In terms of its diameter, the planet Mars is only about half the size of Earth, and is only a third of Earth’s mass. It is surrounded by a thin atmosphere of carbon dioxide that condenses into ice crystals on the planet’s high volcanoes, forming cirrus clouds. The north and south poles of Mars are covered by ice caps that grow and shrink according to the season.
On 21 January 2013, the High Resolution Stereo Camera (HRSC), operated by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) on board ESA's Mars Express spacecraft, imaged the southeast portion of the Olympus Mons shield volcano. This colour image was captured using the nadir channel, which is directed vertically down onto the surface of Mars. Copyright note: As a joint undertaking by DLR, ESA and FU Berlin, the Mars Express HRSC images are published under a Creative Commons licence since December 2014: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO. This licence will also apply to all HRSC images released to date.
ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO.
The more or less parallel fault lines of the Sirenum Fossae clearly show the direction in which tensions in the Martian crust run – geophysicists call this ‘tectonic stress’. The right-hand side of the image is north, and the graben run from northeast to southwest. The forces that stretched the crust, ultimately resulting in its splitting, ran perpendicular to this, i.e. in a southeasterly and northwesterly direction. In this process, the existing three to four billion-year-old plateau was ‘dissected’ in a straight line. The image also shows the result of younger geological processes: for example, asteroid impacts have left craters with barely weathered rims; huge landslides have occurred along the 2000 metre high ridge in the upper third of the image, forming tongue-shaped deposits; and small grooves may possibly be the result of erosion by water running down the slopes.
This colour plan view of Hydraotes Chaos clearly shows the slightly more that 2000-metre-deep valley where many mesas, buttes and hills are arranged in a seemingly chaotic way; these are the result of an intense erosion process. No comparable landforms are found on Earth. It is believed that water in the form of ice was stored in cavities beneath the surface of the highlands early in the history of Mars; this was then heated and thawed out. It was then placed under so much pressure that it escaped to the surface with great force through fissures and fault zones and the overburden collapsed in large slabs. As it flowed out, the water eroded the terrain and left behind the striking landscape visible today. North is to the right in the image. In the hi-res downloadable image, north is up.Copyright note: As a joint undertaking by DLR, ESA and FU Berlin, the Mars Express HRSC images are published under a Creative Commons licence since December 2014: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO. This licence will also apply to all HRSC images released to date.
HRSC nadir channel in full resolution (acquired during the flyby on 12 September 2017 in orbit 17,342).
ESA/DLR/FU Berlin CC BY-SA 3.0 IGO.
In this colour image, acquired using the High Resolution Stereo Camera (HRSC), operated on board Mars Express by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), a collection of several craters of varying ages and at different stages of erosion can be seen. One very large impact crater some 70 kilometres in diameter, with a fairly steep, towering crater rim, dominates the left (southern) half of the image. A large field of black dunes has formed on the crater floor. The dark material in these dunes probably originates from the erosion of basalt, a dark volcanic rock, and was carried there by the wind. This image was created using data acquired by the high-resolution nadir channel and the colour channels of HRSC. North is to the right.
ESA/DLR/FU Berlin – CC BY-SA 3.0 IGO.
The Ascuris Planum region situated northeast of the sprawling volcanic area of Tharsis is a vivid example of a horst and graben landscape. The faults, which run in lines ranging from straight to moderately curved, and that were produced by tectonic expansion, are particularly eye-catching features here.
The Claritas Rupes escarpment on Mars surrounds the Claritas Fossae graben system, which forms the eastern boundary of the gigantic Tharsis volcanic region. This is where the majority of volcanoes on Mars are located, including Olympus Mons. It is believed that the numerous fractures and faults running through the region were formed by stress in the Martian crust during the formation of the Tharsis Bulge, which is up to 10 kilometres high.The images were acquired by the High Resolution Stereo Camera (HRSC) on 30 November 2013, during Mars Express Orbit 12,600. The image resolution is about 14 metres per pixel. The images show a section at 27 degrees south and 254 degrees east.Copyright note: As a joint undertaking by DLR, ESA and FU Berlin, the Mars Express HRSC images are published under a Creative Commons licence since December 2014: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO. This licence will also apply to all HRSC images released to date.
Stereo images from the High Resolution Stereo Camera operated by DLR on ESA’s Mars Express spacecraft can be used to show the landscape from various angles. This image shows the centre of Rabe Crater, which is approximately 100 kilometres across, where a large field of black dunes is located. The dunes are up to 200 metres high. The data used to create this image was acquired by the High Resolution Stereo Camera (HRSC) during orbits 2441 and 12,736 of Mars Express. The image resolution is about 15 metres per pixel.Copyright note: As a joint undertaking by DLR, ESA and FU Berlin, the Mars Express HRSC images are published under a Creative Commons licence since December 2014: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO. This licence will also apply to all HRSC images released to date.
This image has been created using 67 separate image strips acquired using the HRSC stereo camera, operated by DLR on board the ESA Mars Express spacecraft. It covers some 1.5 million square kilometres, which is roughly three times the area of France. The image resolution has been reduced in places and is about 100 metres per pixel. The wide-angle view shows how catastrophic floods carved the Martian highland during the Martian ‘Middle Ages’, and flowed out into Chryse Planitia in the delta region (right). Zooming into the high-resolution image, numerous small-scale geological details are visible that can be traced back to the influence of flowing water.When the ESA Mars Express spacecraft was launched 10 years ago, one of the primary goals of the mission was to create a global map of our planetary neighbour in high resolution, in colour and in 3D. The HRSC system was developed at DLR for this purpose. Since its arrival at Mars, over two thirds of the surface has been recorded by the HRSC at a resolution of 10 to 20 metres per pixel. The real strength of the HRSC lies in its ability to capture wide-angle views of the Martian landscape. Ideally, the individual image strips can be used to create composite image mosaics. However, due to the variable conditions at the times that the images were taken, such as the difference in the angle of the Sun, in the altitude at which the images were taken or variations in the atmospheric conditions, slight variation in the brightness and colouring is unavoidable in such mosaics.Copyright note: As a joint undertaking by DLR, ESA and FU Berlin, the Mars Express HRSC images are published under a Creative Commons licence since December 2014: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO. This licence will also apply to all HRSC images released to date.
This image, looking vertically down at the Ares Vallis outflow channel, was created from data acquired by the nadir and colour channels of the High Resolution Stereo Camera (HRSC) on ESA’s Mars Express spacecraft. The imaged region has an area of about 220 × 70 kilometres; north is to the right. Although the image is dominated by the typical Martian soil colour, variations in both colour and texture are visible. Slightly to the left of the centre is an ‘island’ that has resisted erosion by flowing water. Darker areas suggest the transport of material, possibly by the wind.Copyright note: As a joint undertaking by DLR, ESA and FU Berlin, the Mars Express HRSC images are published under a Creative Commons licence since December 2014: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO. This licence will also apply to all HRSC images released to date.
Mars is the fourth planet from the Sun, and Earth’s outer neighbour. It is similar to Earth in many ways, especially in the geological processes that formed its surface. It is only about half the size of Earth, but it has a shell structure consisting of an iron-rich core, a silicate mantle and an outer crust. At 25.2 degrees, the inclination of its axis of rotation is very similar to that of Earth, and this means that Mars also has seasons. Due to its longer orbit around the Sun (one Mars year lasts roughly as long as two Earth years), each of these seasons last about half an Earth year.
The biggest differences compared to Earth are the very thin atmosphere, the lack of a magnetic field and the extremely low temperatures on the surface. With an average temperature of minus 60 degrees Celsius and an atmospheric pressure of less than one percent of that on Earth, there is no liquid water on Mars, at least not today. Temperatures can rise to almost 27 degrees Celsius near the equator on summer days, while on a winter night they can fall as low as minus 133 degrees Celsius at the poles.
Observation of Mars can be traced back to the time of early civilisations. Due to its reddish hue, which is reminiscent of blood when seen from a distance, the planet was called ‘Horus the Red’ in Ancient Egypt, and named after Ares, the god of war, in Ancient Greece. Today, Mars owes its name to the Roman god of war. In the early 17th century, Johannes Kepler set out various laws based on measurements of the position of Mars conducted by the Danish astronomer Tycho Brahe. These measurements were few in number yet very precise for their time. Kepler’s laws described planetary motion around the Sun and definitively confirmed the Copernican model. In addition, in previous centuries, studies of Mars at opposition tended to use the trigonometrically calculated distance between Earth and Mars to determine the Astronomical Unit (the distance between Earth and the Sun). In 1877, Giovanni Schiaparelli succumbed to an optical illusion, believing that he had seen rift- and groove-like structures on Mars, which he called ‘canali’. Many of Schiaparelli’s contemporaries believed that these could not have been created by natural means, and long after scientists had discovered that such a belief was mistaken, people saw these features as a sign of intelligent life on Earth’s planetary neighbour.
The Martian atmosphere
As on Venus, the Martian atmosphere consists predominantly (up to 95 per cent) of carbon dioxide (CO2). However, the average pressure at the surface is only six millibars, compared to 1013 millibars on Earth. Clouds of water ice and carbon dioxide ice can form in the Martian atmosphere, and during certain seasons massive, long-lasting storms can develop, raising sand and dust to a height of 50 kilometres and blowing it across the whole planet, resulting in a yellowish-brown haze in the sky.
Spectrometers on board the Mars Express orbiter discovered traces of the gases methane and formaldehyde in the atmosphere over some of the large volcanic provinces, leading to some speculation that heat could still be present within these volcanoes, causing these gases to be released. As methane was also detected elsewhere within the Martian atmosphere, there was even speculation that biological processes might be responsible for the methane production, as they are on Earth. However, a number of processes relating to the weathering of minerals in volcanic rock may also explain the formation of methane. The current ExoMars Trace Gas Orbiter (TGO) mission, which has been orbiting Mars since September 2016, is intended to solve the mystery of methane in the Martian atmosphere.
Thanks to numerous Mars missions such as Mariner 9, Viking 1 and 2, Mars Global Surveyor, Mars Odyssey, Mars Express and Mars Reconnaissance Orbiter, there exists a relatively extensive body of knowledge about the planet’s surface characteristics and geological formations. The surface can be roughly divided into two large regions – a northern area of lowland plains and the southern highlands with countless impact craters. Near the equator lies the shield volcano Olympus Mons, which towers at an astonishing 26 kilometres above its surroundings and has a diameter of 600 kilometres. Its three neighbours, Arsia Mons, Ascraeus Mons and Pavonis Mons, are only a little smaller and are located on the six-kilometre-high Tharsis plateau. Another striking feature is the long rift valley system, Valles Marineris (named after the Mariner 9 spacecraft), which is almost 4000 kilometres long and measures up to 700 kilometres from north to south. At its deepest points, the rift valley is almost 10 kilometres deep. The southern hemisphere is home to the Hellas Planitia and Argyre Planitia impact basins, the largest impact structures still visible on Mars.
Volcanic activity shaped the planet during much of its development. Many places on the surface have been shown to contain minerals that are typical of basaltic volcanism, whereby iron- and magnesium-rich rocks rise from the mantle of Mars to the surface. Basalts occur when fairly primitive material from the planet’s mantle is partially melted to form magma, which rises in large bubbles and escapes on the surface as lava. This is the most common form of volcanic activity on Earth-like bodies in the Solar System. It is believed that the Martian crust essentially consists of basalt, which has been altered by processes such as meteorite impacts, weathering and erosion, and in many places is no longer present in its original form.
The volcanoes, which are now extinct, only exist in large numbers in certain regions. The largest volcanic province is Tharsis, where around a dozen large volcanoes and 100 smaller volcanoes have been discovered. In some cases, their activity dates back to a relatively recent time in Mars’ geological past. Another volcanic region is Elysium, where some lava streams are likely to have cooled just a few million years ago – almost yesterday in geological terms. This raises the question of whether Mars may still be volcanically active in some places.
In addition to volcanism, tectonic processes have also shaped the surface of Mars. Numerous faults caused by ruptures in the rigid lithosphere – the brittle, outermost rocky crust of the planet – can be observed on satellite images. Groups of faults can often be several hundred or even 1000 kilometres long. Both extensional and compressional disruptions have been detected, but there are only a few lateral faults. This is not surprising; such faults are primarily caused by plate tectonics on Earth, when the large continental plates slide past one another laterally. Mars, by contrast, is a ‘one-plate planet’ – its lithosphere does not consist of numerous individual plates that shift against one another, as happens on Earth.
The interior of Mars, like all bodies in the Solar System, can essentially be viewed as a heat engine. The main sources of heat within the interior are the decay of radioactive isotopes, such as the elements uranium, thorium and potassium, together with the energy that was generated during the formation of the planet. This heat is released through the planet’s surface, causing the interior to cool down over long geological periods. Convection is one of the most efficient heat transport mechanisms. The steady movement, or ‘circulation’ of the mantle rock due to temperature and pressure differences within the planet ensures the redistribution of heat, and the results are visible as surface structures such as volcanoes and tectonic formations. The large volcanic regions of Tharsis and Elysium, which were active just a few million years ago, show that thermal convection continues to take place inside Mars to this day. Differences in density between hot material from deeper regions within Mars and colder regions near the surface cause what are known as thermal anomalies in the mantle. Mantle plumes or, to use the geological term – diapirs, cause ‘bubbles’ of hot rock to rise slowly through the planet’s mantle towards the surface due to their lower density. The rock can melt more easily near the surface due to the lower pressure of the overlying rock, and thus form magma. This is thought to be the source of the youngest volcanoes on Mars.
During the early years of the planet’s development, convection in the liquid iron core of Mars acted as a ‘dynamo’ and probably produced a weak magnetic field. Today, Mars no longer has an active magnetic field, but the old rocks on the planet’s surface have recorded traces of this once active dynamo. Older surface rocks are magnetised, while younger regions appear to lack magnetisation. This indicates that the self-generated magnetic field of Mars was likely to only have been active for the first 500 million years of the planet’s development.
Wind, water, ice
The surface of Mars was shaped and influenced by water (fluviatile), ice and glaciers (glacial), and winds (aeolian) processes of varying intensity. Branched valley systems, particularly in the older highlands of Mars, stretch across vast areas and attest to the existence of a water cycle on Mars at one point in time. One of the most famous valley systems is Ma’adim Vallis, which once drained into the Gusev impact crater where the Mars rover Spirit searched for traces of water. In addition to flowing water, there were also crater lakes filled with water. Today, these are referred to as paleolakes and are often associated with deltas, characteristic mineral deposits and inflow and outflow channels. One such lake was located in Gale Crater, where the NASA rover Curiosity has been searching for traces of conditions viable for life since 2012. Short-term movement of water in the recent past – in the form of mudslides or debris flows – may have caused the distinctive erosion gullies found at many crater edges.
Traces of glacial processes dating back to Mars’ relatively recent past can be observed at many locations on the planet’s surface. For instance, flow structures reminiscent of rubble-topped rock glaciers in mountainous and polar regions on Earth can be found on the northwestern slopes of the large Tharsis volcanoes. These are interpreted as being the remains of glaciers on Mars. Numerous surface phenomena, especially at medium and higher geographical latitudes, resemble periglacial structures in permafrost areas on Earth. Indeed, ice has been detected at shallow depths in some places. Radar measurements have also shown that Mars has a considerable amount of ground ice. Probably the most impressive ice formations on Mars are the two polar caps, which consist of a mixture of water ice and/or carbon dioxide ice, depending on the season.
Widespread dark dunes testify to the activity of the wind on Mars. In earlier times, when the atmosphere was more dense, this was much more intense than it is today. Large dune fields are predominantly found within impact craters. Unlike on Earth, however, these sand dunes are not made of bright quartz sand, but rather of a dark volcanic ash deposited around three to four billion years ago. There are whole regions covered with streamlined ridges, called yardangs, carved out of the landscape by the constant activity of the wind, almost like the action of a sandblaster. Today, wind activity is particularly dramatic in the form of large dust storms and smaller whirlwinds, known as ‘dust devils’, that move over the surface of Mars at high speed.
Although seven spacecraft have now landed on Mars, no traces of life, or indeed of any complex organic substance, have been found. However, the landers still serve the most important long-term goal of international space travel in terms of searching for existing or extinct life on another celestial body in the Solar System. The Mars Science Laboratory mission landed with the Curiosity rover in the Gale crater on 6 August 2012 and is searching for traces of possible long-past habitats in a thick sedimentary layer. Curiosity has been able to identify the building blocks of life, namely carbon compounds, but no traces of life. This task is to be taken up by the ESA ExoMars mission from 2020. For the first time, a rover called Pasteur will be able to drill up to two metres into the Martian subsurface for this purpose. If life ever did exist on the planet, traces may be more likely to be found under the surface, where they would have been protected from degradation by harmful ultraviolet radiation.
The Martian moons
Mars’ two moons, Phobos and Deimos, were discovered in 1877 by Asaph Hall (1829–1907) and have similar characteristics. Both have a rather irregular shape and a very dark surface that reflects only around five percent of the incident sunlight. Phobos, the larger of the two moons with a diameter of up to 27 kilometres, has a large number of impact craters, of which Stickney is the biggest, at 12 kilometres in diameter, while Hall is five kilometres across. Further from Mars, the small moon Deimos is only up to 15 kilometres across and has far fewer visible craters. Image data from the Viking missions show that its surface has a heavier covering of dust, or regolith, than Phobos.
The origin of the two moons is not yet clear, and several formation models are under discussion. One theory suggests that Mars and its two moons were formed in a common process. Another theory assumes that both moons are small bodies formed in the asteroid belt between Mars and Jupiter and captured by the gravitational field of Mars. Another well-respected model indicates that both moons were formed after a very large asteroid impact on Mars in the earliest stages of the solar system. In this scenario, debris from the impact formed a ring around Mars. Due to interactions between the materials in the ring, several small bodies formed, some of which fell back onto Mars. Phobos and Deimos are the last two remnants of this collection of small bodies.
Phobos also seems to be facing the fate of falling onto Mars. Due to its proximity to its parent planet, Phobos is subject to strong tidal forces. Data from the ESA Mars Express orbiter confirms that it is growing closer to Mars on a spiral course, and that due to the tidal forces exerting ever-greater forces on it, it will break apart in about 40 to 70 million years. The Russian probe Phobos-Grunt, which was intended to examine the moon in detail, failed to leave Earth orbit in late 2011. Other Phobos missions are planned for the early 2020s.
Last modified:05/05/2018 13:59:51