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 rocky 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 approximately as long as two Earth years), each of these seasons lasts about half an Earth year.
The biggest differences compared to Earth are the absence of plate tectonics, the very thin atmosphere, the extremely low temperatures on the surface caused by the thin atmosphere and the absence of a magnetic field. 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 during the day in summer, while during 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 the early advanced 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 describe the motion of the planets around the Sun in elliptical orbits and finally confirmed the Copernican model. In addition, in previous centuries, studies of Mars at opposition were often used to trigonometrically calculate the distance between Earth and Mars to determine the value of 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 reason to believe in the existence of an intelligent civilisation on Earth’s planetary neighbour.
The Martian atmosphere
As on Venus, the Martian atmosphere consists predominantly (up to 95 percent) 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 entire planet, resulting in a yellowish-brown haze in the sky.
Spectrometers on board the European Mars Express orbiter have 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, it was even thought possible 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 but has so far been unable to detect the concentrations measured by Mars Express in 2004.
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 slightly smaller and are located on the six-kilometre-high Tharsis plateau. Another striking feature is the long canyon system, Valles Marineris (named after the Mariner 9 spacecraft), which extends almost 4000 kilometres along the equator and is up to 700 kilometres wide from north to south. At its deepest points, the canyons are almost 10 kilometres deep. The southern hemisphere contains the Hellas Planitia and Argyre Planitia basins, the largest impact structures still visible on Mars.
Volcanic activity shaped the planet during much of its development. Minerals typical of basaltic volcanism, which brings iron- and magnesium-rich silicate rocks from the Martian mantle to the surface, have been identified in many places on the surface. Basalts are formed when relatively pristine material from the planet’s mantle is partially melted to form magma, which rises in large bubbles and emerges at 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 region 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 solidified just a few million years ago – very recently on a geological timescale. 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 and valley structures caused by fractures 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.
Extensional fractures are the most common, but there are also compressional faults in places, but only a few lateral displacements are known. 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 remains of 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 until 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 referred to 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 ancient crustal 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 to 700 million years of the planet's development. To study the thermal evolution, internal structure and tectonic activity of Mars, NASA's InSight geophysical measuring station has been located in Elysium Planitia since the end of 2018.
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 and duration. Branched valley systems, particularly in the older regions of the Martian highlands, 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 and discovered 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 landed in 2012 and identified substances that indicate that conditions suitable for the existence of life once prevailed there. Jezero Crater, where NASA's second large rover, Perseverance, landed in 2021, was also once filled by a lake into which two rivers deposited their sediment load in the form of large delta formations.
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. They are interpreted as remnants of glaciers on Mars whose ice was once completely covered by rock debris and boulders that slid sideways onto the glacial flow. Numerous surface phenomena, especially at medium and higher geographical latitudes, resemble periglacial structures in permafrost areas on Earth. In fact, radar measurements from Mars orbit have detected ice at shallow depths in some places. These measurements have shown that Mars has a considerable occurrence of ground ice, especially in the lowlands of the northern hemisphere. 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 ever-present activity of the wind on Mars. In earlier times, when the atmosphere was denser, 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 light quartz sand, but rather of a dark volcanic ash and crushed volcanic rock, which have been piled up by the wind to form the dune fields visible today. With high-resolution image data from the HiRISE camera on Mars Reconnaissance Orbiter, movements of the countless ripples on the dune surfaces and even displacements of smaller dunes have been observed in recent years. 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 no life forms, not even organic substances or 'biosignatures', have been found to date, Mars remains the most important target of international space exploration with regard to the search for existing or extinct life on another celestial body in the Solar System. Since February 2021, NASA's Perseverance rover has been searching for signs of biosignatures, that is, traces of microbial life, in Jezero Crater. Perseverance will also collect rock samples and deposit them on Mars so that they can be brought back to Earth and studied here in a later mission.
The Martian moons
The two comparatively small Martian 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 approximately 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 consideration. 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 high-profile model indicates that both moons were formed after a very large asteroid impact on Mars in the earliest stages of the formation 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. Their dark appearance could then be explained by the fact that the rocks ejected from Mars came from great depths and represent material from the Martian mantle.
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. The next Phobos mission is planned for the mid-2020s. The Japanese-German-French Martian Moons eXplorer (MMX) lander will carry a small rover.
|Mass||6417 x 1023 kg|
|Rotation period||24.37 h|
|Orbital period||687 Earth days|
|Average distance from the Sun||227.9 x 106 km|
|Mass||1.06 x 1016 kg|
|Dimensions||13 x 11.4 x 9.1 km|
|Orbital period||7.66 hours|
|Average distance from the centre of Mars||9378 km|
|Mass||1.1 x 1015 kg|
|Dimensions||7.8 x 6.0 x 5.1 km|
|Orbital period||30.29 hours|
|Average distance from the centre of Mars||23,459 km|