion molecules. For reference, methane is present in Earth's atmosphere at nearly 2000 ppbv.
- However, the first results from the Trace Gas Orbiter, reported in April 2019, spotted no methane, instead calculating that, if present, the gas must have a maximum concentration of just 0.05 ppbv.
- "We have now used the Trace Gas Orbiter to refine the upper limit for methane at Mars even further, this time gathering data for over 1.4 martian years – 2.7 Earth years," says Franck Montmessin of LATMOS, France, Co-Principal Investigator of the Trace Gas Orbiter's Atmospheric Chemistry Suite (ACS) and lead author of one of a trio of new papers on martian biomarkers.
- "We found no sign of the gas at all, suggesting that the amount of methane at Mars is likely even lower than previous estimates suggest."
- As the orbiter's instruments are highly sensitive, if methane is present it must be at an abundance of less than 0.05 ppbv – and more likely less than 0.02 ppbv, say Franck and colleagues. The scientists also hunted for signs of methane around Curiosity's home, Gale crater, and found nothing, despite the rover reporting the presence of methane there.
- "Curiosity measures right at Mars' surface while the orbiter takes measurements a few kilometers above – so the difference between these two findings could be explained by any methane being trapped to the lower atmosphere or the immediate vicinity of the rover," adds Franck.
- The apparent lack of martian methane reported by Franck and colleagues is supported by a paper using data from the orbiter's NOMAD (Nadir Occultation MArs Discovery) instrument, again spanning a full martian year and searching for methane and two other biomarkers.
- "We also found no sign of methane on Mars, and set an upper limit of 0.06 ppbv, which agrees with TGO's initial findings using ACS," says the paper's lead author Elise Wright Knutsen, previously at NASA's Goddard Space Flight Center, USA, and now at LATMOS, France. "As well as searching for global methane, we also looked for localized plumes at over 2000 locations on the planet and didn't detect anything – so if methane is released in this way, it must be sporadic."
- Alongside methane, Elise and colleagues looked for two other potential biomarker gases: ethane and ethylene. These molecules are expected to occur after methane is broken down by sunlight, and so are exciting both in their own right and in the context of our hunt for methane. Ethane and ethylene molecules also have short lifetimes, meaning that if they are found in a planetary atmosphere they must have been recently released or created via an ongoing process. This makes them excellent tracers of possible biological or geological activity.
- "These are ExoMars' first results hunting for these two gases," says Elise. "We didn't detect either, and so set upper limits for ethane and ethylene at 0.1 and 0.7 ppbv, respectively – low, but higher than our limits for methane."
- The orbiter has also been hunting for phosphine – a gas that caused a splash and huge controversy last year when it was allegedly detected at Venus. Most phosphine on Earth is biologically produced, making it an exciting biomarker in the atmospheres of terrestrial planets.
- "We didn't find any signs of phosphine at Mars," says Kevin Olsen of the University of Oxford, UK, and lead author of the phosphine study. "Our upper limits are similar for those of ethane and ethylene – between 0.1 and 0.6 ppbv."
- The search for life on Mars, or lingering signatures of it, is a central goal of the ExoMars programme, and the hunt for biomarkers in particular is a primary goal of the Trace Gas Orbiter. The forthcoming ExoMars rover Rosalind Franklin, due for launch in 2022, will complement TGO's hunt for biomarkers by digging down into the martian surface; underground samples may be more likely to retain biomarkers, as material is shielded from the harsh radiation environment of space.
- "Whether biomarkers are detected or not, these findings are important for our understanding of which processes occur, and which do not, in the martian atmosphere – essential information when considering where to focus our continued investigation of Mars," adds Håkan Svedhem, ESA Project Scientist for the ExoMars Trace Gas Orbiter.
- "Many key questions remain – for instance, why does Curiosity see methane at Gale Crater, while we find none in orbit? Could this methane have come from elsewhere, or only be found in particular locations across the planet – or could an unexpected process be destroying any methane present before we can detect it?
- It will be exciting to continue working collaboratively with missions like the Curiosity and Rosalind Franklin rovers, both of which have totally different vantage points to an orbiter, to really pin down what is happening in this mysterious planetary environment."
Figure 10: Searching for biomarkers on Mars. The ExoMars Trace Gas Orbiter has set new upper limits on methane, ethane, ethylene and phosphine in the martian atmosphere, finding very little evidence for their existence. The gases are often associated with biological or geological processes, so understanding if they are present or not on another planet is key to understanding what processes may or may not be active on Mars today (image credit: ESA)
• March 15, 2021: At first glance this captivating scene peering through wispy clouds and down onto a dune field is reminiscent of a satellite view of one of Earth's deserts, but this is in fact a beautiful landscape on Mars. 25)
Figure 11: This spectacular dune field sits in the centre of Lomonosov crater, deep in the southern hemisphere of Mars (65ºS, 351ºE). It was imaged by the CaSSIS camera on the ESA-Roscosmos ExoMars Trace Gas Orbiter (TGO) on 2 December, 2020. The image was released on the occasion of the five year launch anniversary of the mission (image credit: ESA/Roscosmos/CaSSIS, CC BY-SA 3.0 IGO)
- At this time, southern summer was coming to an end on Mars. The image was taken as part of a campaign to track the evolution of the dune field through the end of summer and beginning of autumn, when frost would start covering the surface in this region. The crests of the dunes indicate the average wind direction, in this case, the wind comes predominantly from the bottom left to the top right of the image. To the right, darker, more basaltic rich sediments are visible. It is also in the right of the image that bright white clouds stand out against the darker sediments on the ground.
The first five years
- TGO launched from the Baikonur Cosmodrome in Kazakhstan on 14 March 2016, arriving at Mars seven months later. It spent several months aerobraking – using the top of the planet's atmosphere to create drag and slow down – to became the first ESA spacecraft used to enter its science orbit in this way.
- The mission began full science operations in April 2018 with its suite of four instruments. TGO's NOMAD and ACS spectrometers are designed to provide the best ever inventory of the planet's atmospheric gases yet, and have already detected a new gas – hydrogen chloride – for the first time, as well as studying processes linked to atmospheric water escape in greater detail than ever. TGO is also adding to the lively debate surrounding the presence of methane on the planet by revealing a surprising lack of the mysterious gas. The FREND instrument is mapping the distribution of hydrogen in the uppermost meter of the planet's surface, creating a detailed map of possible water-rich oases, relevant for future exploration of Mars. The CaSSIS camera has captured more than 20,000 images documenting the surface and complementing the data returned by the other instruments to help characterize features that may be related to trace gas sources.
- TGO also provides routine data relay for NASA's landers and rovers: Opportunity (until its end of operations in 2018), Curiosity, Insight and Perseverance. It will also be the communication link for the second ESA-Roscosmos ExoMars mission, comprising the Rosalind Franklin rover and Kazachok platform, when it arrives on Mars in 2023.
• February 25, 2021: The ESA-Roscosmos Trace Gas Orbiter has spotted NASA's Mars 2020 Perseverance rover, along with its parachute and back shell, heat shield and descent stage, in the Jezero Crater region of Mars. The images were captured with the orbiter's CaSSIS camera on 23 February 2021. The components are seen as dark or bright pixels. 26)
Figure 12: The CaSSIS camera on ExoMars captured this image of the Jezero Crater region on 23 February 2021. In this image, the colors have been stretched to emphasize the compositional diversity of the surface (image credit: ESA)
Figure 13: ExoMars orbiter images Perseverance landing site (labelled). The ESA-Roscosmos Trace Gas Orbiter has spotted NASA's Mars 2020 Perseverance rover, along with its parachute and back shell, heat shield and descent stage, in the Jezero Crater region of Mars. The images were captured with the orbiter's CaSSIS camera on 23 February 2021. The components are labelled and are seen as dark or bright pixels. In this image, the colors have been adjusted to resemble the typical red color of Mars, as would be seen by a human observer (image credit: ESA/Roscosmos/CaSSIS; acknowledgement A. Valantinas)
• February 22, 2021: Giant dust devils – swirling columns of wind – are constantly scouring the surface of Mars. 27)
Figure 14: In this pair of images, captured 27 February 2019 by the CaSSIS camera on the ESA-Roscosmos ExoMars Trace Gas Orbiter about 45 seconds apart, two dust devils have been caught in action. The bright ‘plumes' were seen travelling across the inside of a 70 km crater in the southern hemisphere of Mars, leaving a dark streak behind them. The shadows of the two dust columns can also be seen in motion. The dust devil in the left part of the image is travelling at about 4 m/s, while the one on the right is travelling at 8 m/s. The dark patch of ground in the left of the image is a large basaltic dune field. Frequent dust devil action leaves this typical web of crisscrossing marks in the surface as they lift the top layer of dust away, leaving darker paths in their wake (image credit: ESA/Roscosmos/CaSSIS)
- Dust devils on Mars form in much the same way as those on Earth: when the ground gets hotter than the air above it, rising plumes of hot air move through cooler denser air, creating an updraft, with the cooler air sinking and setting up a vertical circulation. If a horizontal gust of wind blows through, the dust devil is triggered. Once whirling fast enough, the spinning funnels can pick up dust and push it around the surface.
- Martian dust devils are much larger than their Earth counterparts, though. On the Red Planet they can tower up to eight kilometers high, creating paths that are tens to hundreds of meters wide, stretching out for several kilometers. Their colossal size makes them highly effective at carrying dust high up into Mars' atmosphere. As such, they are interesting – and important – to study in order to understand how they might influence the planet's climate over time.
- One of the scientific goals of the CaSSIS imager is to investigate dynamic surface processes, including aeolian processes like these, and to support the orbiter's suite of spectrometers in documenting trace gases in the planet's atmosphere and their potential sources.
• February 16, 2021: This image shows a portion of the landing ellipse (circled) for NASA's Mars 2020 Perseverance rover, which is expected to land within Jezero crater on 18 February 2021. The complete landing ellipse is 7.7 x 6.6 km, and is centered on an ancient river delta near the rim of Jezero that could hold clues about whether or not Mars was able to harbor life at some point during its ancient past. Jezero crater itself was once the site of a lake, and Perseverance will explore this region looking for signs of fossilized microbial life. 28)
Figure 15: The image was taken by the CaSSIS camera on the ESA-Roscosmos ExoMars Trace Gas Orbiter as part of an imaging campaign of the rover's future neighborhood (image credit: ESA/Roscosmos/CaSSIS, CC BY-SA 3.0 IGO)
• February 16, 2021: This image shows an area close to the landing ellipse for NASA's Mars 2020 Perseverance rover, which is expected to land within Jezero crater on 18 February 2021. Jezero crater was once the site of a lake, and the landing site is centered on an ancient river delta near the rim of the crater. Although the actual landing ellipse is just outside of this image, it was taken as part of an imaging campaign to study the rover's future neighborhood, in preparation for its arrival. 29)
Figure 16: The image was taken by the CaSSIS camera on the ESA-Roscosmos ExoMars Trace Gas Orbiter on 23 April 2020. For scale, the prominent crater left of center is about 2 km across (image credit: ESA/Roscosmos/CaSSIS, CC BY-SA 3.0 IGO)
• February 10, 2021: Sea salt embedded in the dusty surface of Mars and lofted into the planet's atmosphere has led to the discovery of hydrogen chloride – the first time the ESA-Roscosmos ExoMars Trace Gas Orbiter has detected a new gas. The spacecraft is also providing new information about how Mars is losing its water. 30)
- A major quest in Mars exploration is hunting for atmospheric gases linked to biological or geological activity, as well as understanding the past and present water inventory of the planet, to determine if Mars could ever have been habitable and if any water reservoirs could be accessible for future human exploration. Two new results from the ExoMars team published today in Science Advances unveil an entirely new class of chemistry and provide further insights into seasonal changes and surface-atmosphere interactions as driving forces behind the new observations.
Figure 17: The ESA-Roscosmos ExoMars Trace Gas Orbiter studies water vapor and its components as it rises through the atmosphere and out into space. By looking specifically at the ratio of hydrogen to its heavier counterpart deuterium, the evolution of water loss over time can be traced (image credit: ESA)
A new chemistry
- "We've discovered hydrogen chloride for the first time on Mars. This is the first detection of a halogen gas in the atmosphere of Mars, and represents a new chemical cycle to understand," says Kevin Olsen from the University of Oxford, UK, one of the lead scientists of the discovery.
- Hydrogen chloride gas, or HCl, comprises a hydrogen and chlorine atom. Mars scientists were always on the look-out for chlorine- or sulphur-based gases because they are possible indicators of volcanic activity. But the nature of the hydrogen chloride observations – the fact that it was detected in very distant locations at the same time, and the lack of other gases that would be expected from volcanic activity – points to a different source. That is, the discovery suggests an entirely new surface-atmosphere interaction driven by the dust seasons on Mars that had not previously been explored.
Figure 18: Discovering new gases on Mars. The ESA-Roscosmos ExoMars Trace Gas Orbiter is investigating the martian atmosphere. Discovering new gases related to active process and looking for their sources is a key goal of the mission. ExoMars has discovered hydrogen chloride for the first time. It appeared during a global dust storm in 2018 and disappeared again afterwards. The detection was made in both hemispheres simultaneously so it is unlikely to come from volcanic activity. Seasonal change that triggers dust activity is thought to be the driving force behind the observation. Salt in the dusty surface – left over from when Mars had water on its surface earlier in its history – reacts with water vapor in the atmosphere to release chlorine. The chlorine and hydrogen react to create hydrogen chloride gas. The discovery of this new type of gas requires a change in understanding of Mars' global atmospheric chemistry and processes (video credit: ESA - European space Agency)
- In a process very similar to that seen on Earth, salts in the form of sodium chloride – remnants of evaporated oceans and embedded in the dusty surface of Mars – are lifted into the atmosphere by winds. Sunlight warms the atmosphere causing dust, together with water vapor released from ice caps, to rise. The salty dust reacts with atmospheric water to release chlorine, which itself then reacts with molecules containing hydrogen to create hydrogen chloride. Further reactions could see the chlorine or hydrochloric acid-rich dust return to the surface, perhaps as perchlorates, a class of salt made of oxygen and chlorine.
- "You need water vapor to free chlorine and you need the by-products of water – hydrogen - to form hydrogen chloride. Water is critical in this chemistry," says Kevin. "We also observe a correlation to dust: we see more hydrogen chloride when dust activity ramps up, a process linked to the seasonal heating of the southern hemisphere."
- The team first spotted the gas during the global dust storm in 2018, observing it appear simultaneously in both northern and southern hemispheres, and witnessed its surprisingly quick disappearance again at the end of the seasonal dusty period. They are already looking into the data collected during the following dust season and see the HCl rising again.
- "It is incredibly rewarding to see our sensitive instruments detecting a never-before-seen gas in the atmosphere of Mars," says Oleg Korablev, principal investigator of the Atmospheric Chemistry Suite instrument that made the discovery. "Our analysis links the generation and decline of the hydrogen chloride gas to the surface of Mars."
- Extensive laboratory testing and new global atmospheric simulations will be needed to better understand the chlorine-based surface-atmosphere interaction, together with continued observations at Mars to confirm that the rise and fall of HCl is driven by the southern hemisphere summer.
- "The discovery of the first new trace gas in the atmosphere of Mars is a major milestone for the Trace Gas Orbiter mission," says Håkan Svedhem, ESA's ExoMars Trace Gas Orbiter project scientist. "This is the first new class of gas discovered since the claimed observation of methane by ESA's Mars Express in 2004, which motivated the search for other organic molecules and ultimately culminated in the development of the Trace Gas Orbiter mission, for which detecting new gases is a primary goal." 31) 32)
Figure 19: How hydrogen chloride may be created on Mars. This graphic describes a possible new chemistry cycle on Mars following the discovery of hydrogen chloride in the atmosphere by the ESA-Roscosmos ExoMars Trace Gas Orbiter. Salts in the form of sodium chloride (NaCl) – remnants of evaporated oceans and embedded in the dusty surface of Mars – are widespread on the surface of Mars. Winds lift this salty dust into the atmosphere. Sunlight warms the dusty atmosphere causing water vapor released from ice caps to rise. The salty dust reacts with atmospheric water to release chlorine (Cl), which itself then reacts with molecules containing hydrogen (H) to create hydrogen chloride (HCl). A similar process takes place on Earth: sea salt is blown into the air, and if it mixes with water vapor, chlorine becomes available for chemical reactions that form HCl. - Further reactions could see the chlorine or hydrogen chloride-rich dust return to the surface of Mars perhaps as perchlorates, a class of salt made of oxygen and chlorine. The HCl is observed to quickly appear and disappear from the atmosphere so it must be created and destroyed rapidly, with some fraction returned to the surface. -The ExoMars observations suggest this might be an annual process driven by the changing seasons, specifically the warming of the southern hemisphere ice cap during southern summer, which releases water vapor into the atmosphere. The extra warmth also generates strong winds as air moves from warm to cool regions. In turn, the winds lift more dust, triggering regional and global dust storms. - The graphic is simplified to show very broadly one possible way that hydrogen chloride is generated; there are likely additional pathways for the chemical reactions that could also be at play, perhaps with other trace gases that ExoMars hasn't discovered yet (image credit: ESA)
Rising water vapor holds clues to climate evolution
- As well as new gases, the Trace Gas Orbiter is refining our understanding of how Mars lost its water – a process which is also linked to seasonal changes.
- Liquid water is once thought to have flowed across the surface of Mars as evidenced in the numerous examples of ancient dried out valleys and river channels. Today, it is mostly locked up in the ice caps and buried underground. Mars is still leaking water today, in the form of hydrogen and oxygen escaping from the atmosphere.
- Understanding the interplay of potential water-bearing reservoirs and their seasonal and long-term behavior is key to understanding the evolution of the climate of Mars. This can be done through the study of water vapor and ‘semi-heavy' water (where one hydrogen atom is replaced by a deuterium atom, a form of hydrogen with an additional neutron).
Figure 20: Liquid water once flowed across the martian surface. Today it is mostly locked up in the ice caps and buried underground, but water loss still occurs today. The ESA-Roscosmos ExoMars Trace Gas Orbiter is providing new data to learn more about water loss and thus the planet's climate evolution. It is following the vertical path of water through the atmosphere and its changing isotopic composition, a metric used to estimate water loss on Mars, behaving like a water ‘chronometer'. The new data reveal that as water travels and rises to colder regions it condenses and its isotopic signature changes dramatically, impacting the local value of the water chronometer. Yet, when water is fully vaporised, it mostly displays a common isotopic enrichment and a D/H ratio six times greater than Earth's across all reservoirs on Mars, confirming that large amounts of water have been lost over time (video credit: ESA)
- "The deuterium to hydrogen ratio, D/H, is our chronometer – a powerful metric that tells us about the history of water on Mars, and how water loss evolved over time. Thanks to the ExoMars Trace Gas Orbiter, we can now better understand and calibrate this chronometer and test for potential new reservoirs of water on Mars," says Geronimo Villanueva of NASA's Goddard Space Flight Center and lead author of the new result.
Figure 21: The ESA-Roscosmos ExoMars Trace Gas Orbiter studies water vapor and its components as it rises through the atmosphere and out into space. By looking specifically at the ratio of hydrogen to its heavier counterpart deuterium, the evolution of water loss over time can be traced (image credit: ESA)
- With the Trace Gas Orbiter we can watch the path of the water isotopologues as they rise up into the atmosphere with a level of detail not possible before. Previous measurements only provided the average over the depth of the whole atmosphere. It is like we only had a 2D view before, now we can explore the atmosphere in 3D," says Ann Carine Vandaele, principal investigator of the Nadir and Occultation for MArs Discovery (NOMAD) instrument that was used for this investigation.
- The new measurements reveal dramatic variability in D/H with altitude and season as the water rises from its original location."Interestingly, the data show that once water is fully vaporised, it mostly displays a common large enrichment in semi-heavy water, and a D/H ratio six times greater than Earth's across all reservoirs on Mars, confirming that large amounts of water have been lost over time," says Giuliano Liuzzi of American University and NASA's Goddard Space Flight Center and one of the lead scientists of the investigation.
- ExoMars data collected between April 2018 and April 2019 also showed three instances that accelerated water loss from the atmosphere: the global dust storm of 2018, a short but intense regional storm in January 2019, and water release from the south polar ice cap during summer months linked to seasonal change. Of particular note is a plume of rising water vapor during southern summer that would potentially inject water into the upper atmosphere on a seasonal and yearly basis.
- Future coordinated observations with other spacecraft including NASA's MAVEN, which focuses on the upper atmosphere, will provide complementary insights to the evolution of water over the martian year.
- "The changing seasons on Mars, and in particular the relatively hot summer in the southern hemisphere seems to be the driving force behind our new observations such as the enhanced atmospheric water loss and the dust activity linked to the detection of hydrogen chloride, that we see in the two latest studies," adds Håkan. "Trace Gas Orbiter observations are enabling us to explore the martian atmosphere like never before."
Figure 22: How ExoMars studies the atmosphere. The ExoMars Trace Gas Orbiter will analyze the atmosphere of Mars with two spectrometers: the Atmospheric Chemistry Suite (ACS) and the Nadir and Occultation for MArs Discovery, or NOMAD. - The graphic shows a simple representation (not to scale) of the three observing modes that will be used. In nadir mode (left) the spacecraft looks directly at the sunlight reflected from the surface and atmosphere of Mars. In limb mode (centre) it looks across the martian horizon at emission from the atmosphere. In solar occultation mode (right), the instruments point through the atmosphere toward the Sun and observe how different atmospheric ingredients absorb the Sun's light. - Since different chemicals have distinctive fingerprints, these observations provide a detailed inventory of the atmosphere's composition. These observations are critical to detect atmospheric gases that exist in tiny amounts, but which play an important role in determining if Mars is active today – either geologically or biologically speaking. - Note that the orientation of the spacecraft is not true: it is shown here with the instrument panel facing the viewer for illustrative purposes only. ACS is represented by the yellow square shape at the front of the orbiter; NOMAD is the grey box also at the front. Click here for a labelled diagram (image credit: ESA/ATG medialab)
• February 10, 2021: The CaSSIS (Color and Stereo Surface Imaging System) instrument onboard the ExoMars Trace Gas Orbiter took the image on 19 October 2020. 33)
Figure 23: This image features the southeast wall of a small crater located a few hundred kilometers to the north of the giant Hellas impact basin on Mars. The complete crater itself is about 12 km in diameter; this image shows a 5 x 10 km area. When viewed with CaSSIS' color filters, the image shows exceptional diversity in color. This diversity is related to the presence of various minerals that reflect light differently at different wavelengths. The light-toned deposits highlight the bedrock exposures of the area, which probably contain ancient clay-rich minerals that would have formed in the presence of water. Also visible are wind-blown sandy deposits that form ripples on the floor of the crater. Their distinctive tan color implies that they contain iron-oxide minerals. The image was featured by Science Advances online in February 2021 (image credit: ESA/Roscosmos/CaSSIS, CC BY-SA 3.0 IGO)
• February 4, 2021: The CaSSIS (Color and Stereo Surface Imaging System) instrument onboard the ExoMars Trace Gas Orbiter mission returned this image of an area in Melas Chasma, part of the vast Valles Marineris canyon system on Mars. Valles Marineris stretches for more than 4000 km across the planet's surface, and plunges more than 7 km deep in places. 34)
Figure 24: The section seen here is about 5 x 6 km in size. It is a color infrared image (combining the NIR, PAN and BLU filters of CaSSIS), and emphasizes the spectral diversity of landforms and sediments on the surface. It shows details of a blocky deposit on the floor of Melas Chasma that is consistent with an eroded and exposed landslide deposit. Windblown ripples are abundant and interspersed between the blocks. The image was taken on 19 October 2020 and featured on the February 2021 cover of Nature Geoscience (image credit: ESA/Roscosmos/CaSSIS, CC BY-SA 3.0 IGO)
- The CRISM spectrometer on NASA's Mars Reconnaissance Orbiter revealed a variety of minerals and phases that correlate with the light-toned blocks seen here (for example: nontronite, jarosite, aluminum-rich clays, hydrated silica, and/or an acid-leached clay). The tan-colored ripples likely contain ferric iron oxides that gives rise to this distinctive color. There is also evidence of the past presence of water in this region. The bright-white layered materials imply the presence of a hydrated calcium sulphate (possibly gypsum), which is thought to have formed through the ponding and subsequent evaporation of water that may have once occupied portions of the Chasma floor.
• January 28, 2021: The CaSSIS camera onboard the ExoMars Trace Gas Orbiter has captured its 20,000th image of Mars! 35)
Figure 25: The image, taken on 13 December 2020, features Solis Dorsum, a segment of a prominent wrinkle ridge system in a vast volcanic plateau, known as Tharsis. Wrinkle ridges are tectonic features that form in layered basalt lavas due to loading and flexure of the planet's crust and upper mantle. These tectonic stresses are caused by the planet's interior cooling and subsequent contraction (image credit: ESA/Roscosmos/CaSSIS, CC BY-SA 3.0 IGO)
- The study of wrinkle ridges, and in particular their distribution and orientation, can reveal details of the complex and dynamic geological history of Mars.
- The ExoMars program is a joint endeavor between ESA and Roscosmos.
• January 6, 2021: Frosty scenes in martian summer. The CASSIS camera onboard the ExoMars Trace Gas Orbiter captured remnant frost deposits in a region near Sisyphi Tholus, in the high southern latitudes of Mars (74ºS/246ºE). 36)
Figure 26: This image was taken during the early morning of a midsummer day in the southern hemisphere. At these high latitudes, carbon dioxide ice and frost develop. Frost can be seen within polygonal cracks in the terrain, a feature that indicates the presence of water ice embedded in the soil. The black spots observed throughout the scene are due to dark soil being pushed through cracks in the carbon dioxide ice as it sublimates – turns directly from solid ice to vapor – in the summer months (image credit: ESA/Roscosmos/CASSIS, , CC BY-SA 3.0 IGO)
- The ExoMars program is a joint endeavor between ESA and Roscosmos.
• August 4, 2020: A new set of images captured this spring by CASSIS (Color and Stereo Surface Imaging System) on the ESA-Roscosmos ExoMars Trace Gas Orbiter shows a series of interesting geological features on the surface of Mars, captured just as the planet passed its spring equinox. 37)
Figure 27: Dune fields in the Green Crater of Mars. The image, taken on 27 April 2020 and centered at 52.3ºS, 351.8ºE, shows part of an impact crater located inside the larger Green Crater in the Argyre quadrangle in the southern hemisphere of Mars (image credit: ESA/ExoMars/CaSSIS)
- The image of Figure 27 reveals an almost black dune field on the right, surrounded by red soils that are partially covered with bright white ice. Gullies, also partially covered with ice, are visible in the crater wall in the center of the image. Scientists are currently investigating the relationship between this seasonal ice and the presence of the gullies. The image was taken just after the spring equinox in the southern hemisphere of Mars, when the southernmost part of the crater (to the right) was almost completely free of ice while the northern part (center) was still partially covered. The southern crater wall has had a longer exposure to the Sun (like on Earth equator-facing slopes receive more sunlight), so the ice in this area recedes faster.
Figure 28: Leaf-like structures in Antoniadi impact crater. This image, captured on 25 March 2020, shows the bottom of the 400 km in diameter Antoniadi impact crater, which is located in the northern hemisphere of Mars in the Syrtis Major Planum region. The blue color of the image, centered at 21.0ºN, 61.2ºE, does not represent the real color of the crater floor but highlights the diversity of the rock composition inside the impact crater (image credit: ESA/ExoMars/CaSSIS)
- In the center of the image (Figure 28) are dendritic structures which look like the veins on oak leaves. These structures, evidence of ancient river networks in this region, protrude from the surface, unlike channels, which are usually sunken in the surface. This is because the channels were filled with harder material – possibly lava – and over time the softer rocks surrounding these branching channels have been eroded, leaving an inverted imprint of this ancient river system.
Figure 29: Argyre impact basin after spring equinox. This image of the Argyre impact basin in the southern highlands of Mars was taken on 28 April 2020 just as Mars had passed its southern hemisphere spring equinox. The seasonal ice in the 800km-long impact basin is receding while the ridge on the right side of the image is still covered with frost. The image is centered at 57.5°S, 310.2°E. The frost-covered ridge is facing the pole, therefore receiving less solar radiation than the neighboring equator-facing slope. On Mars, incoming solar radiation transforms the ice into water vapor directly without melting it first into water in a process called sublimation. Since the north-facing slope (on the left) has had a longer exposure to solar radiation, its ice has sublimated more quickly (image credit: ESA/ExoMars/CaSSIS)
Figure 30: Rock composition in Ius Chasma canyon. The image taken on 5 May 2020 shows a part of the floor of the Ius Chasma canyon, part of the Valles Marines system of canyons that stretches nearly a quarter of the circumference of Mars south of the planet's equator. The Ius Chasma canyon, which can be seen in the image rising up to a ridge on the right side, is about 1000 km long and up to 8 km deep, which makes it more than twice as long and four times as deep as the famous Grand Canyon in the US state of Arizona. The center of this image is located at 8.8ºS, 282.5ºE (image credit: ESA/ExoMars/CaSSIS)
- The beautiful color variations across the floor of Ius Chasma are caused by changes in rock composition. Scientists theorize that the light rocks are salts left behind after an ancient lake evaporated. The information about the rock's composition is useful to scientists as it allows them to retrace the formation history of the canyon.
• July 27, 2020: ESA's ExoMars Trace Gas Orbiter has spotted new gas signatures at Mars. These unlock new secrets about the martian atmosphere, and will enable a more accurate determination of whether there is methane, a gas associated with biological or geological activity, at the planet. 38)
- The Trace Gas Orbiter (TGO) has been studying the Red Planet from orbit for over two years. The mission aims to understand the mixture of gases that make up the martian atmosphere, with a special focus on the mystery surrounding the presence of methane there.
- Meanwhile, the spacecraft has now spotted never-before-seen signatures of ozone (O3) and carbon dioxide (CO2), based on a full martian year of observations by its sensitive Atmospheric Chemistry Suite (ACS). The findings are reported in two new papers published in Astronomy & Astrophysics, one led by Kevin Olsen of the University of Oxford, UK and another led by Alexander Trokhimovskiy of the Space Research Institute (IKI) of the Russian Academy of Sciences in Moscow, Russia. 39) 40)
- These features are both puzzling and surprising," says Kevin.
- They lie over the exact wavelength range where we expected to see the strongest signs of methane. Before this discovery, the CO2 feature was completely unknown, and this is the first time ozone on Mars has been identified in this part of the infrared wavelength range."
- The martian atmosphere is dominated by CO2, which scientists observe to gauge temperatures, track seasons, explore air circulation, and more. Ozone – which forms a layer in the upper atmosphere on both Mars and Earth – helps to keep atmospheric chemistry stable. Both CO2 and ozone have been seen at Mars by spacecraft such as ESA's Mars Express, but the exquisite sensitivity of the ACS instrument on TGO was able to reveal new details about how these gases interact with light.
- Observing ozone in the range where TGO hunts for methane is a wholly unanticipated result.
- Scientists have mapped how martian ozone varies with altitude before. So far, however, this has largely taken place via methods that rely upon the gas' signatures in the ultraviolet, a technique which only allows measurement at high altitudes (over 20 km above the surface).
- The new ACS results show that it is possible to map martian ozone also in the infrared, so its behavior can be probed at lower altitudes to build a more detailed view of ozone's role in the planet's climate.
Figure 31: Spectral signatures of carbon dioxide and ozone at Mars. This graph shows an example of the measurements made by the Atmospheric Chemistry Suite (ACS) MIR instrument on ESA's ExoMars Trace Gas Orbiter (TGO), featuring the spectral signatures of carbon dioxide (CO2) and ozone (O3). The bottom panel shows the data (blue) and a best-fit model (orange). The top panel shows the modelled contributions from a variety of different gases for this spectral range. The deepest lines come from water vapor (light blue). The strongest O3 feature (green) is on the right, and distinct CO2 lines (grey) appear on the left. The locations of strong methane features (orange) are also shown in the modelled contributions, though methane is not observed in the TGO data (image credit: K. Olsen et al. (2020))
Unravelling the methane mystery
- One of the key objectives of TGO is to explore methane. To date, signs of martian methane – tentatively spied by missions including ESA's Mars Express from orbit and NASA's Curiosity rover on the surface – are variable and somewhat enigmatic.
- While also generated by geological processes, most of the methane on Earth is produced by life, from bacteria to livestock and human activity. Detecting methane on other planets is therefore hugely exciting. This is especially true given that the gas is known to break down in around 400 years, meaning that any methane present must have been produced or released in the relatively recent past.
Figure 32: How methane is created and destroyed on Mars is an important question in understanding the various detections and non-detections of methane at Mars, with differences in both time and location. Although making up a very small amount of the overall atmospheric inventory, methane in particular holds key clues to the planet's current state of activity. This graphic depicts some of the possible ways methane might be added or removed from the atmosphere. One exciting possibility is that methane is generated by microbes. If buried underground, this gas could be stored in lattice-structured ice formations known as clathrates, and released to the atmosphere at a much later time (image credit: ESA)
- Methane can also be generated by reactions between carbon dioxide and hydrogen (which, in turn, can be produced by reaction of water and olivine-rich rocks), by deep magmatic degassing or by thermal degradation of ancient organic matter. Again, this could be stored underground and outgassed through cracks in the surface. Methane can also become trapped in pockets of shallow ice, such as seasonal permafrost.
- Ultraviolet radiation can both generate methane – through reactions with other molecules or organic material already on the surface, such as comet dust falling onto Mars – and break it down. Ultraviolet reactions in the upper atmosphere (above 60 km) and oxidation reactions in the lower atmosphere (below 60 km) acts to transform methane into carbon dioxide, hydrogen and water vapor, and leads to a lifetime of the molecule of about 300 years.
- Methane can also be quickly distributed around the planet by atmospheric circulation, diluting its signal and making it challenging to identify individual sources. Because of the lifetime of the molecule when considering atmospheric processes, any detections today imply it has been released relatively recently.
- But other generation and destruction methods have been proposed which explain more localized detections and also allow a faster removal of methane from the atmosphere, closer to the surface of the planet. Dust is abundant in the lower atmosphere below 10 km and may play a role, along with interactions directly with the surface. For example, one idea is that methane diffuses or ‘seeps' through the surface in localized regions, and is adsorbed back into the surface regolith. Another idea is that strong winds eroding the planet's surface allows methane to react quickly with dust grains, removing the signature of methane. Seasonal dust storms and dust devils could also accelerate this process.
- Continued exploration at Mars – from orbit and the surface alike – along with laboratory experiments and simulations, will help scientists to better understand the different processes involved in generating and destroying methane.
- "Discovering an unforeseen CO2 signature where we hunt for methane is significant," says Alexander Trokhimovskiy. "This signature could not be accounted for before, and may therefore have played a role in detections of small amounts of methane at Mars."
Figure 33: This graph shows a new CO2 spectral feature, never before observed in the laboratory, discovered in the martian atmosphere by the Atmospheric Chemistry Suite (ACS) MIR instrument on ESA's ExoMars Trace Gas Orbiter (TGO). The graph shows the full extent of the magnetic dipole absorption band of the 16O12C16O molecule (one of the various 'isotopologues' of CO2). The top panel shows the ACS MIR spectra (shown in black) along with the modelled contribution of CO2 and H2O (shown in blue); the model is based on the HITRAN 2016 database. The bottom panel shows the difference between data and model, or residuals, revealing the structure of the absorption band in detail. The calculated positions of spectral lines are marked with arrows, in different colors corresponding to different 'branches' of the absorption band (red stands for the P-branch, green for the Q-branch and blue for the R-branch), image credit: A. Trokhimovskiy et al. (2020)
- The observations analyzed by Alexander, Kevin and colleagues were mostly performed at different times to those supporting detections of martian methane. Besides, the TGO data cannot account for large plumes of methane, only smaller amounts – and so, currently, there is no direct disagreement between missions.
- "In fact, we're actively working on coordinating measurements with other missions," clarifies Kevin. "Rather than disputing any previous claims, this finding is a motivator for all teams to look closer – the more we know, the more deeply and accurately we can explore Mars' atmosphere."
Realizing the potential of ExoMars
- Methane aside, the findings highlight just how much we will learn about Mars as a result of the ExoMars program.
- "These findings enable us to build a fuller understanding of our planetary neighbor," adds Alexander.
- "Ozone and CO2 are important in Mars' atmosphere. By not accounting for these gases properly, we run the risk of mischaracterizing the phenomena or properties we see."
- Additionally, the surprising discovery of the new CO2 band at Mars, never before observed in the laboratory, provides exciting insight for those studying how molecules interact both with one another and with light – and searching for the unique chemical fingerprints of these interactions in space.
- "Together, these two studies take a significant step towards revealing the true characteristics of Mars: towards a new level of accuracy and understanding," says Alexander.
Figure 34: Comparing the atmospheres of Mars and Earth. Mars is about half the size of Earth by diameter and has a much thinner atmosphere, with an atmospheric volume less than 1% of Earth's. The atmospheric composition is also significantly different: primarily carbon dioxide-based, while Earth's is rich in nitrogen and oxygen. The atmosphere has evolved: evidence on the surface suggest that Mars was once much warmer and wetter. - The planets in this graphic are not to scale. Mars atmospheric values are as measured by NASA's Curiosity rover. (image credit: ESA)
- Understanding if life could have ever existed in such conditions is one of the hot topics of Mars exploration, and for the ESA–Roscosmos ExoMars mission. The ExoMars Trace Gas Orbiter is capable of sniffing out the composition of the planet's trace gases – which make up less than 1% by volume of a planet's atmosphere – in minute amounts. Although making up a very small amount of the overall atmospheric inventory, methane in particular holds key clues to the planet's current state of activity.
- On Earth, living organisms release much of the planet's methane. It is also the main component of naturally occurring hydrocarbon gas reservoirs, and a contribution is also provided by volcanic and hydrothermal activity. Because of the key role natural biology plays in Earth's methane production, confirming the existence of methane on Mars, and distinguishing between its potential sources, is a top priority of the ExoMars Trace Gas Orbiter.
Successful collaboration in the hunt for life
- As its name suggests, the TGO aims to characterize any trace gases in Mars' atmosphere that could arise from active geological or biological processes on the planet, and identify their origin.
- The ExoMars program consists of two missions: TGO, which was launched in 2016 and will be joined by the Rosalind Franklin rover and the Kazachok landing platform, due to lift off in 2022. These will take instruments complementary to ACS to the martian surface, examining the planet's atmosphere from a different perspective, and share the core objective of the ExoMars program: to search for signs of past or present life on the Red Planet.
- "These findings are the direct result of hugely successful and ongoing collaboration between European and Russian scientists as part of ExoMars," says ESA TGO Project Scientist Håkan Svedhem.
- "They set new standards for future spectral observations, and will help us to paint a more complete picture of Mars' atmospheric properties – including where and when there may be methane to be found, which remains a key question in Mars exploration."
- "Additionally, these findings will prompt a thorough analysis of all the relevant data we've collected to date – and the prospect of new discovery in this way is, as always, very exciting. Each piece of information revealed by the ExoMars Trace Gas Orbiter marks progress towards a more accurate understanding of Mars, and puts us one step closer to unravelling the planet's lingering mysteries."
Figure 35: Artist's impression of the ExoMars 2022 rover (foreground), surface science platform (background) and the Trace Gas Orbiter (top). Not to scale (image credit: ESA/ATG medialab)
• June 15, 2020: ESA's ExoMars Trace Gas Orbiter has detected glowing green oxygen in Mars' atmosphere – the first time that this emission has been seen around a planet other than Earth. 41)
Figure 36: Artist's impression of ESA's ExoMars Trace Gas Orbiter detecting the green glow of oxygen in the martian atmosphere. This emission, spotted on the dayside of Mars, is similar to the night glow seen around Earth's atmosphere from space (image credit: ESA)
- On Earth, glowing oxygen is produced during polar auroras when energetic electrons from interplanetary space hit the upper atmosphere. This oxygen-driven emission of light gives polar auroras their beautiful and characteristic green hue.
- The aurora, however, is just one way in which planetary atmospheres light up. The atmospheres of planets including Earth and Mars glow constantly during both day and night as sunlight interacts with atoms and molecules within the atmosphere. Day and night glow are caused by slightly different mechanisms: night glow occurs as broken-apart molecules recombine, whereas day glow arises when the Sun's light directly excites atoms and molecules such as nitrogen and oxygen.
- On Earth, green night glow is quite faint, and so is best seen by looking from an ‘edge on' perspective – as portrayed in many spectacular images taken by astronauts aboard the International Space Station (ISS). This faintness can be an issue when hunting for it around other planets, as their bright surfaces can drown it out.
Figure 37: Airglow observed from the ISS. Airglow occurs in Earth's atmospheres as sunlight interacts with atoms and molecules within the atmosphere. In this image, taken by astronauts aboard the International Space Station (ISS) in 2011, a green band of oxygen glow is visible over Earth's curve. On the surface, portions of northern Africa are visible, with evening lights shining along the Nile river and its delta (image credit: NASA) .
- This green glow has now been detected for the first time at Mars by the ExoMars Trace Gas Orbiter (TGO), which has been orbiting Mars since October 2016.
- "One of the brightest emissions seen on Earth stems from night glow. More specifically, from oxygen atoms emitting a particular wavelength of light that has never been seen around another planet," says Jean-Claude Gérard of the Université de Liège, Belgium, and lead author of the new study published in Nature Astronomy.
- "However, this emission has been predicted to exist at Mars for around 40 years – and, thanks to TGO, we've found it."
- Jean-Claude and colleagues were able to spot this emission using a special observing mode of the TGO. One of the orbiter's advanced suite of instruments, known as NOMAD (Nadir and Occultation for Mars Discovery) and including the ultraviolet and visible spectrometer (UVIS), can observe in various configurations, one of which positions its instruments to point directly down at the martian surface – also referred to as the ‘nadir' channel.
- "Previous observations hadn't captured any kind of green glow at Mars, so we decided to reorient the UVIS nadir channel to point at the ‘edge' of Mars, similar to the perspective you see in images of Earth taken from the ISS," adds co-author Ann Carine Vandaele of the Institut Royal d'Aéronomie Spatiale de Belgique, Belgium, and Principal Investigator of NOMAD.
- Between 24 April and 1 December 2019, Jean-Claude, Ann Carine and colleagues used NOMAD-UVIS to scan altitudes ranging from 20 to 400 km from the martian surface twice per orbit. When they analyzed these datasets, they found the green oxygen emission in all of them.
Figure 38: Observation and model of green dayglow emission at Mars. The observations, obtained with the TGO's NOMAD instrument using its UVIS channel between April and December 2019, are shown as green dots as a function of altitude, and compared to a theoretical model (red line). Oxygen green dayglow appears to be brightest at 80 km, reaching a second peak around 120 km, and dissipating above 150 km (image credit: J.-C. Gérard et al. (2020))
- "The emission was strongest at an altitude of around 80 kilometers and varied depending on the changing distance between Mars and the Sun," adds Ann Carine.
- Studying the glow of planetary atmospheres can provide a wealth of information about the composition and dynamics of an atmosphere, and reveal how energy is deposited by both the Sun's light and the solar wind – the stream of charged particles emanating from our star.
- To better understand this green glow at Mars, and compare it to what we see around our own planet, Jean-Claude and colleagues dug further into how it was formed.
- "We modelled this emission and found that it's mostly produced as carbon dioxide, or CO2, is broken up into its constituent parts: carbon monoxide and oxygen," says Jean-Claude. "We saw the resulting oxygen atoms glowing in both visible and ultraviolet light."
- Simultaneously comparing these two kinds of emissions showed that the visible emission was 16.5 times more intense than the ultraviolet.
- "The observations at Mars agree with previous theoretical models but not with the actual glowing we've spotted around Earth, where the visible emission is far weaker," adds Jean-Claude. "This suggests we have more to learn about how oxygen atoms behave, which is hugely important for our understanding of atomic and quantum physics."
- This understanding is key to characterizing planetary atmospheres and related phenomena – such as auroras. By deciphering the structure and behavior of this green glowing layer of Mars' atmosphere, scientists can gain insight into an altitude range that has remained largely unexplored, and monitor how it changes as the Sun's activity varies and Mars travels along its orbit around our star.
Figure 39: Oxygen emission detected in dayside limb spectra from the UVIS channel of the NOMAD instrument on ESA's ExoMars Trace Gas Orbiter. Different colors show the measurements at different altitudes in the martian atmosphere. Oxygen dayglow appears to be brightest at 80 km, reaching a second peak around 120 km, and dissipating above 150 km. This is the first time that this emission has been seen around a planet other than Earth (image credit: J.-C. Gérard et al. (2020))
- "This is the first time this important emission has ever been observed around another planet beyond Earth, and marks the first scientific publication based on observations from the UVIS channel of the NOMAD instrument on the ExoMars Trace Gas Orbiter," highlights Håkan Svedhem, ESA's TGO Project Scientist.
- "It demonstrates the remarkably high sensitivity and optical quality of the NOMAD instrument. This is especially true given that this study explored the dayside of Mars, which is much brighter than the nightside, thus making it even more difficult to spot this faint emission."
- Understanding the properties of Mars' atmosphere is not only interesting scientifically, but is also key to operate the missions we send to the Red Planet. Atmospheric density, for example, directly affects the drag experienced by orbiting satellites and by the parachutes used to deliver probes to the martian surface.
- "This type of remote-sensing observation, coupled with in situ measurements at higher altitudes, helps us to predict how the martian atmosphere will respond to seasonal changes and variations in solar activity," adds Håkan. "Predicting changes in atmospheric density is especially important for forthcoming missions, including the ExoMars 2022 mission that will send a rover and surface science platform to explore the surface of the Red Planet." 42)
• January 17, 2020: Water reaches Mars' upper atmosphere. Mars once hosted abundant water on its surface but subsequently lost most of it to space. Small amounts of water vapor are still present in the atmosphere, which can escape if they reach sufficiently high altitudes. Fedorova et al. used data from the ExoMars Trace Gas Orbiter spacecraft to determine the distribution of water in Mars' atmosphere and investigate how it varies over seasons. Water vapor is sometimes heavily saturated, and its distribution is affected by the planet's large dust storms. Water can efficiently reach the upper atmosphere when Mars is in the warmest part of its orbit, and this behavior may have controlled the overall rate at which Mars lost its water. 43)
- Mars once harbored an active hydrological cycle, as demonstrated by geological features on its surface, but it no longer holds the quantity of water required to produce such geological imprints . 44) The planet's bulk inventory of water amounts to a global equivalent layer (GEL) of ~30 m, mostly contained in its polar ice caps. This is less than 10% of the water that once flowed on the surface. Mars' enhanced concentration of heavy water (semiheavy water five or more times the terrestrial standard), strengthens the hypothesis that most of Mars' primordial water has escaped over time.
- Water in the atmosphere is a negligible component of the planet's total water inventory, being equivalent to a global layer 10 µm thick, but nevertheless regulates the dissipation of water over time. Most martian water has been lost to space because its decomposition products (atomic hydrogen and oxygen) reach the upper atmosphere, where they can acquire sufficient thermal energy to overcome the low gravity of Mars (which is about one-third that of Earth's). Water decomposition is theorized to follow a complex reaction chain involving the recombination of H atoms into H2 on a time scale of centuries, buffering any short-term hydrogen abundance variations. This mechanism has been challenged by observations showing that freshly produced H atoms can reach the exosphere (the uppermost layer where the atmosphere thins out and exchanges matter with interplanetary space) on a monthly time scale. The observed short-term variability of the hydrogen atoms populating the exosphere could be caused by direct deposition of water molecules at altitudes high enough to expose them to sunlight, which subsequently triggers a rapid enhancement of hydrogen atoms in the exosphere.
- Testing this hypothesis requires characterizing the mechanisms contributing to upward water propagation through large-scale atmospheric circulation. One such mechanism is the cold trap imposed (as on Earth) by water ice cloud formation at low altitude, subsequent to water condensation. The condensation is predicted to occur whenever the partial pressure of water vapor exceeds saturation. The vapor pressure law causes the cold trap efficiency to depend heavily on temperature, which eventually limits the amount of water that can be transported to higher altitudes.
- We investigate these processes using occultations of the Sun by the martian atmosphere (henceforth, solar occultations), where the vertical distributions of gases and particles can be directly observed. We used the Atmospheric Chemistry Suite (ACS) on the ExoMars Trace Gas Orbiter (TGO) spacecraft. 45) ACS is an assembly of three infrared spectrometers that together provide continuous spectral coverage from 0.7 to 17 µm, with a spectral resolving power ranging from 10,000 to 50,000. Our dataset was assembled by performing solar occultations with the near-infrared (NIR), mid-infrared (MIR), and thermal infrared in honor of professor V. I. Moroz (TIRVIM) channels of ACS. The NIR channel (0.7 to 1.7 µm) encompasses absorption bands of CO2, H2O, CO, and O2, diagnostic of their molecular concentrations over altitudes of 5 to 100 km, with a vertical resolution of 1 to 3 km. TIRVIM (2 to 17 µm) provides simultaneous information on dust and water ice particle abundance.
Figure 40: Example ACS spectra and retrieved profiles. (A) ACS NIR spectra measured during orbit 2580 (Ls = 197.8º, 47.27ºN, 85.2ºW, local time 17:27) at three example altitudes, labeled in each panel. Synthetic models (blue curves) fitted to the data (red dots) account for the water content, CO2 number density, and atmospheric temperature. The residuals are shown with gray lines. (B) Corresponding retrieved profiles of the H2O mixing ratio, temperature, and saturation ratio derived from them. The profiles of the mass of aerosol particles (ice, blue circles; dust, red dots) per cubic centimeter obtained during the same orbit from ACS TIRVIM data are also shown. All altitudes are above areoid (equipotential surface for Mars, the analog of geoid for Earth), image credit: ACS Research Team
• December 20, 2019: Ice capping the northern hemisphere terrain of Mars slowly recedes as summer progresses, revealing the underlying surface. 46)
- At this time, it was mid-summer in the northern hemisphere of Mars: the carbon dioxide ice cover had retreated, revealing the permanent water ice deposits much more clearly, along with details of surfaces previously covered in ice.
Figure 41: This scene was captured by the CaSSIS camera onboard ESA's ExoMars Trace Gas Orbiter as it flew over the ice-coated Korolev crater on 1 November 2019. Korolev crater is an 80 km-wide crater in the northern latitudes of Mars that contains a massive ice sheet in its center – this image focuses on one of the crater walls. The image is centered at 164.90ºE/72.02ºN and was taken on 1 November 2019 (image credit: ESA/Roscosmos/CaSSIS, CC BY-SA 3.0 IGO)
• December 20, 2019: Many craters in the polar regions of Mars hold permanent ice deposits year-round. 47)
Figure 42: In this image, taken by the CaSSIS camera onboard ESA's ExoMars Trace Gas Orbiter, the south-eastern wall of a 35 km-wide crater is seen. The image captures its permanent deposits of water ice, which survive the summer months due to the low average sunlight at high latitudes. The image is centered at 192.99ºE/70.4ºN. It was taken on 29 October 2019 (image credit: ESA/Roscosmos/CaSSIS, CC BY-SA 3.0 IGO)
• December 20, 2019: The rim of this ice-rich crater catches the early morning sunlight in the high northern latitudes of Mars, imaged by the CaSSIS camera onboard ESA's ExoMars Trace Gas Orbiter on 26 October 2019. 48)
Figure 43: This image features a simple 7 km-wide bowl-shaped crater pictured in the early morning. The sunlight falling on the ice deposits on the crater's north-facing walls causes the ice to appear extremely bright. Ice fills much of the crater floor, and coats part of the surrounding terrain (image credit: ESA/Roscosmos/CaSSIS, CC BY-SA 3.0 IGO)
- While the image was taken during the summer months, some shadowed regions receive fewer hours of sunlight on average throughout the year, so they trap permanent deposits of water ice.
- The image is centered at 230.77ºE/73.95ºN. It was taken on 26 October 2019. The scale is indicated on the image.
• September 16, 2019: Dunes come in various characteristic shapes on Mars just as on Earth, providing clues about the prevailing wind direction. Monitoring them over time also gives us a natural laboratory to study how dunes evolve, and how sediments in general are transported around the planet. 49)
Figure 44: This captivating image was taken in the north polar region of Mars by the ESA/Roscosmos ExoMars Trace Gas Orbiter's CaSSIS camera. The image is centered at 74.46ºN/348.3ºE. The image was taken on 25 May 2019 (image credit:ESA/Roscosmos/CaSSIS, CC BY-SA 3.0 IGO)
- During winter in the polar regions, a thin layer of carbon dioxide ice covers the surface and then sublimates – turns directly from ice into vapor – with the first light of spring. In the dune fields, this springtime defrosting occurs from the bottom up, trapping gas between the ice and the sand. As the ice cracks, this gas is released violently and carries sand with it, forming the dark patches and streaks observed in this CaSSIS image.
- The image also captures ‘barchan' dunes – the crescent or U-shaped dunes seen in the right of the image – as they join and merge into barchanoid ridges. The curved tips of the barchan dunes point downwind. The transition from barchan to barchanoid dunes tells us that secondary winds also play a role in shaping the dune field.
• May 30, 2019: On 15 June, the ESA-Roscosmos ExoMars Trace Gas Orbiter (TGO) will follow a different path. An ‘Inclination Change Maneuver' will put the spacecraft in an altered orbit, enabling it to pick up crucial status signals from the ExoMars rover, Rosalind Franklin, due to land on the Red Planet in 2021. 50)
- After completing a complex series of maneuvers during 2017, ExoMars TGO is now orbiting the Red Planet every two hours, collecting scientific data from NASA's surface-bound rover and lander, and relaying it back to Earth. At the same time, the orbiter is gathering its own data on the planet's atmosphere, water abundance and alien surface.
- More than a year before Rosalind even lifts off from Earth's surface, flight dynamics experts at ESA's ESOC mission control center have formulated a long-term plan to ensure ExoMars TGO can communicate with the new ESA rover and surface platform, contained in the entry, descent and landing module.
- Slight changes to a spacecraft's orbit have a large effect over time, so while the upcoming maneuvers will only slightly alter TGO's speed, it will be in the right position to communicate with the then-incoming rover by 2021.
TGO's natural motion
- Mars' uneven gravity field means that TGO's orbit ‘wanders', so it gradually rotates around Mars over time. As illustrated in this image, the spacecraft first follows the black path, then the green, then the red – continuing until it completes an entire rotation around the planet every four and a half months.
- To keep in touch with the descent module as it enters the Martian atmosphere, descends, and lands upon its surface, TGO's orientation needs to change.
- Three maneuvers in the month of June will alter TGO's speed, twice by 30.9 m/s and one final small change of 1.5 m/s, bringing it slightly closer to the Martian poles.
Figure 45: Mars' uneven gravity field means that TGO's orbit 'wanders' (image credit: ESA)
Inclined to fly
- Thanks to these maneuvers, TGO's path will look more like the second graphic shown here, illustrating ‘snapshots in time' during the 2021 descent of the new rover. - The green line of Figure 46 represents Rosalind Franklin's landing approach path. The black line shows the TGO orbit with its optimized orientation, two years after the upcoming maneuvers. The red path shows TGO's original orbit.
Figure 46: ExoMars TGO's adjusted orbit (image credit: ESA)
In-phase with Rosalind Franklin
- Once TGO is set to orbit with its new, optimized orientation around Mars, teams on the ground must also ensure it will be on the correct side of the planet when the rover arrives – ‘in phase' with Rosalind Franklin.
Figure 47: ExoMars TGO monitors the entry, descent and landing of ESA's new rover (image credit: ESA)
- In February 2021, a small maneuver will be performed to ensure TGO is in the right place at the right time for the lander's arrival. The result of all these maneuvers combined can be seen in Figure 47.
- The black line represents TGO's orbit around Mars at the time Rosalind Franklin begins descending, shown by the green line. Blue dots along the orbits of both spacecraft are connected by horizontal lines, illustrating their relative positions at different time intervals, and how they are able to ‘see' each other at every moment, thus ensuring that radio contact can be maintained.
- If teams at mission control were to leave ExoMars TGO in its current orbit, without performing any maneuvers, Mars itself would later get between the orbiting spacecraft and the new Mars explorer.
- In Figure 48, the red line illustrates TGO's un-phased orbit, and again the green line shows Rosalind Franklin's entry path and Blue dots represent moments in time for each spacecraft. Lines between the dots reveal how in this scenario, Mars would block their view of each other.
Figure 48: ExoMars TGO, un-phased (image credit: ESA)
- Without phasing the orbiter with the Mars rover, the two craft will remain invisible to each other at the crucial moment when the rover descends to the surface.
- Not only does the foresight and long-term planning of mission experts ensure communication is maintained between two of ESA's most important Mars missions, it saves fuel – a huge amount of which would be needed to get TGO in the right position in the weeks or even months before the ExoMars rover's arrival.
- ESA has demonstrated expertise in studying Mars from orbit, now we are looking to secure a safe landing, to rove across the surface and to drill underground to search for evidence of life. Our orbiters are already in place to provide data relay services for surface missions. The next logical step is to bring samples back to Earth, to provide access to Mars for scientists globally, and to better prepare for future human exploration of the Red Planet. This week we're highlighting ESA's contribution to Mars exploration as we ramp up to the launch of our second ExoMars mission, and look beyond to completing a Mars Sample Return mission.
• May 30, 2019: The ExoMars rover has a brand new control center in one of Europe's largest Mars yards. The Rover Operations Control Center (ROCC) was inaugurated today in Turin, Italy, ahead of the rover's exploration adventure on the Red Planet in 2021. 51)
- The control center will be the operational hub that orchestrates the roaming of the European-built laboratory on wheels, named after Rosalind Franklin, upon arrival to the martian surface on Kazachok, the Russian surface platform.
- "This is the crucial place on Earth from where we will listen to the rover's instruments, see what she sees and send commands to direct the search for evidence of life on and under the surface," said Jan Wörner, ESA's Director General.
- The ExoMars rover will be the first of its kind to both move across the Mars surface and to study it at depth with a drill able to collect samples from down to two meters into the surface.
- The epicenter of the action for directing Mars surface operations on Earth is at the ALTEC (Aerospace Logistics Technology Engineering Company) SpA premises in Turin, Italy. From here, engineers and scientists will work shoulder to shoulder at mission control, right next to a very special Mars yard. Filled with 140 tons of soil, the Mars-like terrain has sandy areas and rocks of various sizes that will help rehearse possible mission scenarios.
Figure 49: Moving off the landing platform (image credit: ALTEC)
Figure 50: Technology image of the week. Constructed within an 8 m long, 2.5 m diameter pressure chamber, the Aarhus Mars Simulation Wind Tunnel has attracted researchers from all over Europe and the United States, to test instruments and equipment for a wide range of Mars missions, including ESA's ExoMars and NASA's Mars 2020 rovers (image credit: Aarhus University)
- The air pressure within the wind tunnel can be taken down to less than one hundredth of terrestrial sea level and the temperature reduced to as low as -170°C using liquid nitrogen. Fans then blow the scanty atmosphere that remains at up to 30 m/s, along with Mars-style dust.
- Researchers can evaluate how items such as sensors, solar panels and mechanical parts stand up to the clingy, abrasive particles, sourced from Mars-like, oxide-rich soil found in central Denmark.
- "We've been in operation all through this decade," comments Jonathan Merrison of Aarhus University's Department of Physics and Astronomy, overseeing the facility. "We're the only wind tunnel that not only reproduces the low pressure and low temperatures of Mars, but also allows the introduction of particulates of sand and dust.
- "Probably about a third of the testing carried out here has been ExoMars related, then there have been users related to other Mars missions, as well as industrial testing of high altitude terrestrial equipment. We are also a member of the Europlanet network, a grouping of planetary scientists supported by the European Union, supporting the usage of various planetary simulation facilities and analogues."
- The Aarhus Mars Simulation Wind Tunnel was based on a smaller, earlier version, which remains in use. Its development was supported by ESA's Technology Development Element program for promising new technologies as well as the philanthropic Villum Kann Rasmussen Foundation.
• May 28, 2019: The ExoMars rover, named Rosalind Franklin, will be the first of its kind to both roam the Mars surface and to study it at depth. Rosalind Franklin will drill down to two meters into the surface to sample the soil, analyze its composition and search for evidence of past – and perhaps even present – life hidden underground. A miniature laboratory inside the rover – the ALD (Analytical Laboratory Drawer) – will analyze the samples with three different instruments, with some baked in the onboard oven to release gases for analysis, a technique used to search for traces of organic compounds. 53)
- The rover will relay its data back to Earth via the ExoMars Trace Gas Orbiter, which is already conducting its science mission from Mars orbit.
Figure 51: The ExoMars rover's Analytical Laboratory Drawer (ALD) was integrated into the rover at Airbus, Stevenage, UK in May 2019. The video is shown at 18 times real speed; in reality the sequence of events took around 11.5 minutes (video credit: ESA, Airbus, Published on May 28, 2019)
• May 28, 2019: There are few things better than the smell of freshly baked bread from the oven, this is because molecules in the bread disperse in the heat to reach your nose. In a similar way, the ExoMars rover Rosalind Franklin will ‘bake' and ‘sniff' martian samples in miniature ovens, imaged above, as part of its investigation of the extra-terrestrial world. 54)
- Set to land on Mars in 2021, Rosalind Franklin will scout areas of interest and drill up to 2 m below the surface and report back its findings to scientists on Earth. Nothing short of a miniature laboratory on wheels, the dirt that Rosalind Franklin collects will pass through different steps in an intricate process allowing for many types of analysis to get the best possible overview of the composition of Mars so far.
- MOMA (Mars Organic Molecule Analyzer) will heat samples to unlock the organic molecules from the Martian dust and transform them into the gas phase. The gas produced will then flow past a receptor that ‘sniffs' the molecules to learn more about the sample, thanks to its gas chromatograph.
- Baked to up to 800°C in the pyrolysis ovens, the investigations are a one-shot affair. The samples are arranged around the circumference of a rotating carrousel, so that each tube can be placed under the sample funnel and positioned in the tapping station where the samples are ‘cooked' and ‘sniffed'.
- The thumb-sized gold-colored tubes are hollow to hold the samples. At the tapping station a sphere pushes down on the oven rim to ensure an airtight seal during heating. The double golden pins are the connectors that send electricity running into the ovens.
- The silver-colored rod is a calibration target for a second component of MOMA dubbed ‘LDMS' that uses laser heating (desorption) and mass spectrometry to analyze the samples. The rod is used to create a standard value for the laser on the Red Planet to ensure that LDMS is working correctly. Together MOMA's gas chromatographer and LDMS will target biomarkers to answer questions related to the potential origin, evolution and distribution of life on Mars.
- Choosing when and where to take a Martian sample, and choosing which instrument to analyze the sample with, will be a discussion of interplanetary proportions for scientists, but that discussion will need to reach conclusions quickly: the ExoMars rover has 31 tubes to fill and analyze and is designed to work for 218 "sols" or Martian days.
- MOMA is built by a scientific consortium led by the Max-Planck-Institut für Sonnensystemforschung in Göttingen, Germany, with the gas chromatograph built by LISA (Laboratoire Interuniversitaire des Systèmes Atmosphériques) in Paris, France, and the LDMS by NASA's Goddard Spaceflight Center in Greenbelt, USA. These miniature ovens are part of the rover on-board laboratory "ultra clean zone" that was designed by Thales Alenia Space in Italy, and mounted on a carrousel developed by OHB in Munich, Germany.
Figure 52: Illustration of Martian ovens mounted on a carrousel (image credit: Thales Alenia Space)
• April 10, 2019: New evidence of the impact of the recent planet-encompassing dust storm on water in the atmosphere, and a surprising lack of methane, are among the scientific highlights of the ExoMars Trace Gas Orbiter's first year in orbit. 55)
Figure 53: Overview of the three new results presented today by the ExoMars TGO (Trace Gas Orbiter teams). While results from the imaging system CaSSIS have been presented previously, today's release covers the first analysis of the Mars atmosphere and subsurface (image credit: ESA; spacecraft: ESA/ATG medialab)
- TGO's main science mission began at the end of April 2018, just a couple of months before the start of the global dust storm that would eventually lead to the demise of NASA's Opportunity rover after 15 years roving the martian surface.
- Spacecraft in orbit, however, were able to make unique observations, with TGO following the onset and development of the storm and monitoring how the increase in dust affected the water vapor in the atmosphere – important for understanding the history of water at Mars over time. 56) 57)
Exploiting the dust storm
- Two spectrometers onboard – NOMAD and ACS – made the first high-resolution solar occultation measurements of the atmosphere, looking at the way sunlight is absorbed in the atmosphere to reveal the chemical fingerprints of its ingredients.
- This enabled the vertical distribution of water vapor and ‘semi-heavy' water – with one hydrogen atom replaced by a deuterium atom, a form of hydrogen with an additional neutron – to be plotted from close to the martian surface to above 80 km altitude. The new results track the influence of dust in the atmosphere on water, along with the escape of hydrogen atoms into space.
- "In the northern latitudes we saw features such as dust clouds at altitudes of around 25–40 km that were not there before, and in southern latitudes we saw dust layers moving to higher altitudes," says Ann Carine Vandaele, principal investigator of the NOMAD instrument at the Royal Belgian Institute for Space Aeronomy.
- "The enhancement of water vapor in the atmosphere happened remarkably quickly, over just a few days during the onset of the storm, indicating a swift reaction of the atmosphere to the dust storm."
Figure 54: TGO's main science mission began at the end of April 2018, just a couple of months before the start of the global dust storm that engulfed the planet. TGO followed the onset and development of the storm and monitored how the increase in dust affected the water vapor in the atmosphere [image credit: ESA; spacecraft: ATG/medialab; data: A-C Vandaele et al (2019)]
Legend to Figure 54: TGO made the first high-resolution solar occultation measurements with its two spectrometers ACS and NOMAD, by looking at the way sunlight is absorbed in the atmosphere to reveal the chemical fingerprints of its ingredients. This enabled the vertical distribution of water vapor and ‘semi-heavy' water, to be plotted from close to the martian surface to above 80 km altitude – important for understanding the history of water at Mars over time. - The new results track the influence of dust in the atmosphere on water, and provide further insight into the escape of hydrogen atoms into space. The instruments also recorded dust and ice clouds appearing at different altitudes, and a quick enhancement of water vapor in the atmosphere.
- The observations are consistent with global circulation models. Dust absorbs the Sun's radiation, heating the surrounding gas and causing it to expand, in turn redistributing other ingredients – like water – over a wider vertical range. A higher temperature contrast between equatorial and polar regions is also set up, strengthening atmospheric circulation. At the same time, thanks to the higher temperatures, fewer water-ice clouds form – normally they would confine water vapor to lower altitudes.
- The teams also made the first observation of semi-heavy water simultaneously with water vapor, providing key information on the processes that control the amount of hydrogen and deuterium atoms escaping to space. It also means the deuterium-to-hydrogen (D/H) ratio can be derived, which is an important marker for the evolution of the water inventory on Mars.
- "We see that water, deuterated or not, is very sensitive to the presence of ice clouds, preventing it from reaching atmospheric layers higher up. During the storm, water reached much higher altitudes," says Ann Carine."This was theoretically predicted by models for a long time but this is the first time we have been able to observe it."
- Since the D/H ratio is predicted to change with the season and with latitude, TGO's continued regional and seasonal measurements are expected to provide further evidence of the processes at play.
Methane mystery plot thickens
- The two complementary instruments also started their measurements of trace gases in the martian atmosphere. Trace gases occupy less than one percent of the atmosphere by volume, and require highly precise measurement techniques to determine their exact chemical fingerprints in the composition. The presence of trace gases is typically measured in ‘parts per billion by volume' (ppbv), so for the example for Earth's methane inventory measuring 1800 ppbv, for every billion molecules, 1800 are methane.
- Methane is of particular interest for Mars scientists, because it can be a signature of life, as well as geological processes – on Earth, for example, 95% of methane in the atmosphere comes from biological processes. Because it can be destroyed by solar radiation on timescales of several hundred years, any detection of the molecule in present times implies it must have been released relatively recently – even if the methane itself was produced millions or billions of years ago and remained trapped in underground reservoirs until now. In addition, trace gases are mixed efficiently on a daily basis close to the planet's surface, with global wind circulation models dictating that methane would be mixed evenly around the planet within a few months.
- Reports of methane in the martian atmosphere have been intensely debated because detections have been very sporadic in time and location, and often fell at the limit of the instruments' detection limits. ESA's Mars Express contributed one of the first measurements from orbit in 2004, at that time indicating the presence of methane amounting to 10 ppbv.
- Earth-based telescopes have also reported both non-detections and transient measurements up to about 45 ppbv, while NASA's Curiosity rover, exploring Gale Crater since 2012, has suggested a background level of methane that varies with the seasons between about 0.2 and 0.7 ppbv – with some higher level spikes. More recently, Mars Express observed a methane spike one day after one of Curiosity's highest-level readings.
Figure 55: The ExoMars Trace Gas Orbiter's first analysis of the martian atmosphere at various points around the globe finds an upper limit of methane 10–100 times less than all previous reported detections. The measured data show the sensitivity of the ACS and NOMAD instruments when looking at other molecules, such as water, while methane is apparently absent: the results suggest an upper limit of 0.05 parts per billion (ppbv), — The difference between TGO's dataset and that of NASA's Curiosity, which previously reported a seasonal background variation of methane, is presented, noting that the highest sensitivity of TGO's measurements was achieved before the global dust storm that engulfed the planet in mid 2018, soon after the start of TGO's science mission. A map with the locations where TGO's detection attempts were made is also provided, with the majority of measurements taken over high latitudes. - To reconcile the differing results, a better understanding of the different mechanisms able to destroy methane close to the surface of the planet is needed [image credit: ESA; spacecraft: ATG/medialab; data: O. Korablev et al (2019)]
- The new results from TGO provide the most detailed global analysis yet, finding an upper limit of 0.05 ppbv, that is, 10–100 times less methane than all previous reported detections. The most precise detection limit of 0.012 ppbv was achieved at 3 km altitude.
- As an upper limit, 0.05 ppbv still corresponds to up to 500 tons of methane emitted over a 300 year predicted lifetime of the molecule when considering atmospheric destruction processes alone, but dispersed over the entire atmosphere, this is extremely low.
- "We have beautiful, high-accuracy data tracing signals of water within the range of where we would expect to see methane, but yet we can only report a modest upper limit that suggests a global absence of methane," says ACS principal investigator Oleg Korablev from IKI (Space Research Institute), Russian Academy of Sciences, Moscow.
- "The TGO's high-precision measurements seem to be at odds with previous detections; to reconcile the various datasets and match the fast transition from previously reported plumes to the apparently very low background levels, we need to find a method that efficiently destroys methane close to the surface of the planet."
- "Just as the question of the presence of methane and where it might be coming from has caused so much debate, so the issue of where it is going, and how quickly it can disappear, is equally interesting," says Håkan.
- "We don't have all the pieces of the puzzle or see the full picture yet, but that is why we are there with TGO, making a detailed analysis of the atmosphere with the best instruments we have, to better understand how active this planet is – whether geologically or biologically."
Figure 56: This graphic summarizes significant measurement attempts of methane at Mars. Reports of methane have been made by Earth-based telescopes, ESA's Mars Express from orbit around Mars, and NASA's Curiosity located on the surface at Gale Crater; they have also reported measurement attempts with no or very little methane detected. More recently, the ESA-Roscosmos ExoMars Trace Gas Orbiter reported an absence of methane, and provided a very low upper limit. - In order to reconcile the range of results, which show variations in both time and location, scientists have to understand better the different processes acting to create and destroy methane (image credit: ESA)
Best map of shallow subsurface water
- While the lively debate on the nature and presence of methane continues, one sure thing is that water once existed on Mars – and still does in the form of water-ice, or as water-hydrated minerals. And where there was water, there might have been life.
- To help understand the location and history of water on Mars, TGO's neutron detector FREND is mapping the distribution of hydrogen in the uppermost meter of the planet's surface. Hydrogen indicates the presence of water, being one of the constituents of the water molecule; it can also indicate water absorbed into the surface, or minerals that were formed in the presence of water.
- The instrument's mapping task will take about one Mars year – almost two Earth years – to produce the best statistics to generate the highest quality map. But the first maps presented based on just a few month's data already exceed the resolution of previous measurements.
Figure 57: TGO's first map of shallow subsurface water distribution on Mars. The FREND neutron spectrometer on the ExoMars Trace Gas Orbiter has started mapping the distribution of hydrogen in the uppermost meter of the martian's surface. Hydrogen indicates the presence of water, being one of the constituents of the water molecule; it can also indicate water absorbed into the surface, or minerals that were formed in the presence of water. A map produced from 131 days data, from 3 May to 10 September 2018, is presented here, covering the globe from 70ºN to 70ºS. - Aside from the obviously water-rich permafrost of the polar regions, the new map provides more refined details of localized ‘wet' and ‘dry' regions. It also highlights water-rich materials in equatorial regions that may signify the presence of water-rich permafrost in present times, or the former locations of the planet's poles in the past [image credit: ESA; spacecraft: ATG/medialab; data: I. Mitrofanov et al (2018)]
- "In just 131 days the instrument had already produced a map that has a higher resolution than that of the 16 years data from its predecessor onboard NASA's Mars Odyssey – and it is set to continue getting better," says Igor Mitrofanov, principal investigator of the FREND instrument at the Space Research Institute, Russian Academy of Sciences, Moscow.
- Aside from the obviously water-rich permafrost of the polar regions, the new map provides more refined details of localized ‘wet' and ‘dry' regions. It also highlights water-rich materials in equatorial regions that may signify the presence of water-rich permafrost in present times, or the former locations of the planet's poles in the past.
- "The data is continually improving and we will eventually have what will become the reference data for mapping shallow subsurface water-rich materials on Mars, crucial for understanding the overall evolution of Mars and where all the present water is now," adds Igor. "It is important for the science on Mars, and it is also valuable for future Mars exploration."
- "We have already been enjoying beautiful images and stereo views of Mars thanks to the TGO's imaging system and now we are delighted to share the first look at data from the other instruments," concludes Håkan. "We have a promising future in contributing to the many fascinating aspects of Mars science, from the distribution of subsurface water, to active surface processes and to the mysteries of the martian atmosphere."
- For dust devil details, see Figure 58.