Minimize Mars Express

Mars Express Mission

Spacecraft     Launch    Mission Status     Sensor Complement    Ground Segment    References

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

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

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

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

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

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

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

Scientific objectives:

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

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

- global IR mineralogical mapping of the surface;

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

- global atmospheric circulation and mapping of the atmospheric composition;

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

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

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

MarsExpress_Auto93

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


Spacecraft

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

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

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

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

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

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

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

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

Spacecraft item

Mass at launch

Spacecraft bus

439 kg

Lander

71 kg

Payload

116 kg

Propellant

427 kg

Launch mass

1223 kg

Typical mean power demand

Observation

Maneuver

Communication

Spacecraft

270 W

310 W

445 W

Payload

140 W

50 W

55 W

Total

410 W

360 W

500 W

Table 1: Spacecraft mass and power budget 3)

Dimensions

Spacecraft bus dimensions

1.5 x 1.8 x 1.4 m

Thrust of main spacecraft engine

400 N

Attitude thrusters

8 at 10 N each

Propellant tank volume

2 x270 = 540 liter

Pointing accuracy

Better than 0.05º

Power source

Solar array area

11.42 m2

Lithium batteries

3 at 22.5 Amp hour each (at launch)

Thermal specification

Spacecraft bus

10-20ºC

PFS, OMEGA

-180ºC

Thermal blanket

Gold-plated AISn alloy

Table 2: Spacecraft parameters

MarsExpress_Auto92

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


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

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

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

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

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

Nominal Operational Orbit Parameters:

• Orbital inclination - 86.9°

• Apocenter - 10,530 km

• Pericenter - 330 km

• Period - 7 hr 00 m

• Observational phase at pericenter - about 1 hour

• Communications phase - 6.5-7.0 hours minimum

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


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

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

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

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




Mission status

• On 18 February 2021, NASA’s Perseverance rover is expected to arrive at Jezero impact crater, the site of a former lake on Mars. The High Resolution Stereo Camera on ESA’s Mars Express has provided important context for the landing site and its surrounds. 5)

- Mars Express has been imaging the Red Planet in three dimensions and in color since 2004. The data it collects make it possible to create images of large areas of the martian surface – not only as color mosaic images, but also as digital terrain models. These provide researchers with important information about the terrain and elevation of the land. Accurate topographical information is critical for ensuring a safe landing. The detailed data from Mars Express has been used to support the selection of numerous landing site candidates, including that of NASA’s Mars 2020 mission that will see Perseverance rover land in Jezero Crater this week.

- The images shown here are derived from ‘map sheets’ covering millions of square kilometers, depicting the wider geographical context around the landing site. But the images not only provide a large-scale overview of the geographic location; thanks to their high resolution, they can be greatly enlarged for a closer look at individual details of the landscape in exceptional quality.

Figure 5: Jezero crater flyover. NASA’s Perseverance rover is expected to arrive at Jezero impact crater, the site of a former lake on Mars, on 18 February 2021. The High Resolution Stereo Camera on ESA’s Mars Express has provided important context for the landing site and its surrounds. A taste of this imagery is provided in this short video clip (video credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

Jezero in context

- Jezero crater lies exactly on the border between the ancient highland region of Terra Sabaea, where rocks from 4.1–3.7 billion years ago can still be found, and the 3.9 billion year old Isidis impact basin. The Nili Fossae graben system, which roughly traces the curved shape of the rim of Isidis basin, was formed by the impact event as a result of tectonic fractures. The Isidis Planitia (plains) is made up of much younger deposits and formed in the martian ‘middle ages’ some 3.7-3 billion years ago, modified up to the present day. To the southwest of Jezero Crater is the volcanic region of Syrtis Major, where lava flowed around 3.7-3 billion years ago. Thus, the rocks and deposits in and around Jezero Crater span the geological history of Mars.

Jezero mineralogy in depth

- Jezero Crater has a diverse mineralogy, which can be used to infer the environmental conditions at the time of the minerals’ formation. Perseverance rover will be studying them directly.

- The detailed map of Jezero Crater (Figure 6) shows that the crater rim is breached by three valleys that were former rivers. Neretva Vallis and Sava Vallis were inflow channels that have created two deltas on the western and northwestern rim of the crater. Perseverance will examine the larger of the two in the west in more detail.

- Pliva Vallis in the east of Jezero was an outflow channel through which water was discharged from the crater. As such, Jezero is known as an ‘open basin lake’, a type of lake that is thought to have once been numerous on Mars. Compared to closed basins (with inflow but no outflow), they are interesting because they hosted freshwater lakes with a stable water level. Lakes in closed basins, on the other hand, were subjected to more frequent periods of drying out, which turned them into salt lakes, thus making them less promising in the search for conditions that are conducive to life.

- The large map shows the water catchment area of the two inflow channels, from which material from the surrounding area was transported by the rivers into the crater and deposited in the two deltas. Spacecraft orbiting Mars have used spectrometers to detect a variety of minerals in this catchment area. These are mainly silicates from the olivine and pyroxene groups, both mineral classes that originate from magma within the martian mantle and indicate basaltic volcanic deposits that were not subject to long-term weathering by water. Carbonates identified on the inner rim of Jezero Crater, together with the clay minerals, testify to weathering by water of rock with a volcanic source.

- Volcanic minerals, carbonates and clay minerals are observed both in the delta and elsewhere in the crater. Some carbonates (limestones) are thought to have been formed directly in the lake. Such lake carbonates, and especially the clay minerals, indicate freshwater conditions and have the potential to preserve traces of life.

- However, other types of minerals, such as sulphates that contain iron oxide, amorphous silicon oxides, and hydroxides, have also been observed; these tend to form in acidic waters that gradually dried up. These minerals indicate that the environmental conditions in Jezero Crater became drier and less conducive to life at a later stage.

MarsExpress_Auto91

Figure 6: Topographic map of Jezero crater and surrounds (annotated). This elevation map of Jezero crater and its surrounds shows the topography of the broader region, from the highlands (red and browns) to the lower lying floor of the Isidis impact basin (green). The height difference in this area of 1.5 million square kilometers is over 6800 meters, with the floor of Jezero crater lying at an elevation of approximately minus 2600 meters below the ‘Mars Areoid’, a notional plane of equal gravitational attraction, analogous to sea level on Earth. Jezero crater, the landing site of NASA’s 2020 Perseverance rover mission, is marked on the map. It hosts two river deltas from inflow channels that once brought water into Jezero, which is thought to have once hosted a lake. - This elevation map was created from ESA Mars Express data. The High Resolution Stereo Camera’s nine sensors, arranged at right angles to the north-south flight direction, record the surface of Mars from different angles and in four color channels. From the four inclined stereo channels and the nadir channel, which is directed perpendicular to the surface of Mars, scientists at the DLR Institute of Planetary Research and the Freie Universität Berlin compute digital terrain models, which assign elevation information to each pixel. The high resolution of the data processed for this image allows for greater enlargement of the images for a closer look at individual details of the landscape (ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

MarsExpress_Auto90

Figure 7: Jezero crater, the touchdown site for NASA’s Mars 2020 Perseverance rover, in context of its surroundings. It is situated between highlands, an impact basin, a volcanic province and an ancient river delta. The dark bluish-black areas are layers of ancient volcanic ash that is widely dispersed by the wind, often piling up into impressive dune fields. - This image was created from the red, green and blue channels of the High Resolution Stereo Camera on ESA’s Mars Express, combined with high-resolution data from its nadir channel, which is directed perpendicular to the surface of Mars. The high resolution of the data processed for this image allows for greater enlargement, enabling a closer look at individual details of the landscape. Small gaps in the image mosaic were interpolated (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

Tour the viewpoints

- The 49 km Jezero Crater (viewpoint crater center) once hosted an ancient lake fed by two prominent valleys (viewpoints Naretva and Sava Vallis), with Perseverance targeting a region close to one of the river deltas (viewpoints Delta top and basement). Within the delta deposits, numerous water-bearing minerals have been observed from orbit, indicating that liquid water was once present for an extended period of time, perhaps up to 250 m depth.

- The deltas are a prime target to look for ancient biomarkers. Meanwhile an outflow channel (viewpoint Outflow channel) breaks through the eastern rim of Jezero. The presence of both inflow and outflow channels suggest the crater was constantly replenished with fresh water.

- The Mountain View viewpoint on top of a larger hill in the southeast offers a perfect vista into the crater. From here, the observer can see that the northern part of the crater floor is sloping and that the northern crater rim is clearly less defined, compared to the flat, smooth crater floor in the south and the much steeper southern crater rim flanks. This appearance originates from the erosion of material in the catchment areas to the north of the crater, which was then transported into the crater basin itself and deposited in the deltas. Also contributing to the asymmetrical topography is the erosion of the northern crater rim, caused by the river valleys breaking through the flank of Jezero.

• February 11, 2021: NASA’s Mars 2020 Perseverance rover is due to land on the Red Planet at 21:43 CET on 18 February 2021. In order to communicate with Earth from its landing site in Jezero Crater, the rover will rely on spacecraft orbiting Mars to relay the images and other data it collects back to Earth and pass on the commands from engineers beamed across space in the other direction. 6)

- On 18 February, NASA’s Mars 2020 Perseverance rover will land on the Red Planet

- ESA’s Mars orbiters – the ESA-Roscosmos ExoMars Trace Gas Orbiter (TGO) and Mars Express – are supporting the landing

- TGO will relay important data from Perseverance to Earth as soon as four hours after landing

- Mars Express is monitoring the local conditions at the landing site, Jezero Crater

- Both ESA orbiters are providing context images of the region

- TGO will attempt to image the rover in the weeks after landing.

The ESA-Roscosmos ExoMars Trace Gas Orbiter (TGO) is one of these spacecraft. As its orbit takes it over the landing site, TGO will enter communication windows with Perseverance and relay data between Earth and the rover via a network of deep space ground stations on Earth, including ESA’s Estrack network.

Helping Perseverance phone home

The data transmitted by Perseverance in its first hours and days on Mars will be vital to the mission. Did the rover land safely? Are all of its systems functional?

To ensure that this information gets to the engineers on Earth as quickly as possible, TGO and NASA’s mars orbiters will be able to communicate with deep space ground stations on Earth almost twenty four hours a day, seven days a week for the first two weeks after landing. ESA’s ground station network will provide roughly 14 hours a day of this ‘low-latency’ coverage.

“TGO will provide low-latency data relay support to Perseverance during this period, and continue to provide routine relay support afterwards,” says ESA’s Peter Schmitz, TGO Spacecraft Operations Manager. “Our first relay session with TGO will start at 02:07 CET on 19 February, just four hours after landing.”

MarsExpress_Auto8F

Figure 8: ESA's Trace Gas Orbiter will relay data from NASA's Perseverance rover to ground stations on Earth. The data transmitted by Perseverance in its first hours and days will be vital to the mission. Did the rover land safely? Are all of its systems functional? To ensure that this information gets to the engineers on Earth as quickly as possible, TGO and NASA’s mars orbiters will be able to communicate with deep space ground stations on Earth almost twenty four hours a day, seven days a week for the first two weeks after landing. ESA’s ground station network will provide roughly 14 hours a day of this ‘low-latency’ coverage. - “TGO will provide low-latency data relay support to Perseverance during this period, and continue to provide routine relay support afterwards,” says ESA’s Peter Schmitz, TGO Spacecraft Operations Manager. “Our first relay session with TGO will start at 02:07 CET on 19 February, just four hours after landing.”- The Trace Gas Orbiter is the first of two missions of ESA’s ExoMars programme that is attempting to determine whether life has ever existed on Mars. TGO arrived at Mars in October 2016 and is conducting a detailed study of the atmosphere and mapping signatures of water below the planet’s surface. The orbiter is operated from ESA’s European Space Operations Centre in Darmstadt, Germany and mission controllers already have a lot of experience relaying data from existing Mars landers (image credit: ESA)

Monitoring conditions at Jezero

Mars Express is Europe’s first mission to the Red Planet. Since beginning operations in 2004, the durable orbiter has helped answer fundamental questions about the geology, atmosphere, surface environment, history of water and potential for life on Mars.

The high-resolution camera on board Mars Express has sent back thousands of dramatic, 3D views of the martian surface, including those used to produce a virtual flight over Jezero Crater, the planned landing site for Perseverance.

The spacecraft’s Visual Monitoring Camera – also known as the ‘Mars Webcam’ – is providing additional wide context views of the landing region.

The Mars Express Planetary Fourier Spectrometer (PFS) is monitoring local conditions at Jezero Crater. The information it collects is passed on to the Perseverance Entry, Descent and Landing (EDL) team at NASA and is included in their daily report during the two weeks leading up to the landing.

“The engineers working with EDL systems need precise information on the density of the Martian atmosphere above the site at the time of landing and how it changes with altitude,” says Marco Giuranna, PFS Principal Investigator from Istituto Fisica Spazio Interplanetario in Rome, Italy.

“Having up-to-date knowledge of the temperature, pressure, dust and ice conditions in the atmosphere is crucial for understanding its density and predicting and analyzing the trajectory of the rover’s descent to the surface of the Red Planet.”

• February 8, 2021: A new Mars year began on 7 February 2021. 7)

MarsExpress_Auto8E

Figure 9: The image on the left was taken on 6 February and the image on the right on 7 February – one of the first images returned from the new martian year. The images were captured by the Visual Monitoring Camera onboard ESA’s Mars Express, which takes regular snapshots of the planet from orbit. The images are automatically shared to the camera’s Twitter account and posted to Flickr. One year on Mars equals 687 Earth days, so it takes almost twice as long to orbit the Sun. Your birthday would instead be every 23 Earth months! The martian new year begins with the northward equinox (northern spring, southern autumn) and the coming year is designated Mars Year 36 (image credit: ESA, CC BY-SA 3.0 IGO)

• February 3, 2021: On Mars, there are no yearly tax returns, but as the planet also orbits around our Sun, time on Mars is similarly measured in years. However, there are some significant differences between a year on Mars and a year on Earth. As we approach New Year’s Eve on Mars, let’s look at some similarities and differences between a year on the two planets. 8)

MarsExpress_Auto8D

Figure 10: On Mars, there are no yearly tax returns, but as the planet also orbits around our Sun, time on Mars is similarly measured in years. However, there are some significant differences between a year on Mars and a year on Earth. 7 February 2021 marks the start of Year 36 on Mars. As we approach this New Year’s Eve, let’s look at some similarities and differences between a year on the two planets (image credit: ESA)

• One year on Mars equals 687 Earth days. It takes almost twice as long as our Earth to orbit the Sun. This means your age would be a lot less if you lived on Mars! If you would like to feel younger, just divide your current age by 1.88 and casually mention to your friends that that’s your real age ... on Mars.

• A martian day is defined, like on Earth, as the time it takes for the planet to make one revolution around its axis. This is called a sol. A sol is only slightly longer than an Earth day: 24 hours and 39 minutes.

• Mars has four seasons: winter, spring, summer and autumn. They are defined by the planet’s position along its orbit around the Sun. The martian New Year begins with the northward equinox (northern spring, southern autumn). As Mars travels through its yearly trajectory, the planet’s axial tilt causes the northern hemisphere to receive more sunlight during the northern summer, and the southern hemisphere to receive more sunlight in northern winter – just like on Earth. Unlike Earth’s seasons however, the seasons on Mars are not of equal lengths. This is because the orbit of Mars around the Sun is more elliptical than that of Earth. For example, the northern hemisphere spring (southern hemisphere autumn) lasts the longest, 194 sols, and the northern hemisphere autumn (southern hemisphere spring) is the shortest season at 142 sols.

• Mars’ elliptical orbit can have important consequences. During southern spring and summer, Mars swings by the sun closer and faster. The resulting increase in luminosity heats up the atmosphere, causing turbulence to lift up very fine particles from the martian soil. For this reason, the second half of a martian year is often marked by fierce dust storms that can sometimes become planet-wide.

• Like on Earth, winters are cold and summers are warm on Mars, but the planet’s overall temperature is a lot cooler, it has a yearly average temperature of minus 60 degrees Celsius. The planet experiences different weather phenomena throughout the seasons. A weather phenomenon that reappears every year around the southern spring and summer is the Arsia Mons Elongated Cloud, a cloud of ice crystals that can reach up to 1800 km in length. It repeats for at least 80 sols and then disappears again during the rest of the year.

• The martian calendar began fairly recently compared to the one on Earth. The count started in Earth year 1955. This first martian year coincided with a very large dust storm in its second half, aptly named ‘the great dust storm of 1956’.

Table 3: Some background information on planet Mars

MarsExpress_Auto8C

Figure 11: This image from ESA’s Mars Express shows a beautiful slice of the Red Planet from the northern polar cap, and highlights cratered, pockmarked swathes of the Terra Sabaea and Arabia Terra regions. It comprises data gathered on 17 June 2019 during orbit 19550. The ground resolution at the center of the image is approximately 1 km/pixel and the images are centered at about 44ºE/26ºN. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (image credit: ESA/DLR/FU Berlin (G. Neukum), CC BY-SA 3.0 IGO)

• December 17, 2020: As the holiday season swiftly approaches, even our planetary neighbors are getting into the spirit – as shown by this perfect pair of festive silhouettes spotted by ESA’s Mars Express. 9)

- The defined wings of an angelic figure, complete with halo, can be seen sweeping up and off the top of the frame in this image from Mars Express’ High Resolution Stereo Camera, while a large heart sits just right of centre. These shapes appear to jump out of the light tan — or, in the spirit of the season, eggnog-colored! — surface of Mars; their dark color is a result of the composition of the constituent dune fields, which largely comprise sands rich in dark, rock-forming minerals that are also found on Earth (namely pyroxene and olivine).

MarsExpress_Auto8B

Figure 12: This image provides a perspective view of a pair of festive silhouettes – an angel (left) and a heart (right) spotted by ESA’s Mars Express near Mars’ south pole. It comprises data gathered by ESA’s Mars Express on 8 November 2020 during orbit 21305. The ground resolution is approximately 15 m/pixel and the image is centered at about 148ºE/78ºS. This view was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. HRSC stereo imaging was then used to derive the digital elevation model (DTM) upon which this oblique view is based (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

MarsExpress_Auto8A

Figure 13: This image from ESA’s Mars Express shows a region near Mars' south pole in wider context. The area outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera (HRSC) on 8 November 2020 during orbit 21305 (image credit: NASA MGS MOLA Science Team)

- This ethereal scene is found in the south polar region of Mars, with the pole itself located directly out of frame to the right (south). The south pole is typically covered in a 1.5 km-thick ice cap measuring around 400 km across and with a volume of 1.6 million km3, just over 12% of which is water ice. The rest of the cap is largely composed of ‘dry ice’ (solid carbon dioxide), which freezes from the atmosphere during winter and then sublimates (turns from a solid to a gas) in the summer.

- As Mars’ southern hemisphere is currently experiencing summer, this image shows the planet’s southern polar ice stores at their lowest annual levels.

- The ‘angel’ and ‘heart’ are both composed of various interesting features. Firstly, the angel’s hand, seen as if reaching to the left, is thought to be a large sublimation pit, a type of feature that forms as ice turns to gas and leaves empty pockets and depressions in the planetary surface (a process that often occurs as the seasons change). Sublimation pits have been seen on other planets in the Solar System, such as Pluto, and can also be seen scattered across the terrain to the right.

MarsExpress_Auto89

Figure 14: This color-coded topographic image shows a pair of festive silhouettes spotted by ESA’s Mars Express near Mars’ south pole, based on data gathered by the Mars Express High Resolution Stereo Camera (HRSC) during orbit 21305 (8 November 2020). This view is based on a digital terrain model of the region, from which the topography of the landscape can be derived; lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows and reds, as indicated on the scale to the top right. North is to the left (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- Moving on to one of the angel’s most distinctive features, its halo, reveals yet more intriguing processes at play. The ‘head’ and halo are formed of an impact crater, created as a body from space flew inwards to collide with Mars’ crust. As this impactor hit it dug down into the surface, revealing the numerous layered deposits that make up the southern polar region. These subsurface layers can be glimpsed in other areas where the surface has been disturbed – areas that are clearly identifiable in the associated topographic view due to their notably low elevation – and hint at the long, complex, interesting history of this part of Mars.

MarsExpress_Auto88

Figure 15: This image provides a perspective view of a heart-shaped silhouette spotted by ESA’s Mars Express near Mars’ south pole. It comprises data gathered by ESA’s Mars Express on 8 November 2020 during orbit 21305. The ground resolution is approximately 15 m/pixel and the image is centered at about 148ºE/78ºS. This view was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. HRSC stereo imaging was then used to derive the digital elevation model (DTM) upon which this oblique view is based (image credit: ESA/DLR/FU Berlin, , CC BY-SA 3.0 IGO)

- Finally comes the heart, which is underscored by a steep escarpment – a line of cliffs or steep slopes created by erosive processes – and separated from the dark expanse of dunes below. The origin of this dark material, which is found all over Mars, remains unclear, but scientists posit that it once existed deeper below the surface in layers of material formed by ancient volcanic activity. Although this material was initially buried, it has since been brought to the surface by ongoing impacts and erosion, and then distributed more widely across the planet by martian winds.

- This landscape also shows signs of dust devils in the dark, scratched, cross-hatched pattern to the left of the frame. Dust devils are common on Mars, and form as dust is whipped up from the Sun-warmed surface by wind. Here, dust devils have lifted surface material and carried it away, leaving dark marks in their wake.

MarsExpress_Auto87

Figure 16: A festive scene near Mars’ south pole – in 3D. This image shows a pair of festive silhouettes spotted by ESA’s Mars Express near Mars’ south pole in 3D when viewed using red-green or red-blue glasses. This anaglyph was derived from data obtained by the nadir and stereo channels of the High Resolution Stereo Camera (HRSC) on ESA’s Mars Express during spacecraft orbit 21305 (8 November 2020). It covers a part of the martian surface centered at 148ºE/78ºS. North is to the left (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

• November 19, 2020: Elevation can be deceiving in satellite imagery of Mars, even when differences are extreme – as demonstrated by this image of Pyrrhae Regio from ESA’s Mars Express. A chunk of terrain has collapsed and dropped more than four kilometers below its surroundings, illustrating the incredible contrast and dynamism seen across the martian surface. 10)

MarsExpress_Auto86

Figure 17: This image from ESA’s Mars Express shows craters, valleys and chaotic terrain in Mars’ Pyrrhae Regio. This image comprises data gathered by ESA’s Mars Express using its High Resolution Stereo Camera (HRSC) on 3 August 2020 (orbit 20972). The ground resolution is approximately 16 m/pixel and the images are centered at about 322°E/16°S. This image was created using data from the nadir and color channels of the HRSC. The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- This slice of Mars, seen here as imaged by Mars Express’ High Resolution Stereo Camera (HRSC), shows signs of various intriguing processes.

- A scattering of impact craters, formed as incoming bodies from space collided with Mars’ surface, can be seen to the left of the frame; the floor of the largest and uppermost basin spans about 40 km, and contains some fractures and markings that formed just after the crater itself. Hot, molten rock is thought to have been thrown up during the crater-forming collision, after which it cooled and settled to form the scar-like features visible here.

MarsExpress_Auto85

Figure 18: This image provides a perspective view of chaotic terrain in Mars’ Pyrrhae Regio. Chaotic terrain forms as a shifting subsurface layer of melting ice and sediment causes the surface above to collapse. This view comprises data gathered by ESA’s Mars Express on 3 August 2020 during orbit 20972. The ground resolution is approximately 16 m/pixel and the image is centered at about 322ºE/16ºS. This view was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. HRSC stereo imaging was then used to derive the digital elevation model (DTM) upon which this oblique view is based (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- Towards the middle of the frame, the surface is relatively smooth and featureless – however, two broad channels have worked their way through the landscape, and can be seen as meandering, branching indentations in the surrounding terrain. These channels are reminiscent of so-called ‘sapping valleys’ on Earth, which form as water consistently seeps and flows through sediment to carve out a natural drainage network.

- The valleys are attached at their rightward end to the real star of this image: a sunken, uneven, scarred patch of ground known as chaotic terrain.

- Chaotic terrain, as the name suggests, looks irregular and jumbled, and is thought to form as sub-surface ice and sediment begins to melt and shift. This shifting layer causes the surface above to collapse – a collapse that can happen quickly and catastrophically as water drains away rapidly through the regolith (the near-surface layer of rocky planets). Ice can be triggered to melt by heating events such as volcanic lava flows, subsurface magmatism, impacts by large meteorites, or changes in climate.

MarsExpress_Auto84

Figure 19: This color-coded topographic image shows craters, valleys and chaotic terrain in Mars’ Pyrrhae Regio, based on data gathered by the Mars Express High Resolution Stereo Camera (HRSC) during orbit 20972 (3 August 2020). This view is based on a digital terrain model (DTM) of the region, from which the topography of the landscape can be derived; lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows and reds, as indicated on the scale to the top right. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- In the chaotic terrain seen here, ice has melted, the resulting water drained away, and a number of disparate broken ‘blocks’ have been left standing in now-empty cavities (which once hosted ice). Remarkably, the floors of these cavities lie some four kilometers below the flatter ground near the craters to the left, as seen clearly in the associated topographic view – a colossal difference in height (for reference, the highest mountain peaks of the Pyrenees and the Alps top out at just over 3.4 km and 4.8 km, respectively).

- Considering the broader landscape containing and surrounding Pyrrhae Regio, the chaotic nature of this area is unsurprising. To the west of this patch of ground lies one of the most extreme features in the Solar System: a colossal canyon system named Valles Marineris.

- Valles Marineris is roughly ten times longer and five times deeper than the Grand Canyon on Earth, and comprises myriad smaller rifts, channels, outflows, fractures and signs of flowing material (such as water, ice, lava or debris). It is home to many substantial chaotic terrains, including Aurorae Chaos and Erythraeum Chaos.

- Valles Marineris is an unmissable scar on the face of Mars, and thought to have formed as the planet’s crust was stretched by nearby volcanic activity, causing it to rip and crack open before collapsing into the deep troughs we see today. These troughs have been further shaped and eroded by water flows, landslides, and other erosive processes, with spacecraft including Mars Express spying signs that water existed in parts of Valles Marineris in the relatively recent past (‘mere’ hundreds of millions of years ago).

MarsExpress_Auto83

Figure 20: This image from ESA’s Mars Express shows Mars’ Pyrrhae Regio in wider context. The area outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera (HRSC) on 3 August 2020 during orbit 20972 (image credit: NASA MGS MOLA Science Team)

- Valles Marineris is an unmissable scar on the face of Mars, and thought to have formed as the planet’s crust was stretched by nearby volcanic activity, causing it to rip and crack open before collapsing into the deep troughs we see today. These troughs have been further shaped and eroded by water flows, landslides, and other erosive processes, with spacecraft including Mars Express spying signs that water existed in parts of Valles Marineris in the relatively recent past (‘mere’ hundreds of millions of years ago).

MarsExpress_Auto82

Figure 21: This image shows craters, valleys and chaotic terrain in Mars’ Pyrrhae Regio in 3D when viewed using red-green or red-blue glasses. This anaglyph was derived from data obtained by the nadir and stereo channels of the High Resolution Stereo Camera (HRSC) on ESA’s Mars Express during spacecraft orbit 20972 (3 August 2020). It covers a part of the martian surface centered at 322ºE/16ºS. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

• October 29, 2020: Mars is covered in intriguing scars – some of the most prominent being impact craters. A particularly unusual example is shown in this new image from ESA’s Mars Express: an ancient triplet comprising not one but three overlapping craters. 11)

- The crater triplet is located in an especially old part of Mars’ southern hemisphere known as Noachis Terra. This region was heavily cratered during the Noachian era, an ancient time about four billion years ago in Mars’ history in which huge numbers of asteroids and comets flew inwards to crash into the planet's surface. Some of the features created by these collisions remain intact on Mars today and, as they formed during the very earliest days of the Solar System, are of particular interest to scientists seeking to know more about our planetary neighbor and its past.

- Signs of chaotic Noachian processes and events are seen especially clearly in Mars’ southern highlands, which are peppered with old, time-worn craters. ESA’s Mars Express has imaged many craters in this region, from the severely eroded Greeley Crater, named after the American geologist Ronald Greeley, to the dune-patterned Neukum Crater, named after one of the founders of the Mars Express mission (and the former Principal Investigator of the spacecraft’s High Resolution Stereo Camera (HRSC), the camera responsible for this new image).

MarsExpress_Auto81

Figure 22: In context: Triple crater east of Le Verrier. This image shows a triple crater in the ancient martian highlands – more specifically, the region of Noachis Terra – in a wider context. The area outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera (HRSC) on 6 August 2020 during orbit 20982 (image credit: NASA MGS MOLA Science Team)

MarsExpress_Auto80

Figure 23: Perspective view of triple martian crater. This image provides a perspective view of a triple crater in the ancient martian highlands. It comprises data gathered by ESA’s Mars Express on 6 August 2020 during orbit 20982. The ground resolution is approximately 15 m/pixel and the images are centered at about 19ºE/37ºS. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. HRSC stereo imaging was then used to derive the digital elevation model (DTM) upon which this oblique view is based (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- This image shows a triple crater found just east of a better-known feature named Le Verrier Crater, which spans nearly 140 km across. By contrast, the three depressions seen here are somewhat smaller; the largest measures 45 km across, and the smallest 28 km.

- How would such a crater triplet form? One possible explanation – and that thought to be most likely – is that the impactor broke into three before hitting the ground, forming a crater trio upon impact. Not all ‘multiple impactors’ leave such clear and neat features in their wake, with many instead showing elongated troughs, non-circular hollows lying closely side-by-side, or only partially overlapping basins. Another explanation could be coincidence: at different points in time, three separate impactors could have hit Mars’ surface in this location, creating a neat superposition of craters completely by chance.

- Interestingly, if the impactor did indeed fragment and break apart, this may imply that the atmosphere of Noachian Mars was far denser – and harder to penetrate – than it is now. This points towards an early Mars that was far warmer and wetter than the cold, arid world we see today. Observations from numerous missions are supporting this view and returning evidence that water once flowed across the Red Planet in large amounts, revealing features such as old river valley networks and large lake basins thought to have formed in the Noachian period.

MarsExpress_Auto7F

Figure 24: Topographic view of triple crater on Mars. This color-coded topographic image shows a triple crater in the ancient martian highlands, based on data gathered by the Mars Express High Resolution Stereo Camera (HRSC) during orbit 20982 (6 August 2020). This view is based on a digital terrain model (DTM) of the region, from which the topography of the landscape can be derived; lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows and reds, as indicated on the scale to the top right. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- Like many of the ancient and eroded craters in Mars’ southern highlands, these three craters have flattened rims, shallow floors, and have been filled with sediment in the four billion years since their formation. There is also evidence of ice here – the smallest crater has marks that are typically created as ice and debris creep across a surface, similar to how mixed rock-and-ice glaciers or debris-covered ice glaciers move in alpine regions of Earth.

- This frame may once have contained other craters, as indicated by the round patches of sunken surface to the top right and bottom left. In fact, despite the cratered nature of Noachis Terra, the environment around this triplet is surprisingly smooth for such ancient terrain. Only a handful of small surrounding craters appear to have clear, sharply defined rims and bowls, indicating that they are relatively young and have not yet begun to erode in earnest. Overall, it seems that older craters in this area have ‘melted down’ into the surface – a phenomenon that is, again, due to ice.

- As ice just under the surface of Mars flows and melts over many millions of years, the soil becomes softer. This soft, ice-rich soil subsides more quickly and fills up indentations and depressions more readily, contributing to the smooth appearance of this part of Noachis Terra. This suggests that there must have been a large amount of water present on Mars, at least during the Noachian period, capable of producing a glacier-like flow of abundant ice.

MarsExpress_Auto7E

Figure 25: Triple crater east of Le Verrier in 3D. This image shows a triple crater in the ancient martian highlands in 3D when viewed using red-green or red-blue glasses. This anaglyph was derived from data obtained by the nadir and stereo channels of the High Resolution Stereo Camera (HRSC) on ESA’s Mars Express during spacecraft orbit 20982 (6 August 2020). It covers a part of the martian surface centered at 19ºE/37ºS. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

- Understanding the history of Mars, and mapping the features covering the planet’s surface in detail, is a key objective of Mars Express. The spacecraft has been exploring the Red Planet since its launch in 2003, and is gearing up to collaborate with a number of new missions that have joined – or will soon join – the spacecraft at Mars. The ESA-Roscosmos ExoMars Trace Gas Orbiter (TGO) arrived in 2016, and the ExoMars Rosalind Franklin rover and its accompanying surface science platform are scheduled for launch in September 2022.

- Together with Mars Express, these missions will work to fully characterize our neighbor, to help us not only understand more about Mars, but, by comparison, more about the history and nature of our home in the Universe.

• October 16, 2020: Volcanoes, ice caps, dust storms, curious clouds and a double vortex are just some of the delights captured by ESA’s Mars Express ‘webcam’, and now the data behind the images are available in the Planetary Science Archive, the central repository for all scientific data returned by ESA's Solar System missions. 12)

- Originally intended as an engineering instrument to watch the release of the Beagle 2 lander in 2003, the Visual Monitoring Camera (VMC) onboard ESA’s Mars Express was repurposed to observe the Red Planet since 2007, and became a recognized science instrument in 2016. Now the data behind the images are available in the Planetary Science Archives alongside that of the spacecraft’s other seven science instruments.

- The data comes from observations taken of the Red Planet between 2007 to mid-2020, as well as of the release of the Beagle 2 lander in 2003, calibrated for scientific use and available in different formats.

- Data from the VMC has already been instrumental for a number of scientific studies and publications. Previously, VMC images were only available via a Flickr page, but today’s release is the first ‘science data’ release for the instrument, and is the culmination of several years of hard work by the VMC team.

- Stunning views of the Red Planet will continue to be provided to the general public through Flickr in near real-time – usually within a few days of when the images were taken at Mars.

MarsExpress_Auto7D

Figure 26: The image collage presented here highlights some of the beautiful views already captured by the camera and which are available in the archive and gallery. From left to right, row by row: Dust/water ice over the north pole (4 October 2019); Arsia Mons Elongated Cloud (12 November 2008); Olympus Mons caldera (19 October 2019); Double cyclone at the edge of the north pole (16 June 2012); Full disk of Mars with south pole visible (17 November 2016); Part of Valles Marineris with hazes present (12 November 2018); Tharsis Volcanoes and Olympus Mons (9 June 2010): Cloud over Olympus Mons (9 January 2019); Textured patterns in north polar cap (1 January 2020); Polar hoods (water ice clouds) over the north polar cap (28 December 2010): Valles Marineris canyon system (1 July 2008); Full disk of Mars with Tharsis volcanos visible (22 October 2017); Local dust storm over the north polar cap (6 September 2016); Syrtis Major (9 March 2020); Twilight clouds on Mars (25 November 2019); Close up of the south pole of Mars (8 August 2010), image credit: ESA/MEX/VMC, CC BY-SA 3.0 IGO)

• September 29, 2020: NASA's 2020 Mars mission has been en route to the Red Planet for two months now. The Perseverance rover on board the spacecraft is set to land in Jezero crater on 18 February 2021. There, the rover, which weighs almost one tonne, will search for signs of past microbial life. Jezero crater was chosen for good scientific reasons. Within the crater lie two ancient river deltas in which many hydrated minerals have been discovered – proof that liquid water existed there for a very long period of time. Precise maps showing the geographical conditions are essential both for the selection of the landing site and for planning exploration on the ground. A topographical map has now been created using numerous stereo images acquired by the HRSC camera operated by the DLR (German Aerospace Center) board ESA’s Mars Express spacecraft. This is the most accurate surface model of the entire crater to date. 13)

Water – the most important prerequisite for life

- Jezero crater was formed during the Noachian era, approximately 3.9 billion years ago. The landing site planned for the Perseverance rover is marked with an ellipse labelled 'Mars 2020 LS'. Jezero was once home to a lake and two deltas that were fed with water by tributaries from the large catchment areas in the region. The larger and more clearly defined delta is marked on the map with a dashed ellipse. Its two former tributary valleys (Neretva Vallis and Sava Vallis) can be seen on the western and north-western edges of the crater. An outflow valley (Pliva Vallis) breaks through the crater rim to the east.

- The floor of Jezero crater has an elevation of 2700 meters below the notional 'sea level' on Mars. Strictly speaking, this reference level, which is used on the map shown here, is a surface of equal gravitational attraction, referred to as an areoid. The lake in Jezero crater is thought to have been at least 250 meters deep, a figure derived from the altitude measurements for the crater rim, the crater floor, the river valley profile and the upper edges of the deltas. However, it is no longer possible to make precise estimates today, as the deltas have been severely eroded since water activity ceased approximately 3.8 billion years ago. In addition, the crater was filled with very extensive lava flows from the nearby Syrtis Major region about 300 million years later.

- The topographical map clearly shows how the northern part of the crater floor is sloping and this results in a less clearly defined crater rim when compared to the flat, southern part of the crater floor and the steeper southern flanks. This is due to the erosion of material in the catchment areas to the north of the crater, which was then transported into the crater itself and deposited as sediments in the delta. The erosion of the northern crater rim by the three river valleys that break through it also contributed to the creation of Jezero's asymmetrical topography.

- Clay minerals, particularly carbonates, which have been discovered in comparatively large quantities in the western delta, are significant as potential repositories of biosignatures – that is, traces of microbial life. The early presence of liquid water and the enormous carbonate deposits here make Jezero a perfect destination for the search for possible past life on Mars.

From image data to a mosaic and topographic map

- The map shown here was generated using an image mosaic assembled from individual images acquired by the High Resolution Stereo Camera (HRSC) on board ESA’s mission Mars Express orbiter. The color mosaic on which the map is based was derived from two image strips acquired during orbits 5252 and 5270. It covers an area located at approximately 18 degrees north and 77 degrees east. The color-coded topographical map is based on a Digital Terrain Model (DTM) of the region, from which the topography of the landscape can be derived. The reference for the HRSC-DTM is an equipotential surface of Mars (areoid). The HRSC camera on board Mars Express is operated by the German Aerospace Center (DLR). Systematic processing of the camera data was carried out at the DLR Institute of Planetary Research in Berlin-Adlershof. Personnel at the Department of Planetary Sciences and Remote Sensing at Freie Universität Berlin used these data to create the image products shown here.

The HRSC experiment on Mars Express

- The High Resolution Stereo Camera (HRSC) was developed by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) and built in collaboration with partners in industry (EADS Astrium, Lewicki Microelectronic GmbH and Jena-Optronik GmbH). The science team, which is headed by Principal Investigator (PI) Ralf Jaumann, consists of 50 co-investigators from 35 institutions in 11 countries. The camera is operated by the DLR Institute of Planetary Research in Berlin-Adlershof.

MarsExpress_Auto7C

Figure 27: The most accurate topographic model of Jezero crater today – created using data from DLR's High Resolution Stereo Camera (HRSC). Jezero crater is the landing site for NASA's Perseverance rover (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

• September 28, 2020: ESA’s Mars Express spacecraft has discovered several ponds of liquid water buried under the ice in the south polar region of Mars. The spacecraft’s radar instrument, MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding), revealed one underground reservoir in 2018, buried about 1.5 km below the ice. Now, taking into account more data and analyzing it in a different way, three new ponds have been discovered. The largest underground lake measures about 20 x 30 km, and is surrounded by several smaller ponds. The water is thought to be very salty in order for it to remain liquid at cold temperatures. 14)

Figure 28: Mars was once warmer and wetter with water flowing across the surface, much like early Earth. While it is not possible for water to remain stable on the surface today the new result opens the possibility that an entire system of ancient lakes might exist underground, perhaps millions or even billions of years old. They would be ideal locations to search for evidence of life on Mars, albeit very difficult to reach (video credit: ESA)

- Subglacial lakes are also known on Earth, like Lake Vostok in Antarctica. They may harbor unique ecosystems, providing useful analogies for astrobiologists exploring how life can survive in extreme environments. The techniques used to analyze the radar data on Mars are similar to those used in investigations of subglacial lakes in Antarctica, Canada and Greenland.

• September 3, 2020: A true-color image of the Nereidum Mountain Range in the southern hemisphere of Mars captured by the High Resolution Stereo Camera (HRSC) aboard ESA's Mars Express orbiter in 2015. The recently reprocessed image reveals evidence of a variety of geological processes that have shaped the area north of the large Argyre impact basin over billions of years. 15)

- Branch-like structures of valleys were carved into the slopes either by the melting snow and ice or runoff from rain in the early period of Mars’ evolution when the planet had abundant water resources.

- Inside the craters in the image, one can see deposits, in some cases concentric, probably created by the sublimation of ice that used to cover the region in the past. Scientists believe water ice might still be present in the region deep under the surface.

- Mass wasting deposits clearly visible in the canyons captured in this image are also a result of glacial processes that had taken place in the area in the past.

MarsExpress_Auto7B

Figure 29: The dark brown streak on the left-hand side of the image represents dune fields created by the effects of wind as it transports sand grains over large distances on the surface of Mars. On the north-facing slope of the ridge between the northern part of the dune field and the large filled crater towards the south, one can clearly see recently created alcoves and gullies. These structures are often linked to the melting of ground ice and could point to still existing water ice deposits in the subsurface (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

MarsExpress_Auto7A

Figure 30: A three-dimensional rendering of the Nereidum Mountain Range in the southern hemisphere of Mars created from data captured by the High Resolution Stereo Camera (HRSC) on ESA's Mars Express orbiter in 2015. The rendering shows a dune field and two large craters with a canyon between them viewed from the west. The rendering was generated by the combination of data from the Mars digital terrain model (DTM), the nadir (downward-looking) channel and the color channels of the HRSC instrument (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

• July 29, 2020: Flight over the Mars Jezero Crater, the landing site of NASA's Mars 2020 Perseverance rover, based on data of the HRSC on ESA's Mars Express mission. 16) 17)

Figure 31: This video shows Jezero crater, the landing site of the NASA Mars 2020 Perseverance rover on the Red Planet, based on images from ESA’s Mars Express mission. The planned landing area is marked with an orange ellipse. The animation was created using an image mosaic made from four single orbit observations obtained by the High Resolution Stereo Camera (HRSC) on Mars Express between 2004 and 2008. The mosaic combines data from the HRSC nadir and color channels; the nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. The mosaic image was then combined with topography information from the stereo channels of HRSC to generate a three-dimensional landscape, which was then recorded from different perspectives, as with a movie camera, to render the flight shown in the video (animation credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO ; Music: Björn Schreiner ; Soundtrack logo: Alicia Neesemann)

- Scheduled for launch from Cape Canaveral, Florida on 30 July 2020 on board an Atlas V rocket, the Perseverance rover will land on 18 February 2021 in Jezero crater.

- An impact crater with a diameter of about 45 km, Jezero is located at the rim of the giant Isidis impact basin. Morphological evidence suggests that the crater once hosted a lake, some 3.5 billion years ago.

- Jezero possesses an inlet- and an outlet channel. The inlet channel discharges into a fan-delta deposit, containing water-rich minerals such as smectite clays. Scientists believe that the lake was relatively long lived because the delta may have required 1 to 10 million years to reach its thickness and size. Other studies conclude that the lake did not experience periods of important water-level fluctuations and that it was formed by a continuous surface runoff. This makes Jezero crater to a prime target for the search for potential signs of microbial life, because organic molecules are very well preserved in river deltas and lake sediments.

- A recent study of the ancient lakeshores, diverse minerals and violent volcanism of Jezero crater based on data from ESA’s Mars Express mission is available here: Mars Express helps uncover the secrets of Perseverance landing site.

• July 29, 2020: A mysteriously long, thin cloud has again appeared over the 20-km high Arsia Mons volcano on Mars. 18)

- A recurrent feature, the cloud is made up of water ice, but despite appearances it is not a plume linked to volcanic activity. Instead, the curious stream forms as airflow is influenced by the volcano’s ‘leeward’ slope − the side that does not face the wind.

MarsExpress_Auto79

Figure 32: These images of the cloud, which can reach up to 1800-km in length, were taken on 17 and 19 July by the Visual Monitoring Camera (VMC) on Mars Express, which has been studying the Red Planet from orbit for the past 16 years (image credit: ESA/GCP/UPV/EHU Bilbao)

- “We have been investigating this intriguing phenomenon and were expecting to see such a cloud form around now,” explains Jorge Hernandez-Bernal, PhD candidate at the University of the Basque Country (Spain) and lead author of the ongoing study.

- “This elongated cloud forms every martian year during this season around the southern solstice, and repeats for 80 days or even more, following a rapid daily cycle. However, we don’t know yet if the clouds are always quite this impressive”.

- A martian day, or sol, is slightly longer than an Earth day at 24 hours, 39 minutes and 35 seconds long. A year at the Red Planet consists of 668 sols, approximately 687 days, so the seasons last for twice as long.

- The southern solstice is the period of the year when the Sun is in the southernmost position in the martian skies, just like 21 December on Earth. In the early mornings during this period, this fleeting cloud grows for approximately three hours, quickly disappearing again just a few hours later.

- Most spacecraft in orbit around the Red Planet tend to observe in the afternoon, however Mars Express is in a privileged position to gather and provide crucial information on this unique effect.

- “The extent of this huge cloud can't be seen if your camera only has a narrow field of view, or if you're only observing in the afternoon,” says Eleni Ravanis, a Young Graduate Trainee for the Mars Express mission who works specifically for the VMC instrument.

- “Luckily for Mars Express, the highly elliptical orbit of the spacecraft, coupled with the wide field of view of the VMC instrument, lets us take pictures covering a wide area of the planet in the early morning. That means we can catch it!”

- The Mars Express science team have now named the cloud the Arsia Mons Elongated Cloud, AMEC. So, for how long has it been disappearing and reappearing? Why does it only form in the early morning? Stay tuned as scientists continue to investigate, and we reveal more mysteries from Mars.

• July 2, 2020: This movie, based on images taken by ESA’s Mars Express, showcases the 82 km wide Korolev crater on Mars. 19)

- Located in the northern lowlands of the Red Planet, south of the large Olympia Undae dune field that partly surrounds Mars’ north polar cap, this well-preserved impact crater is filled with water ice all year round. The crater’s floor lies two kilometers below its rim, enclosing a 1.8 km thick domed deposit that represents a large reservoir of non-polar ice on Mars.

- Water ice is permanently stable within Korolev crater because the deepest part of this depression acts as a natural cold trap. The air above the ice cools and is thus heavier compared to the surrounding air: since air is a poor conductor of heat, the water ice mound is effectively shielded from heating and sublimation.

- The crater is named after chief rocket engineer and spacecraft designer Sergei Pavlovich Korolev (1907-1966), dubbed the father of Russian space technology. Korolev developed the first Russian intercontinental rocket R7, the precursor of the modern Soyuz rockets that are still operated today. With his rocket and spacecraft design, he was also responsible for the first human-made satellite (Sputnik in 1957) and for the first human spaceflight (Yuri Gagarin in 1961).

Figure 33: This movie was created using an image mosaic made from single orbit observations from the High Resolution Stereo Camera (HRSC) on Mars Express, which was first published in December 2018. The mosaic combines data from the HRSC nadir and color channels; the nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. The mosaic image was then combined with topography information from the stereo channels of HRSC to generate a three-dimensional landscape, which was then recorded from different perspectives, as with a movie camera, to render the flight shown in the video (video credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

• July 1, 2020: An elevation map of Jezero Crater on Mars, the landing site for NASA's 2020 Mars Perseverance rover. Lighter colors represent higher elevation. 20)

MarsExpress_Auto78

Figure 34: This image was created using data from a combination of instruments and spacecraft: NASA's Mars Global Surveyor and its Mars Orbiter Laser Altimeter (MOLA); NASA's Mars Reconnaissance Orbiter and its Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and Context Camera (CTX); and the High Resolution Stereo Camera (HRSC) on ESA's Mars Express. It was originally published in November 2019 (image credit: NASA/JPL-Caltech/MSSS/JHU-APL/ESA)

- Two recent studies based on ESA's Mars Express observations of Jezero crater have shed light on how and when this intriguing area formed – and identified the regions most likely to reveal signs of ancient life.

- Full story: Mars Express helps uncover the secrets of Perseverance landing site.

MarsExpress_Auto77

Figure 35: This topographical map shows the Nili Fossae region of Mars, a part of the planet that hosts the future landing site for NASA's 2020 Mars Perseverance rover: Jezero crater (marked by a white star). The region's wider context on the martian surface is shown to the right of the frame. The colors represent the altitude of the surface, ranging from lower blues and greens to higher oranges and reds (as shown in the key to the top right). This image uses topographic and altimetric data from NASA's Mars Global Surveyor's Mars Orbiter Laser Altimeter (MOLA), image credit: L. Mandon et al. (2020)

- NASA's Mars 2020 Perseverance rover will search for signs of past life on Mars and collect samples for a future flight to Earth. NASA and ESA are currently working together on mission concepts to bring these samples to Earth by 2031.

- Two studies based on data from another space mission – ESA's Mars Express, orbiting the Red Planet since 2003 – now identify which parts of the rover's landing site are most likely to have preserved ancient signs of life, climate, water, and volcanism. Both studied a part of Mars' surface known as Nili Fossae and, more specifically, a crater within this area named Jezero crater.

- The Jezero crater contains a delta, clear evidence that water once flowed there in the form of an ancient lake, and holds large quantities of both olivine and carbonate minerals. Carbonates form in the presence of water and are known to trap biosignatures, the signatures of life, whereas olivine is present in magmatic rocks and can be used to explore and precisely date Mars' volcanic past.

- "We've known for decades now that Nili Fossae is quite a unique part of Mars, and Jezero crater was chosen as a landing site for the Perseverance rover given this uniqueness," says Lucia Mandon of the Laboratoire de Géologie de Lyon (Terre, Planètes, Environnement), France, and lead author of a study into the mineralogy, age, and evolution of the Nili Fossae region.

- "However, while this part of Mars has been well-studied, scientists were still unsure about how and when it formed, or how it came to contain all of this olivine- and carbonate-rich material. In fact, at least six different formation scenarios have been proposed in the last couple of decades."

MarsExpress_Auto76

Figure 36: This image of the martian surface maps the olivine-rich rock of the area surrounding Jezero crater, the future landing site for NASA's 2020 Mars Perseverance rover. It combines spectral data in the near-infrared, thermal radiance, and altimetry from ESA's Mars Express, NASA's Mars Reconnaissance Orbiter and NASA's Mars Odyssey missions. The altitude of the surface is represented as shown in the key to the bottom left, with the depth of the olivine-bearing material absorption bands represented as shown in the key to the top left (image credit: L. Mandon et al. (2020))

- To resolve this uncertainty, Lucia and colleagues analyzed observations of the Nili Fossae region gathered by ESA's Mars Express and NASA's Mars Reconnaissance Orbiter (MRO): a mix of high-resolution images, topographic data, mineralogical data, and thermal data. They found that the olivine-rich bedrock in the region around Jezero crater extends for at least 18,000 km2, and formed around 3.8 billion years ago.

- "One of the most popular formation theories suggests that the olivine-bearing material formed as a sheet of melted rock, created by the giant impact that produced the huge nearby Isidis basin," adds Lucia, "but our timeline reveals that the olivine-rich bedrock formed tens of millions of years or more after this impact."

- "However, we think that this impact made the crust fragile and more prone to volcanism. After reviewing all plausible scenarios, we found that the Nili Fossae region was most likely sculpted by massive eruptions of ash and other material thrown out from giant volcanoes. The erupted volume is colossal: more than 1000 times larger than that of the Vesuvian event that destroyed Pompeii in 79 AD."

- This suggests that the common view of Martian volcanism, in which volcanic activity broadly occurs via lava flows with only a few instances of explosive activity, may not be wholly accurate.

MarsExpress_Auto75

Figure 37: This high-resolution 3D model shows terrain on the surface of Mars in the vicinity of the Jezero crater, the future landing site for NASA's 2020 Mars Perseverance rover (image credit: Mars Express, NASA/JPL/University of Arizona; image processing: L. Mandon)

• May 18, 2020: Lava mud flows on Mars. Scientists have long suspected that the 'fire-breathing' volcanoes that spread large quantities of flowing lava over Mars were not the only kind. The numerous mountain cones in the northern hemisphere of the Red Planet may be the result of mud volcanoes. However, until now, researchers have lacked knowledge about the behavior of water-rich mud on the surface of Mars. An unusual laboratory experiment involving the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) has now been able to show how mud flows at very low temperatures and under reduced atmospheric pressure. It behaves in a similar way to very specific lava flows on Earth. The results, which have now been published in the journal Nature Geoscience, add important details to the existing knowledge of Mars and its history, which has been shaped by volcanic activity. 21) 22)

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

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

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

c) Focus: Space, exploration.

MarsExpress_Auto74

Figure 38: A mud volcano on Mars? (image credit: NASA/JPL-Caltech/University of Arizona)

MarsExpress_Auto73

Figure 39: Active mud volcanoes on Earth (image credit: CAS/Petr Brož (CC BY-SA 4.0)

MarsExpress_Auto72

Figure 40: Mud volcanoes on Mars? (image credit: ESA/DLR/FU Berlin CC BY-SA 3.0 IGO)

Are small volcanic cones the result of mud extrusions?

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

Figure 41: How does watery-mud move on Mars? Like pahoehoe lava! Exploration of Mars has revealed the presence of large outflow channels which have been interpreted as the products of catastrophic flood events during which a large quantity of water was released from the subsurface. The rapid burial of water-rich sediments following such flooding may have promoted an ideal setting to trigger sedimentary volcanism, in which mixtures of rock fragments and water erupt to the surface in the form of mud (video credit: Geofyzikální ústav AV ČR v. v. i)

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

Major impact of low atmospheric pressure

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

MarsExpress_Auto71

Figure 42: Did water-rich sediments also reach the Martian surface? Water that flowed over the surface of Mars billions of years ago transported large quantities of sediments to the northern lowlands, where they were later covered by younger sediments and volcanic rocks. Some Mars researchers suspect that these water-rich sediments became liquefied underground and rose back to the surface under pressure – similar to this hot ‘mud spring’ at Bakhar in Azerbaijan (diameter approximately 1.5 meters). Experiments in a low-pressure chamber, in which DLR scientist Ernst Hauber was involved, have now shown that the flow behavior is similar to that of what is referred to as ‘ropy lava’ (or, according to the Hawaiian term for smooth, unbroken lava, also known as ‘ pāhoehoe’ lava), which is at a temperature of several hundred degrees Celsius. This implies that mud flows on Mars take a completely different course than those on Earth. This observation could support the assumption that many of the conical hills with central craters discovered in the north of Mars are also mud volcanoes (image credit: CAS/Peter Brosž CC BY-SA 4.0)

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

• May 14, 2020: Nature is a powerful sculptor – as shown in this image from ESA’s Mars Express, which portrays a heavily scarred, fractured martian landscape. This terrain was formed by intense and prolonged forces that acted upon Mars’ surface for hundreds of millions of years. 23)

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

MarsExpress_Auto70

Figure 43: Topographic view of Tempe Fossae on Mars. This color-coded topographic image shows part of Mars’ surface located northeast of the Tharsis volcanic province, based on data gathered by the Mars Express High Resolution Stereo Camera on 30 September 2019 during orbit 19913. This is a portion of Tempe Fossae – a series of tectonic faults that cuts across Tempe Terra in Mars’ northern highlands. This view is based on a digital terrain model (DTM) of the region, from which the topography of the landscape can be derived; lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows and reds, as indicated on the scale to the top right. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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

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

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

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

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

MarsExpress_Auto6F

Figure 44: This image shows part of Mars’ surface, located northeast of the Tharsis volcanic province, in a wider context. This is Tempe Fossae – a series of tectonic faults that cuts across Tempe Terra in Mars’ northern highlands. The area outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera on 30 September 2019 during orbit 19913 (image credit: NASA MGS MOLA Science Team)

MarsExpress_Auto6E

Figure 45: Located just northeast of the colossal Tharsis volcanic-tectonic province on Mars, the landscape shown in this image from ESA’s Mars Express is a mix of faults, elevated ground, deep valleys, and largely parallel ridges, extending both down into the surface and up above the martian crust. This is a portion of Tempe Fossae – a series of tectonic faults that cuts across Tempe Terra in Mars’ northern highlands. This region is a great example of terrain featuring two key martian features: grabens and horsts. In a way, these are opposites of one another – grabens are slices of ground that have dropped down between two roughly parallel faults, while horsts are ground that has been uplifted between faults. Both were created by tremendous volcanic and tectonic forces acting across the surface of Mars, which fractured the ground and manipulated it into new configurations. The surface to the right of the frame is smoother, created as lava flooded the region before cooling and solidifying, and some perpendicular slices across the predominantly parallel ridges can be seen to the left of the frame. As the nearby Tharsis province grew larger, it stretched and stressed the surrounding crust – and these features are evidence of a change in the direction of stress. This image comprises data gathered on 30 September 2019 during orbit 19913. The ground resolution is approximately 15 m/pixel and the images are centered at about 279°E/36°N. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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

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

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

MarsExpress_Auto6D

Figure 46: This image shows part of Mars’ surface located northeast of the Tharsis volcanic province in 3D when viewed using red-green or red-blue glasses. This is a portion of Tempe Fossae – a series of tectonic faults that cuts across Tempe Terra in Mars’ northern highlands. This anaglyph was derived from data obtained by the nadir and stereo channels of the High Resolution Stereo Camera (HRSC) on ESA’s Mars Express during spacecraft orbit 19913. It covers a part of the martian surface centered at 279ºE/36ºN. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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

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

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

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

MarsExpress_Auto6C

Figure 47: This image shows a part of Mars’ surface located northeast of the Tharsis volcanic province. This is a portion of Tempe Fossae – a series of tectonic faults that cuts across Tempe Terra in Mars’ northern highlands. It comprises data gathered on 30 September 2019 during orbit 19913. The ground resolution is approximately 15 m/pixel and the images are centered at about 279ºE/36ºN. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. This HRSC stereo imaging was then used to derive a digital elevation model, upon which this oblique view is based (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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

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

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

• March 5, 2020: Known for its wide swathes of rippling, textured, gently sloping dunes, the Terra Sabaea region on Mars is home to many fascinating geological features – including the prominent Moreux crater, the star of a new image from ESA’s Mars Express. 24)

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

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

MarsExpress_Auto6B

Figure 48: This image comprises data gathered on 30 October 2019 during orbit 20014. The ground resolution is approximately 16 m/pixel and the images are centered at about 44ºE/42ºN. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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

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

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

MarsExpress_Auto6A

Figure 49: This color-coded topographic image shows a feature on Mars’ surface named Moreux crater, based on data gathered by the Mars Express High Resolution Stereo Camera on 30 October 2019 during orbit 20014. This view is based on a digital terrain model (DTM) of the region, from which the topography of the landscape can be derived; lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows and reds, as indicated on the scale to the bottom left. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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

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

MarsExpress_Auto69

Figure 50: Perspective view of the Moreux crater on Mars observed on 30 October 2019 during orbit 20014 (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

• February 13, 2020: Mars is very much a world of two halves, as this new image from ESA’s Mars Express highlights, showing where these dramatically different regions come together as one. 25)

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

MarsExpress_Auto68

Figure 51: Where north meets south: fragmented terrain on Mars. This color-coded topographic image shows a region of Mars’ surface named Nilosyrtis Mensae, based on data gathered by the Mars Express High Resolution Stereo Camera on 29 September 2019 during orbit 19908. This view is based on a digital terrain model (DTM) of the region, from which the topography of the landscape can be derived; lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows and reds, as indicated on the scale to the top right. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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

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

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

- The large crater to the right of Figure 52 is an example of this degradation: it has a smooth, rounded appearance, with gently sloping walls, softened edges, and a flat bottom that has been widened and filled by sedimentary material over time. This worn-away morphology reflects both the crater’s advanced age, and the levels of erosion it has undergone since it formed.

MarsExpress_Auto67

Figure 52: This image shows shows a region of Mars’ surface named Nilosyrtis Mensae in 3D when viewed using red-green or red-blue glasses. This anaglyph was derived from data obtained by the nadir and stereo channels of the HRSC on ESA’s Mars Express during spacecraft orbit 19908. It covers a part of the martian surface centered at about 69ºE/31ºN. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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

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

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

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

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

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

MarsExpress_Auto66

Figure 53: This image shows a region of Mars’ surface named Nilosyrtis Mensae. It comprises data gathered on 29 September 2019 during orbit 19908. The ground resolution is approximately 15 m/pixel and the images are centered at about 69ºE/31ºN. This image was created using data from the nadir and color channels of the HRSC. The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. This perspective looks over the region from north to south (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

MarsExpress_Auto65

Figure 54: This image shows shows a region of Mars’ surface named Nilosyrtis Mensae in wider context. The area outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera on 29 September 2019 during orbit 19908 (image credit: NASA MGS MOLA Science Team)

• January 13, 2020: ESA’s Mars Express has captured beautiful images of the icy cap sitting at Mars’ north pole, complete with bright swathes of ice, dark troughs and depressions, and signs of strong winds and stormy activity. 26)

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

MarsExpress_Auto64

Figure 55: This image shows part of the ice cap sitting at Mars’ north pole, complete with bright swathes of ice, dark troughs and depressions, and signs of strong winds and stormy activity. The landscape here is a rippled mix of color. Dark red and ochre-hued troughs appear to cut through the icy white of the polar cap; these form part of a wider system of depressions that spiral outwards from the very center of the pole. Visible to the left of the frame are a few extended streams of clouds, aligned perpendicularly to a couple of the troughs. These are thought to be caused by small local storms that kick up dust into the martian atmosphere, eroding scarps and slopes as they do so and slowly changing the appearance of the troughs over time. - This image comprises data gathered on 16 November 2006 during orbit 3670. The ground resolution is approximately 15 m/pixel and the images are centered at about 244ºE/85ºN. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is to the upper right (image credit: ESA/DLR/FU Berlin , CC BY-SA 3.0 IGO)

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

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

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

MarsExpress_Auto63

Figure 56: Mars' north pole in context. This image shows shows the ice cap at Mars' north pole. The area outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera on 16 November 2006 during orbit 3670 (image credit: Topography data by MOLA Science team. Reference body for elevations: Mars sphere map compilation by Freie Universität Berlin)

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

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

MarsExpress_Auto62

Figure 57: This image shows part of the ice cap at Mars' north pole in 3D when viewed using red-green or red-blue glasses. This anaglyph was derived from data obtained by the nadir and stereo channels of the HRSC on ESA’s Mars Express during spacecraft orbit 3670. It covers a part of the martian surface centered at about 244ºE/85ºN. North is to the upper right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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

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

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

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

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

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

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

• December 12, 2019: ESA’s Mars Express has captured detailed views of the small, scarred and irregularly shaped moon Phobos from different angles during a unique flyby. 27)

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

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

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

Figure 58: ESA’s Mars Express recently tracked Phobos as the moon passed in front of the spacecraft's camera, capturing detailed views of the small, irregularly shaped body at different angles and stages of the flyby. This sequence comprises 41 images taken by the HRSC on 17 November 2019 during orbit 20,076, when Phobos passed Mars Express at a distance of roughly 2400 km. The images have a resolution of 21 m/pixel. This opportunity allowed the spacecraft to capture many features across the moon’s surface; alongside a number of impact craters (including the large and prominent Stickney crater), one can see a number of linear marks and furrows (video credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

Figure 59: Phobos phase angles explained. This image sequence nicely demonstrates the concept of ‘phase angle’: the angle between a light source (in this case, the Sun) and the observer (Mars Express’ HRSC), as viewed from the target object itself (Phobos). The initial phase angle is 17 degrees, drops to almost 0 degrees mid-way through (when Phobos is at its brightest), and then rises to 15 degrees by the end of the animation. - Phase angle, and how it corresponds to what we see, is represented in the animation to the right of the image sequence (video credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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

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

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

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

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

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

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

MarsExpress_Auto61

Figure 60: This schematic accompanies a new sequence of Phobos images, created as the small martian moon passed in front of ESA’s Mars Express. The images were captured at different phase angles. The phase angle (marked as 'ϕ' in the graphic) is the angle between a light source (in this case, the Sun) and the observer (Mars Express), as viewed from the target object itself (Phobos). In the movie of Phobos, the initial phase angle is 17 degrees (A), drops to almost zero degrees mid-way through (when Phobos is at its brightest, B), and then rises to 15 degrees by the end of the animation (B), image credit: DLR

• November 21, 2019: Where the two hemispheres of Mars meet, the planet is covered in broken-up terrain: a sign that slow-but-steady flows of icy material once forged their way through the landscape, carving out a fractured web of valleys, cliffs and isolated mounds of rock. 28)

MarsExpress_Auto60

Figure 61: This image shows a region of Mars named Deuteronilus Mensae. The area outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera on 25 February 2018 during orbit 17913 (image credit: NASA MGS MOLA Science Team)

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

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

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

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

MarsExpress_Auto5F

Figure 62: This color-coded topographic view shows a region of Mars named Deuteronilus Mensae. Lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows and reds, as indicated on the scale to the top right. This view is based on a digital terrain model of the region, from which the topography of the landscape can be derived. It comprises data gathered on 25 February 2018 during orbit 17913. The ground resolution is approximately 13 m/pixel and the images are centered at about 25.5ºE/44ºN. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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

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

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

MarsExpress_Auto5E

Figure 63: Perspective view of Deuteronilus Mensae. This image from ESA’s Mars Express shows a region of Mars named Deuteronilus Mensae. This oblique perspective view was generated using a digital terrain model and Mars Express data gathered on 25 February 2018 during orbit 17913. The ground resolution is approximately 13 m/pixel and the images are centered at about 25.5ºE/44ºN. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface.

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

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

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

MarsExpress_Auto5D

Figure 64: This image from ESA’s Mars Express shows a region of Mars named Deuteronilus Mensae. This oblique perspective view was generated using a digital terrain model and Mars Express data gathered on 25 February 2018 during orbit 17913. The ground resolution is approximately 13 m/pixel and the images are centered at about 25.5ºE/44ºN. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera (HRSC). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

MarsExpress_Auto5C

Figure 65: This image shows shows a region of Mars named Deuteronilus Mensae in 3D when viewed using red-green or red-blue glasses. This anaglyph was derived from data obtained by the nadir and stereo channels of the High Resolution Stereo Camera (HRSC) on ESA’s Mars Express during spacecraft orbit 17913. It covers a part of the martian surface centered at about 25.5ºE/44N. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

• November 13, 2019: In June, NASA’s Curiosity rover reported the highest burst of methane recorded yet, but neither ESA’s Mars Express nor the ExoMars Trace Gas Orbiter recorded any signs of the illusive gas, despite flying over the same location at a similar time. 29)

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

MarsExpress_Auto5B

Figure 66: Key methane measurements at Mars. This graphic summarizes significant measurement attempts of methane at Mars. Reports of methane have been made by Earth-based telescopes, ESA’s Mars Express from orbit around Mars, and NASA’s Curiosity located on the surface at Gale Crater; they have also reported measurement attempts with no or very little methane detected. More recently, the ESA-Roscosmos ExoMars Trace Gas Orbiter reported an absence of methane, and provided a very low upper limit. -In order to reconcile the range of results, which show variations in both time and location, scientists have to understand better the different processes acting to create and destroy methane (image credit: ESA)

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

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

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

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

MarsExpress_Auto5A

Figure 67: How methane is created and destroyed on Mars is an important question in understanding the various detections and non-detections of methane at Mars, with differences in both time and location. Although making up a very small amount of the overall atmospheric inventory, methane in particular holds key clues to the planet’s current state of activity. - This graphic depicts some of the possible ways methane might be added or removed from the atmosphere. One exciting possibility is that methane is generated by microbes. If buried underground, this gas could be stored in lattice-structured ice formations known as clathrates, and released to the atmosphere at a much later time. - Methane can also be generated by reactions between carbon dioxide and hydrogen (which, in turn, can be produced by reaction of water and olivine-rich rocks), by deep magmatic degassing or by thermal degradation of ancient organic matter. Again, this could be stored underground and outgassed through cracks in the surface. Methane can also become trapped in pockets of shallow ice, such as seasonal permafrost. -Ultraviolet radiation can both generate methane – through reactions with other molecules or organic material already on the surface, such as comet dust falling onto Mars – and break it down. Ultraviolet reactions in the upper atmosphere (above 60 km) and oxidation reactions in the lower atmosphere (below 60 km) acts to transform methane into carbon dioxide, hydrogen and water vapor, and leads to a lifetime of the molecule of about 300 years. - Methane can also be quickly distributed around the planet by atmospheric circulation, diluting its signal and making it challenging to identify individual sources. Because of the lifetime of the molecule when considering atmospheric processes, any detections today imply it has been released relatively recently. - Continued exploration at Mars – from orbit and the surface alike – along with laboratory experiments and simulations, will help scientists to better understand the different processes involved in generating and destroying methane (image credit: ESA)

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

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

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

MarsExpress_Auto59

Figure 68: The story of methane on Mars is a subject of intense debate. On Earth, methane is mainly created by living organisms, but also through natural geological processes. It has a relatively short lifetime of around 400 years – because it is broken down by ultraviolet light – so detecting it on another planet raises exciting questions as to how it is produced. Previous observations of Mars, by both Earth-based telescopes and ESA’s Mars Express, hint at seasonal variations in methane abundance, with concentrations varying with location and time. NASA’s Curiosity rover has also reported methane ‘spikes’, with one corresponding to a detection by Mars Express. Curiously, the ExoMars Trace Gas Orbiter, the most sensitive atmosphere analyzer at Mars, has not yet detected any. In order to reconcile the range of results, which show variations in both time and location, scientists have to understand better the different processes acting to create and destroy methane. - This set of infographics highlight’s ESA’s contribution to Mars exploration as we ramp up to the launch of our second ExoMars mission, and look beyond to completing a Mars Sample Return mission (image credit: ESA, S. Poletti)

• 28 October 2019: Mars Express, ESA's first planetary mission, is a true marathon runner among spacecraft. Launched on 2 June 2003, the spacecraft arrived at Mars during the night of 25 December that same year. On 26 October 2019, this spacecraft completed its twenty-thousandth orbit around Mars. Mars Express is in good company in Martian orbit: NASA’s Mars 2001 Odyssey and Mars Reconnaissance Orbiter have also been studying the Red Planet for more than 10 years. Odyssey has been in orbit since 2001 and the Reconnaissance Orbiter since March 2006. 30)

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

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

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

• 10 October 2019: Mars may seem to be an alien world, but many of its features look eerily familiar – such as this ancient, dried-up river system that stretches out for nearly 700 km across the surface, making it one of the longest valley networks on the planet. 31)

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

MarsExpress_Auto58

Figure 69: This image from ESA’s Mars Express shows a dried-up river valley on Mars named Nirgal Vallis. It comprises data gathered on 16 November 2018 during Mars Express Orbit 18818. The ground resolution is approximately 14 m/pixel and the images are centered at about 315ºE/27ºS. This image was created using data from the nadir and color channels of the High Resolution Stereo Camera. The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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

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

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

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

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

MarsExpress_Auto57

Figure 70: Nirgal Vallis in context. This image shows a dried-up river valley on Mars named Nirgal Vallis. The area outlined by the bold white box indicates the area imaged by the Mars Express High Resolution Stereo Camera on 16 November 2018 during orbit 18818 (image credit: NASA MGS MOLA Science Team)

MarsExpress_Auto56

Figure 71: Topographic view of Nirgal Vallis. This color-coded topographic view shows a dried-up river valley on Mars named Nirgal Vallis. Lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows, and reds, as indicated on the scale to the top right. This view is based on a digital terrain model of the region, from which the topography of the landscape can be derived. It comprises data obtained by the High Resolution Stereo Camera on Mars Express on 16 November 2018 during orbit 18818. The ground resolution is approximately 14 m/pixel and the images are centered at about 315ºE/27ºS. North is to the right (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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

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

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

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

MarsExpress_Auto55

Figure 72: Perspective view of Nirgal Vallis. This image from ESA’s Mars Express shows a dried-up river valley on Mars named Nirgal Vallis. This oblique perspective view was generated using a digital terrain model and Mars Express data gathered on 16 November 2018 during Mars Express orbit 18818. The ground resolution is approximately 14 m/pixel and the images are centered at about 315°E/27°S. This image was created using data from the nadir and color channels of the HRSC (High Resolution Stereo Camera). The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

MarsExpress_Auto54

Figure 73: Nirgal Vallis in 3D. This image shows Nirgal Vallis, a dried-up river valley on Mars, in 3D when viewed using red-green or red-blue glasses. This anaglyph was derived from data obtained by the nadir and stereo channels of the HRSC on ESA’s Mars Express during spacecraft orbit 18818. It covers a part of the martian surface centered at about 315ºE/27ºS. North is to the right (image credit: ESA/DLR/FU Berlin, , CC BY-SA 3.0 IGO)

• 19 September 2019: This beautiful view from ESA’s Mars Express stretches from the bright, cloud-covered north pole of Mars to the contrasting hues of the northern hemisphere and the cratered terrain in the south. 32)

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

MarsExpress_Auto53

Figure 74: This image from ESA’s Mars Express shows a beautiful slice of the Red Planet from the northern polar cap downwards, and highlights cratered, pockmarked swathes of the Terra Sabaea and Arabia Terra regions. It comprises data gathered on 17 June 2019 during orbit 19550. The ground resolution at the center of the image is approximately 1 km/pixel and the images are centered at about 44ºE/26ºN. This image was created using data from the nadir and color channels of the HRSC. The nadir channel is aligned perpendicular to the surface of Mars, as if looking straight down at the surface. North is up (image credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)

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

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

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

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

MarsExpress_Auto52

Figure 75: Topographic context. This color-coded topographic image shows a slice of the Red Planet from the northern polar cap downwards, and highlights cratered, pockmarked swathes of the Terra Sabaea and Arabia Terra regions. The area outlined in the center of the image indicates the area imaged by the Mars Express High Resolution Stereo Camera on 17 June 2019 during orbit 19550. This context map is based on data gathered by NASA’s Viking and Mars Global Surveyor missions; lower parts of the surface are shown in blues and purples, while higher altitude regions show up in whites, yellows, and reds, as indicated on the scale to the bottom left (image credit: NASA/MGS/MOLA Science Team, FU Berlin)

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

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

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

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

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

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

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

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

MarsExpress_Auto51

Figure 76: A slice of Mars in context: Terra Sabaea and Arabia Terra. This image shows a slice of the Red Planet from the northern polar cap downwards, and highlights cratered, pockmarked swathes of the Terra Sabaea and Arabia Terra regions. The area outlined in the center of the image indicates the area imaged by the Mars Express High Resolution Stereo Camera on 17 June 2019 during orbit 19550. This context map is based on data gathered by NASA’s Viking and Mars Global Surveyor missions (image credit: NASA/Viking, FU Berlin)