Mars

Mars is the fourth planet from the Sun in the Solar System. Almost half the size of the Earth, Mars is a differentiated solid body with a thick crust of an average thickness of 50 km, a fairly thick lithosphere - 150 to 200 kilometres - and a core of a radius of 1700 kilometres. No magnetic field has been detected by the magnetometers on board orbital probes; either the core contains little nickel or iron, or movements within it are too slow to cause a dynamo effect.

Like Earth and Venus, Mars has an atmosphere which is, however, very thin; it is composed essentially of carbon dioxide CO² (95.32 %) and a very small amount of water vapour (0.03 %).

Mars is at a mean distance of 1.524 astronomical units from the Sun; because of this, its period of revolution around the Sun is nearly twice that of Earth (one Martian year = 687 terrestrial days). On the other hand, the Martian sidereal rotation period (24 h 37 min 23 s) is very close to that of our planet. The orbit of Mars is highly elliptic, and its high eccentricity (0.093 in comparison to 0.017 for Earth) causes big differences in the lengths of the seasons (spring and summer are much longer in the northern hemisphere than in the southern hemisphere). But the seasonal differences in temperature, caused by the 24° inclination of the axis of rotation in the plane of its orbit, vary inversely. Because of the distance of the planet from the Sun, surface temperatures are much lower than on Earth, and vary on average between -133°C and +17°C. During summer in the southern hemisphere, Mars is about 20% closer to the Sun than during summer in the northern hemisphere. As a result there is an increase of around 45% in insolation, producing a substantial increase (30°C) in summer temperatures in the southern hemisphere compared with summer in the northern hemisphere. These seasonal temperature variations have important consequences on the exchanges between the Martian atmosphere and the surface, especially at the poles. The slow precessional movements of the planet's axis of rotation and the axis of its orbit, together with the variations in eccentricity and inclination of the plane of the orbit and the oscillations of the axis of rotation, cause modifications in the climate conditions of the two hemispheres in the long term. Thus, every 25,000 years, precession movements cause a change in orientation of the poles relative to the Sun and consequently a reverse of the climate conditions between the two hemispheres.

Mars has two natural satellites: Phobos (from the Greek for fear) and Deimos (panic). These two very small bodies are very dark and close to the planet, and are therefore difficult to observe from Earth (they were discovered in 1877 by Asaph Hall). They revolve around Mars in a direct movement, in circular orbits in the equatorial plane of the planet and rotate synchronously with it, i.e. their periods of rotation about their own axes are equal to their periods of revolution around the planet; because of this, they always face Mars on the same side, and their major axis points towards it. They are irregular in shape.

The two main differences between the atmospheres of Mars and Earth are the very low mass of the Martian atmosphere, mainly composed of carbon dioxide CO² (the ground pressure is about 6 hectopascals, compared with 1013 hectopascals on Earth) and its mean temperature, significantly lower (-50 °C), due to the fact that Mars is further than the Earth from the Sun. The pressure is too low for water to exist in the liquid state and, because the climate is generally cold, most of the water vapour is in ice form in the atmosphere (cirrus, freezing mist) or on the ground (polar caps). During the Martian year, about 20% of the atmospheric carbon dioxide condenses alternately on each of the poles, causing a substantial annual variation in pressure.

The low atmospheric mass results in very large diurnal temperature fluctuations (more than 50°C). Like Earth, Mars has an atmosphere which is transparent to most solar radiation; it is therefore heated mainly at the base. The inclination of the axis of rotation of Mars with respect to the plane of its orbit and the length of the Martian day are very close to the values on Earth, and as a result there is the same type of global atmospheric circulation system, with trade winds in the inter-tropical zone and a system of high and low pressures at medium latitudes. The absence of oceans - which on Earth act as a thermal regulator - results in a greater seasonal contrast in temperature and therefore winds. Violent storms occur in spring at the edge of the south polar cap, carrying large amounts of dust into the atmosphere which can ultimately cover the entire planet. Some of this dust falls on the polar caps during the periods when carbon dioxide and water are condensing. The dynamics of the Martian atmosphere are therefore determined by a strong ground-atmosphere interaction through the carbon dioxide, water and dust cycles.

Morphology and topography

The surface of Mars is on the one hand marked by a wide variety of relief features (meteorite craters, giant volcanoes, deep canyons, immense networks of river valleys, dune fields, large fault systems, icecaps at the poles), and on the other, by a major morphological and topographical dissymmetry between the north and south hemispheres.

The dissymmetry between the hemispheres occurs on either side of a large circle inclined at 35° to the equator. Morphologically, this dissymmetry is marked by a large number of meteorite craters that make the southern hemisphere of the planet look like lunar landscapes, in contrast to the small numbers of craters on the plains of the north. Topographically, the dissymmetry is shown by a difference in altitude that can reach 2 to 3 kilometres, the northern plains being substantially lower than the highly cratered terrains of the southern hemisphere. The origin of this dissymmetry is still not known; it could be the result of a structural boundary or an erosion boundary.

There are also large differences in altitude that can reach 30 kilometres. Altitudes are defined in relation to a reference level (level 0) which, in the absence of seas as on the Earth, corresponds to an atmospheric pressure of 6.1 hPa at ground level, determined at the equator using infrared measurements made by the Mariner 9 probe. These altitude measurements were completed using radar observations from Earth. The highest region is the dome of Tharsis, on which there are three giant volcanoes averaging 26 kilometres in altitude. This region is a bulge about 6 kilometres high and about 5000 kilometres in diameter. Compared with the Earth, this Martian region is the size of a continent. Another region, Elysium Planitia, overlooks the surrounding plains from a height of 4 to 5 kilometres. This too is a wide dome, 1500 kilometres in diameter, also with volcanoes, but not as big as those in the Tharsis region. To the south of the Martian equator, the Valles Marineris canyon system is made up of deeply embedded valleys (6 km deep on average) that stretch from east to west over a distance of more than 5000 kilometres.

Martian volcanism

Apart from the great volcanic eruptions that cover most of the plains in the northern hemisphere, the planet's two main volcanic concentrations are in the region of the dome of Tharsis and the dome of Elysium Planitia.

Tharsis supports some of the biggest volcanic structures on Mars, the Tharsis Montes mountain chain and the giant Olympus Mons volcano. In this region, there is quite a strong correlation between topography and gravity. Under the dome of Tharsis, there is good isostatic compensation of the volcanic relief, and the crust can reach a thickness of 130 kilometres. The Tharsis Montes structures are 350 to 400 kilometres in diameter at their base; the Olympus Mons volcano, 1600 kilometres north-west of the dome of Tharsis, has a diameter of 550 kilometres. Their mean altitude varies between 24 and 27 kilometres and their summits are huge calderas that can reach 110 to 220 kilometres in diameter and 3 to 4 kilometres in depth. These volcanoes have a morphology that is typical of "shield" volcanoes, i.e. large cones with gentle slopes, similar to the Hawaiian volcanoes on Earth. Their surfaces are relatively young (a hundred million years old).

Olympus Mons seen by Mars Global Surveyor

Apart from the giant volcanoes in the Tharsis and Elysium regions, there are other much smaller volcanoes (from 60 to 180 km in diameter) in these two regions. They are also shield-shaped, but with much steeper slopes than the giant volcanoes. Their sides are often ravined by channels several hundred metres wide of which the origin has been attributed to pyroclastic flows or phreatic eruptions caused by the interaction between magma and water or ice in the Martian crust. There are also other older or much smaller shapes (diameters less than 5 km). The oldest volcanoes are especially in the southern hemisphere, and their highly eroded morphology appears to indicate a quite friable material similar to ash. The smallest volcanoes are mainly cone-shaped aligned with major faults, and are on the rims of the Tharsis and Elysium domes. Despite their relatively young surfaces, the Martian volcanoes are probably old.

Phobos and Deimos

Like other bodies with no atmosphere or geological activity, the surfaces of Phobos and Deimos are peppered with craters and covered in regolith. In the case of Phobos, the regolith could be around 300 metres thick. On Phobos, the Stickney crater is nearly 10 kilometres in diameter; on Deimos, the largest crater is 3 kilometres in diameter. From a distance, the surfaces of Phobos and Deimos may look similar, but high resolution images sent by the Viking space probes show that they are very different. On a scale of a few hundred metres, the surface of Phobos is consistent, whereas that of Deimos is scattered with spots about 30% brighter than the surrounding environment. Unlike Deimos, Phobos has a system of long parallel grooves and streaks; these were seen by some researchers as fissures produced by tidal forces from the planet, and by others as fractures caused by the impact that created the crater Stickney.

Phobos

Deimos

The surfaces of Phobos and Deimos are very dark; their reflective power is very low: less than 6 % of the visible light from the Sun is reflected. The density of Phobos is of the order of 2. Deimos probably has a similar density. The low density of Phobos suggests that it must be composed of a material rich in lightweight elements and perhaps water, similar to the composition of type C meteorites (carbonated chondrites).

The spectacular difference in the chemical composition of the surfaces of Mars and its satellites makes it very unlikely that they were formed at the same time as the planet. It would seem that Phobos and Deimos were formed in the outer part of the asteroid belt and then later captured by Mars.

The Martian Canals

Slight straight or curved lines, visible on the surface of Mars to some observers on Earth, were the subject of much controversy at the end of the 19th and beginning of the 20th century. Giovanni Schiaparelli observed about a hundred of them from 1877 and described them as "canals". Other observers had already noticed similar features, but it was Schiaparelli who aroused general interest with his articles. The American astronomer Percival Lowell led those who assigned these features to stretches of vegetation several kilometres wide, enclosing irrigation channels dug by intelligent beings to bring water from the planet's polar caps.

Map of Mars with its "canals" drawn by Schiaparelli in 1888.

Lowell and other astronomers described the networks of canals, dotted with darker coloured intersections, baptised oases, covering a large part of the planet's surface. From time to time, the lines appeared to split into two. Most astronomers failed to see canals, and there were many who questioned their objective reality. Experiments in perception carried out using untrained observers showed that disjointed details on diagrams or drawings can be perceived as forming rectilinear networks when observed from a certain distance. Photographs taken through the Earth's atmosphere provided no certainty, the width of the canals being close to the resolution limit of the human eye and less than that of a photographic plate. The controversy was finally settled when the American space probes Mariner 4, in 1965, the Mariner-6 and Mariner 7, in 1969, succeeded in sending images of the Martian surface from an altitude of a few thousand kilometres. These images showed numerous craters and other topographical features, but nothing that looked like a network of canals. This was confirmed by the later space missions, starting with Viking, launched in 1976.

Data

Physical characteristics

Mass (kg) 6.421 ²³
Mass (Earth = 1) 1.0745 ^-01
Volume (km³) 1.639 ¹¹
Total surface area (m²) 1.44 ¹⁴
Equatorial radius (km) 3397.2
Equatorial radius (Earth = 1) 5.3264 ^-1
Polar radius (km) 3375
Mean volumetric radius (km) 3390
Radius of core (km) 1700
Polar ellipse 0.00648
Mean density (gm/cm³) 3.933
Mean gravitational constant at the surface (m/s2) 3.69
Escape velocity (km/s) 5.03
Albedo 0.15
Visual magnitude V(1.0) -1.52
Solar energy on the ground (W/m²) 589.2
Atmospheric pressure (bars) 0.007

Orbit

Major axis (10⁶ km) 227.92
Sidereal orbital period (days) 686.980
Tropical orbital period (days) 686.973
Perihelion (10⁶ km) 206.62
Aphelion (10⁶ km) 249.23
Mean distance from the Sun (km) 227 940 000
Mean distance from the Sun (Earth = 1) 1.5237
Minimum distance from Earth (10⁶ km) 54.5
Maximum distance from Earth (10⁶ km) 401.3
Synodic orbital period (days) 779.94
Mean orbital velocity (km/sec) 24.13
Maximum orbital velocity (km/sec) 26.50
Minimum orbital velocity (km/sec) 21.97
Inclination of the plane of the orbit (degrees) 1.850
Orbital eccentricity (degrees) 0.0934
Rotation period (hours) 24.6229
Rotation period (days) 1.025957
Inclination of the axis of rotation (degrees) 25.19

Atmosphere

Pressure at the surface (mb) 6.1
Density of the atmosphere at the surface (kg/m³) 0.20
Average molecular weight (g/mole) 43.34

Temperature

Mean atmospheric temperature (black body) (K) 210
Minimum surface temperature (°C) -140
Average surface temperature (°C) -63
Maximum surface temperature (°C) -20

Composition of the atmosphere

Carbon dioxide (C0²) 95.32%
Nitrogen (N²) 2.7%
Argon (Ar) 1.6%
Oxygen (O²) 0,13%
Carbon monoxide (CO) 0.07%
Water (H²O) 0.03%
Neon (Ne) 0.00025%
Krypton (Kr) 0.00003%
Xenon (Xe) 0.000008%
Ozone (O3) 0.000003%

Phobos

Orbit: 9378 km from the centre of the planet
Diameter: 22.2 km (27 x 21.6 x 18.8)
Mass: 1.08 ¹⁶ kg

Deimos

Orbit: 23,459 km from the centre of the planet
Diameter: 12.6 km (15 x 12.2 x 11)
Mass: 1.8 ¹⁵ kg