Mars (planet)

Mars is the fourth planet in the Solar System in ascending order of distance from the Sun and the second in ascending order of size and mass. Its distance from the Sun is between 1,381 and 1,666 AU (206.6 to 249.2 million kilometers), with an orbital period of 669.58 Martian days (686.71 days or 1.88 Earth years).

It is a terrestrial planet, as are Mercury, Venus and Earth, about ten times less massive than Earth but ten times more massive than the Moon. Its topography presents analogies both with the Moon, through its craters and impact basins, and with the Earth, with formations of tectonic and climatic origin such as volcanoes, rifts, valleys, mesas, dune fields and polar caps. The highest volcano in the Solar System, Olympus Mons (which is a shield volcano), and the largest canyon, Valles Marineris, are on Mars.

Mars has now lost almost all of its internal geological activity, and only minor events would still occur episodically on its surface, such as landslides, probably GEYSERS OF CO2 in the polar regions, perhaps earthquakes, or even rare volcanic eruptions in the form of small lava flows.

The rotation period of Mars is of the same order as that of the Earth and its obliquity gives it a cycle of the seasons similar to the one we know; these seasons are however marked by an orbital eccentricity five and a half times higher than that of the Earth, resulting in a seasonal asymmetry significantly more pronounced between the two hemispheres.

Mars can be observed with the naked eye, with a brightness much weaker than that of Venus but which can, in close oppositions, exceed the maximum brightness of Jupiter, reaching an apparent magnitude of -2.91, while its apparent diameter varies from 25.1 to 3.5 arcseconds. depending on whether its distance to Earth varies from 55.7 to 401.3 million kilometers.

Mars has always been visually characterized by its red color, due to the abundance of amorphous hematite — iron(III) oxide — on its surface. This is what has made it associated with war since ancient times, hence its name in the West after the god Mars of war in Roman mythology, assimilated to the god Ares of Greek mythology. In English, Mars is often nicknamed “the red planet” because of this particular color.

Before Mariner 4 flew by Mariner 4 in 1965, it was thought that there was liquid water on the surface and that life forms similar to those existing on Earth may have developed there, a very fruitful theme in science fiction. Seasonal variations in albedo on the planet’s surface were attributed to vegetation, while straight formations perceived in astronomical glasses and telescopes of the time were interpreted, notably by the American amateur astronomer Percival Lowell, as channels of irrigation crossing desert expanses with water from the polar ice caps. All these speculations have been swept away by the space probes that have studied Mars: as early as 1965, Mariner 4 made it possible to discover a planet devoid of a global magnetic field, with a cratered surface reminiscent of that of the Moon, and a tenuous atmosphere.

Since then, Mars has been the subject of more ambitious exploration programs than for any other object in the Solar System: of all the stars we know, it is indeed the one that has the environment with the most similarities with that of our planet. This intensive exploration has given us a much better understanding of Martian geological history, revealing, in particular, the existence of a remote era – the Noachian – where surface conditions must have been quite similar to those of the Earth at the same time, with the presence of large quantities of liquid water; the Phoenix probe discovered in the summer of 2008 water ice at a shallow depth in the soil of Vastitas Borealis.

Mars has two small natural satellites, Phobos and Deimos.

Physical and orbital characteristics

The fourth planet in the Solar System in order of increasing distance from the Sun, Mars is a terrestrial planet half the size of Earth and nearly ten times less massive, whose area is a little smaller than that of the land surface of our planet (144.8 against 148.9 million square kilometers). The gravity is one-third of that of the Earth, twice that of the Moon, while the duration of the Martian solar day, called soil, exceeds that of the Earth day by just under 40 minutes.

Mars is one and a half times farther from the Sun than Earth, in a significantly more elliptical orbit, and receives, depending on its position in this orbit, between two and three times less solar energy than our planet. Since Mars’ atmosphere is more than 150 times less dense than ours, and therefore produces only a very limited greenhouse effect, this low solar radiation explains why the average temperature on Mars is about −65 °C.

The table below compares the values of some physical parameters between Mars and Earth:

Property	Martian value	Land value	% Mars / Earth
Equatorial radius	3,396.2 ± 0.1 km	6,378.1 km	53,3%
Polar radius	3,376.2 ± 0.1 km	6,356.8 km	53,1%
Volumetric mean radius	3,389.5 km	6,371.0 km	53,2%
Surface	144,798,500 km	510,072,000 km	28,4%
Volume	1,631 8 × 10 km	1,083 207 3 × 10 km	15,1%
Mass	6.418 5 × 10 kg	5.973 6 × 10 kg	10,7%
Average density	3,933.5 ± 0.4 kg/m	5,515 kg/m	71,3%
Surface gravity at the equator	3.711 m/s	9.780 327 m/s	37,9%
Release speed	5,027 m/s	11,186 m/s	44,9%
Sidereal rotation period	1,025 956 75 d ≈ 88 642,663 s	86,164,098 903,691 s	102,9%
Duration of the solar day	1 floor ≈ 1.027 491 25 d ≈ 88 775.244 s	1 d = 86,400 s	102,75%
Axis tilt	25,19°	23,439281°	–
Bond’s Albedo	0,25	0,29	–
Visual geometric albedo	0,15	0,367	–
Semi-major axis of the orbit	227,939,100 km	149,597,887.5 km	152,4%
Orbital eccentricity	0,093315	0,016710219	558,4%
Orbital period	668,599 1 soil ≈ 686,971 d	365,256 366 d	188,1%
Aphelion	249,209,300 km	152,097,701 km	163,8%
Perihelion	206,669,000 km	147,098,074 km	140,5%
Solar radiation	492 to 715 W/m	1,321 to 1,413 W/m	–
Average ground temperature	−63 °C ≈ 210 K	14 °C ≈ 287 K	–
Highest temperature	20 °C ≈ 293 K	58 °C ≈ 331 K	–
Lowest temperature	-133 °C ≈ 140 K	−89 °C ≈ 184 K	–

The thin Martian atmosphere, where locally abundant clouds appear, is the seat of a particular meteorology, dominated by dust storms that sometimes obscure the entire planet. Its orbital eccentricity, five times more marked than that of the Earth, is at the origin of a very sensitive seasonal asymmetry on Mars: in the northern hemisphere, the longest season is spring (198.6 days), which exceeds the shortest (autumn,146.6 days) by 35.5%; on Earth, the summer of the northern hemisphere, the longest season, exceeds the duration of winter by only 5%. This peculiarity also explains why the area of the southern polar cap is much smaller in summer than that of the boreal polar cap.

The average distance from Mars to the Sun is about 227.937 million kilometers, or 1.523 7 AU. This distance varies between a perihelion of 1.381 AU and an aphelion of 1.666 AU, corresponding to an orbital eccentricity of 0.093 315. The orbital period of Mars is 686.96 Earth days, or 1,880 8 Earth years, and the solar day lasts 24 h 39 min 35,244 s.

Variations in eccentricity

Of the other seven planets in the Solar System, only Mercury has a higher eccentricity than Mars. However, in the past, Mars’ orbit would have been more circular than it is today, with an eccentricity of about 0.002 1.35 million years ago. The eccentricity of Mars would evolve in two superimposed cycles, the first of a period of 96,000 years and the second of a period of 2,200,000 years, so that it is expected to grow further over the next 25,000 years.

Variations in obliquity

Obliquity refers to the tilt of a planet’s axis of rotation on its orbital plane around the Sun. The obliquity of Mars is currently 25.19°, close to that of Earth, but experiences periodic variations due to gravitational interactions with the other planets of the Solar System. These cyclical variations have been evaluated by computer simulations since the 1970s as having a periodicity of 120,000 years in a super-cycle of 1.2 million years with extreme values of 14.9° and 35.5°.

An even longer cycle would be superimposed on this set, of the order of 10 million years, due to an orbital resonance between the rotation of the planet and its orbit around the Sun, likely to have brought to 40 ° the obliquity of Mars, only 5 million years ago. More recent simulations, carried out in the early 1990s, have also revealed chaotic variations in Martian obliquity, the possible values of which would range from 11° to 49°.

Further refined using data collected by Martian probes from the 1990s and 2000s, these numerical simulations have highlighted the preponderance of chaotic variations in Martian obliquity as soon as one goes back more than a few million years, which makes any evaluation of the value of obliquity beyond a few tens of millions of years in the past or future uncertain. A European team has estimated that mars’ obliquity has reached at least 60° in the last billion years at 63%, and more than 89% in the last three billion years.

These variations in obliquity induce very significant climatic variations on the surface of the planet, affecting in particular the distribution of water ice according to latitudes. Thus, ice tends to accumulate at the poles in periods of low obliquity as at present, while it tends to migrate to low latitudes in periods of high obliquity. The data collected since the beginning of the century tend to show that Mars would emerge at this very moment from an “ice age”, in particular, because of the observation of glacial structures (glaciers, fragments of sea ice and permafrost in particular) up to latitudes as low as 30 °, and which seem to be experiencing active erosion.

Since the average atmospheric pressure on the ground depends on the amount of carbon dioxide frozen at the poles, variations in obliquity also have an impact on the total mass of Mars’ atmosphere, with the average atmospheric pressure even falling, in periods of low obliquity, to only 30 Pa (barely 5% of the current standard atmospheric pressure) and induce a warming of 20 to 30 K of the Martian subsoil in reducing the thermal conductivity of the regolith whose average pore size would be comparable to the average free path of gas molecules in such a rarefied atmosphere, which would block the dissipation of the “aerothermal flow”, i.e. the geothermal flux Martian. Such warming could explain many geological formations involving a subsoil laden with liquid water, without the need to invoke a past increase in atmospheric pressure or the planet’s heat flux.

Earth-Mars Oppositions

Mars is the closest outer planet to Earth. The distance between the two planets is the smallest when Mars is in opposition, that is, when the Earth intercalates between Mars and the Sun. However, given the orbital inclination and eccentricity, the precise moment when Mars is closest to Earth may differ by a few days from the time of astronomical opposition. Thus, the opposition of August 28, 2003, took place precisely at 17:58 min 49 s UTC. while the greatest proximity between the two planets had taken place the day before, on August 27, 2003, at 9:51:14 UTC (IMCCE data).

These objections occur approximately every 780 days, the latest of which occurred on October 13, 2020.

Given the respective eccentricity of the orbits of Mars and Earth, the Earth-Mars distance is not constant with each opposition. The eccentricity of Mars being more important than that of Earth, it is when Mars is at perihelion that the rapprochement is most favorable. This situation occurs every fifteen years or so, after seven oppositions. Thus, on August 27, 2003, at 9:51:14 UTC, Mars was 55.758 million kilometers away from Earth, or 0.372 7 in the nineteenth century; it is the closest proximity between Mars and Earth in 59,618 years. An even closer rapprochement is planned for August 28, 2287, with a distance of 55.688 million kilometers.

Date	Distance (to)	Distance (10 m)	Apparent diameter
August 27, 2003	0,372719	55,758	25,13″
August 15, 2050	0,374041	55,957	25,04″
August 30, 2082	0,373564	55,884	25,08″
August 19, 2129	0,373276	55,841	25,10″
August 24, 2208	0,372794	55,769	25,13″
August 28, 2287	0,372254	55,688	25,16″

Taking into account the gravitational influences of other planets on the orbital eccentricity of Mars, which will continue to grow slightly over the next 25,000 years, it is possible to predict even closer proximities: 55.652 million kilometers on September 3, 2650, and 55.651 million kilometers on September 8, 2729.

Mosaic of images in almost true colors giving a panoramic view of the Victoria crater, about 730 m wide, obtained in autumn 2006 by the Opportunity rover on Meridiani Planum.

Geography of Mars

The study of Martian geography dates back to the early 1970s with the Mariner 9 probe, which made it possible to map almost the entire Martian surface with excellent resolution for the time. These are the data collected on this occasion on which the Viking program was based for the development of its Viking 1 and Viking 2 missions. Knowledge of Martian topography made a dramatic leap in the late 1990s thanks to the Mars Orbiter Laser Altimeter (MOLA) instrument of the Mars Global Surveyor probe, which provided access to very accurate elevation surveys over the entire Martian surface.

Repositories

On Mars, the meridian 0 (zero) is the one that passes through the center of the Crater Airy-0.

In the planetocentric system, developed from data acquired by the Mars Global Surveyor’s MOLA and now the most widely used, geographic coordinates are expressed on Mars in the decimal system — not in the sexagesimal system used on Earth — with longitudes increasing eastward from 0 to 360° E, angles being calculated from the equatorial plane for latitudes and from meridian 0 for Longitudes.

In the planetographic system, developed from the data collected by Mariner 9 and today less and less used, the coordinates are expressed decimally with longitudes increasing westward from 0 to 360 ° W depending on a mesh projected on the surface of the planet. In practice, planetographic and planetocentric longitudes are easily deduced from each other, but planetographic latitudes can be higher than planetocentric latitudes by more than a third of a degree in absolute value.

The reference level of Martian altitudes has been arbitrarily defined as the altitude at which the average atmospheric pressure is 610 Pa. This makes it possible to formally define a global equipotential surface from which it is possible to calculate the altitudes at each point of the planet, although in practice the determination of this surface is quite imprecise due to the large seasonal fluctuations in atmospheric pressure resulting from the fact that carbon dioxide, constituting the majority of the atmosphere of Mars., is in equilibrium with frozen carbon dioxide at the poles, a state of equilibrium that varies throughout the year with the seasons.

Quadrangles

To structure the study, the surface of Mars was divided by the USGS into 30 regions of similar size,15 per hemisphere, whose topography established by the MOLA of Mars Global Surveyor and then THEMIS of Mars Odyssey is available on the Internet in the form of maps at ⁄_5,000,000. Each of these quadrangles has been named after one of its characteristic reliefs, but in the literature they are often referenced by their number, prefixed with the code “MC” meaning Mars Chart.

This division into quadrangles is a general method of mapping, first developed on Earth at varying scales, then gradually extended to planets in the Solar System for which geographic data is sufficient to need to be structured. Venus was thus divided into eight quadrangles at ⁄_10,000,000 and 62 quadrangles at ⁄_5,000,000.

Notable features

The map opposite allows you to identify the major Martian regions, including:

• the crustal dichotomy between the northern and southern hemispheres,
• the large impact basins of the southern hemisphere, Argyre by 50° S and 316° E, and Hellas by 42.7° S and 70° E, and to the north Utopia Planitia by 49.7° N and 118° E,
• the bulge of Tharsis and the three volcanoes of Tharsis Montes as well as Olympus Mons and Alba Mons, in the northern hemisphere on the left, and on the right the volcanoes of Elysium, near Utopia,
• the Valles Marineris canyon system, from the Tharsis region to the small impact basin of Chryse Planitia, centered around 15° S and 300° E.

The most striking feature of Martian geography is its “crustal dichotomy”, that is to say the very clear opposition between on the one hand a northern hemisphere consisting of a vast smooth plain at an altitude of half a dozen kilometers below the reference level, and on the other hand a southern hemisphere formed of plateaus often high and very cratered with relief that can be locally quite rugged.

These two geographical areas are separated by a very clear boundary, slightly oblique on the equator. Two volcanic regions close to each other lie precisely on this geological boundary, one of which is a huge uplift of 5,500 km in diameter, the bulge of Tharsis, whose northwest half includes a dozen major volcanoes among which Olympus Mons, while the southern region consists of a vast set of volcanic highlands such as Syria Planum and Solis Planum, and the eastern part is marked by the valles Marineris canyon system extending the Noctis Labyrinthus network from the east.

Two large impact basins are clearly visible in the southern hemisphere, Argyre Planitia and especially Hellas Planitia, at the bottom of which was recorded the greatest depth on the surface of Mars, with an altitude of −8,200 m above the reference level. The highest point is at the top of Olympus Mons, at 21,229 m above the reference level; five of the six highest mountains in the Solar System are Martian volcanoes, four of which are on the Tharsis bulge and the fifth in the second volcanic region of Mars, Elysium Planitia.

Origin of the Martian dichotomy

Two types of scenarios have been proposed to reflect this situation. The former are based on the internal dynamics of the planet, the convection movements of the mantle and a draft of plate tectonics, in the manner of the formation of terrestrial supercontinents at the dawn of the history of our planet.

The latter are based on one or more large impacts resulting in the melting of the bark in the northern hemisphere. The study of the impact basins buried below the surface has also established that the Martian crustal dichotomy dates back more than four billion years before the present, and is, therefore, a structure inherited from the early ages of the planet. Some more recent formations at the boundary between the two domains also suggest an isostatic relaxation of the southern highlands after the volcanic filling of the depression of the northern hemisphere, which also argues for the great antiquity of this dichotomy.

Sunset seen from Gusev crater by the Spirit rover on May 19, 2005 in true colors rendered through filters at 750,530 and 430 nm. The apparent diameter of the Sun seen from Mars is only two-thirds of that seen from Earth. The glow of twilight extends two good hours after the sun passes under the horizon due to the large amount of dust present up to a high altitude in the atmosphere of Mars.

Atmosphere, climates and radiation on Mars

Atmosphere

The exact pressure and composition of Mars’ atmosphere are known from the first in situ analyses carried out in 1976 by the landers of the Viking 1 and Viking 2 probes. The first observer to speculate the existence of an atmosphere around Mars was the German-British astronomer (and composer) William Herschel, who in 1783 attributed to Martian meteorology certain changes observed on the planet’s surface, including white dots interpreted as clouds.

This hypothesis had been challenged at the beginning of the following century with the progress of mirror telescopes, which provided better quality images seeming to show instead a more static surface, until the debate arose at the end of the nineteenth century on the reality of the Mars channels observed in Italy and popularized by the American amateur astronomer Percival Lowell.

Another American, William Wallace Campbell, an astronomer by profession and a pioneer of spectroscopy, remained skeptical of the existence of a large atmosphere around Mars, and announced on the occasion of the opposition of 1909 that he had not been able to detect any trace of water vapor in this possible atmosphere; his compatriot Vesto Slipher, who supported the theory of channels (see Martian Canals), announced the opposite. Based on the albedo variations of the Martian disk, Percival Lowell estimated in 1908 the atmospheric pressure on the ground at 87 mbar (8,700 Pa), a value that would remain more or less the reference until the measurements made by the Mariner 4 probe in 1965.

The difficulty in analyzing the composition of the Martian atmosphere by spectroscopy was then generally attributed to the presence of dinitrogen, difficult to characterize by this technique, and this is how the French astronomer Gérard de Vaucouleurs, who was then working in England, put forward in 1950 the idea that the Martian atmosphere was made up of 98.5% dinitrogen,1.2% argon and 0.25% carbon dioxide.

At the McDonald Observatory in Texas, the Dutch-born American astronomer Gerard Kuiper established in 1952 from the infrared spectrum of Mars that carbon dioxide was at least twice as abundant in the Martian atmosphere as in the Earth’s atmosphere, the bulk of this atmosphere being, like ours, made up of dinitrogen.

Physical and chemical properties

It is now known that Mars has a tenuous atmosphere whose average pressure at the Martian reference level is by definition 610 Pa, with an average temperature of 210 K (−63 °C). It is composed mainly of carbon dioxide CO₂ (96.0 ± 0.7%), argon Ar (1.93 ± 0.01%) and dinitrogenN₂ (1.89 ± 0.03%). This is followed by oxygen _O2 (0.145 ± 0.009%), carbon monoxide CO (< 0.1%), water vapor H₂O (0.03%) and nitric oxide NO (0.013%).

Various other gases are present in trace amounts, at concentrations never exceeding a few parts per million, including neon Ne, krypton Kr, methanal (formaldehyde) HCHO, xenon Xe, ozone O₃ and methane CH₄, the average atmospheric concentration of the latter being of the order of 10.5 ppb. The average molar mass of the gaseous constituents of Mars’ atmosphere would be 43.34 g/mol.

Given the low gravity on the surface of Mars, the height of scale of this atmosphere is 11 km, more than one and a half times that of the Earth’s atmosphere, which is only 7 km. The pressure at the surface varies from just 30 Pa at the top of Olympus Mons and up to 1,155 Pa at the lowest point of the Hellas Planitia impact basin.

In early {2004}, the PFS infrared spectrometer of the European Mars Express probe detected low concentrations of methane (10 ppb) and formaldehyde (130 ppb) in the Martian atmosphere. Since methane is destroyed by ultraviolet radiation after only 340 years, its presence implies the existence of an internal source. Deep geothermal activity, permafrost bombarded by high-energy particles of cosmic radiation, and a methanogenic microbial life form are all plausible sources. In addition, if we consider that formaldehyde, whose lifespan is only 7 hours, is produced by the oxidation of methane, these sources must be even more abundant. Thus, according to this hypothesis, the annual production of methane is estimated at 2.5 million tons.

Clouds

Very pure water can only exist in the liquid state below the Martian reference level, which corresponds roughly to the pressure of the triple point of water, i.e.611.73 Pa: at this level, provided that the temperature is sufficient (0 °C for pure water, but only 250 K (−23 °C) for many saline solutions, or even 210 K (−63 °C) for some mixtures of sulfuric acid solutions H₂SO₄), water can be in its three physical states (gaseous, liquid, and solid).

Above this level, on the other hand, and especially in the atmosphere, it can only exist in the state of water vapor, which sometimes condenses into ice to form clouds of crystals of H₂Od’appearance very similar to that of our cirrus, typically at an altitude of 10 to 20 km; we observe such clouds for example on the flanks of the large volcanoes of the bulge of Tharsis or Elysium Planitia: visible with a telescope from Earth as early as the nineteenth century, the clouds clinging to the top of Olympus Mons had been mistaken for snow, hence the name Nix Olympica which had been given to this region by Giovanni Schiaparelli.

But carbon dioxide also forms clouds, made up of CO2 crystals exceeding 1 μm in diameter, at altitudes higher than those made up of water ice; the OMEGA instrument of the Mars Express probe determined in 2007 that these clouds are likely to absorb up to 40% of solar radiation, causing a 10 K drop in temperature under these clouds, which is not without consequences on the Martian climate, especially on its wind regime.

Dust

The special feature of the Martian atmosphere is that it is constantly loaded with dust, the grains of which have an average diameter of about 1.5 μm, responsible for the ochre hue of the Martian sky. This dust is continuously injected into the atmosphere by dust vortices (commonly referred to as dust devils), such as the one observed below by the Spirit rover on March 12, 2005; the shots last a total of 575 s (as indicated by the counter in the lower left corner), and three other vortices are briefly visible in the distance in the right half of the view, at the beginning of the sequence, then near the main vortex, then at the very end:

Such whirlwinds are far from anecdotal; both their permanence and their accumulation lead to dusting considerable volumes of atmosphere, as illustrated by a striking photograph (opposite), where we see a multitude of black trails left by eddies that have washed away the surface dust layer, of orange-red color characteristic of iron(III) oxide Fe₂O₃ (hematite) amorphous, revealing the deeper layers of darker sand, possibly related to the nearby volcanic region of Syrtis Major Planum.

The layer of dust thus raised is never very massive; the study of the great storm of 2001, during which the dust spread to all atmospheric layers up to 60 km altitude, led to the estimate that if all the dust then raised was deposited uniformly between 58 ° N and 58 ° S, it would form only a film 3 μm thick. The dynamics of dust in the Martian atmosphere are conditioned by the tenuousness of this atmosphere and the low gravity on the surface of the planet. Thus, while Martian dust grains typically have a few micrometers in diameter, it has been calculated that grains of 20 μm can be lifted by winds of as little as 2 m/s and held indefinitely in suspension by turbulences of only 0.8 m/s.

The dust grains suspended in the atmosphere are responsible for the rust color of the latter, which turns blue around the sun at sunset, as discovered by the Viking 1 and Viking 2 probes and that the following probes have illustrated well thereafter:

Observing The atmospheric activity of Mars using the Hubble Space Telescope between 1996 and 1997, when the planet exposed its north pole in early spring, highlighted the role of polar cap sublimation in the generation of air masses that cause winds that raise large amounts of dust and are likely to trigger real dust storms on the scale of an entire planet, like the one that affected the entire Martian atmosphere in the summer of 2001.

Climate

Because of its greater distance from the Sun than from the Earth, Mars receives from the Sun an energy varying from 492 to 715 W/m depending on its position in its orbit, against from 1,321 to 1,413 W/m for the Earth, that is to say from 37.2% to 50.6% between aphelions and perihelions respectively. The Martian atmosphere is also 150 times less dense than that of the Earth, it produces only a negligible greenhouse effect, resulting in an average temperature of about 210 K (−63 °C) on the surface of Mars, with significant daytime variations due to the low thermal inertia of this atmosphere: Viking 1 Lander had thus recorded diurnal variations typically ranging from 184 to 242 K, i. e. from −89 to −31 °C, while the extreme temperatures — quite variable according to the sources — would be about 130 and 297 K, that is to say of the order of −145 and 25 °C.

Seasons

Season (Northern Hemisphere)	Duration on Mars (days)	Duration on Earth (days)
Spring	198,614	92,764
Summer	183,551	93,647
Autumn	146,623	89,836
Winter	158,182	88,997
Year	686,97	365,25

The obliquity of Mars is close to that of the Earth (respectively 25.19° against 23.44°) but the eccentricity of the Martian orbit is significantly higher (0.09332 against 0.01671 for the Earth) so that, if Mars has seasons similar to those of the Earth, these are of very unequal intensity and duration during the Martian year (see table opposite).

The northern hemisphere thus experiences less marked seasons than the southern hemisphere, because Mars is at its aphelion in late spring and perihelion in late autumn, resulting in short and mild winters and long and cool summers; spring thus lasts 52 days longer than autumn. Conversely, the southern hemisphere experiences very marked seasons, with long and very cold winters while summers are short and warmer than those of the northern hemisphere. It is therefore in the southern hemisphere that the highest temperature differences are observed.

NASA’s Mars24 Sunclock simulator gives the following dates for the northern hemisphere for the start of each season:

Spring	January 21, 2006	December 9, 2007	October 26, 2009	September 13, 2011	July 31, 2013	June 18, 2015
Summer	August 7, 2006	June 24, 2008	May 12, 2010	March 29, 2012	February 14, 2014	January 2, 2016
Autumn	February 7, 2007	December 25, 2008	November 12, 2010	September 29, 2012	August 17, 2014	July 4, 2016
Winter	July 4, 2007	May 21, 2009	April 7, 2011	February 22, 2013	January 10, 2015	November 27, 2016

Towards the end of the austral spring, when Mars is closest to the Sun, local and sometimes regional storms appear. Exceptionally, these storms can become global and last several months as was the case in 1971 and, to a lesser extent, in 2001.

Tiny grains of dust are then lifted, making the surface of Mars almost invisible. These dust storms usually originate over the Hellas Basin. The large thermal differences observed between the pole and the surrounding regions cause strong winds causing fine particles to rise in the atmosphere. During global storms, this phenomenon causes significant climatic changes: suspended dust absorbs solar radiation, thus warming the atmosphere and at the same time reducing ground insolation. Thus, during the storm of 2001, the atmospheric temperature rose by 30 K while the ground temperature dropped by 10 K.

There is only one Hadley cell on Mars but much more marked in altitude and amplitude, joining the two hemispheres and which reverses twice a year.

Finally, the obliquity of the planet, which is not stabilized by the presence of a massive satellite as is the case for the Earth, follows a chaotic regime according to cyclicity of about 120,000 years. It oscillates between 0° and 60° and experiences relatively stabilized phases interspersed with sudden changes, which completely upsets the Martian climate.

Winter condensation of the atmosphere at the poles

One of the characteristics of Mars is that a significant fraction of its atmosphere condenses alternately at the South Pole and the North Pole during the austral winter and the boreal winter respectively. The winter conditions at the poles — pressure and temperature — are indeed favorable to the condensation of carbon dioxide: the saturating vapor pressure of CO₂ at 150 K (−123 °C) is close to 800 Pa, and falls to only 400 Pa to 145 K (−128 °C), which are common temperatures during the austral winter; there is condensation of CO₂ as soon as the partial pressure of this gas exceeds the saturating vapor pressure corresponding to the temperature at which it is located.

The Viking 1 probe measured atmospheric pressure over a full year at its landing point at 22.697° N and 312.778° E in the Chryse Planitia Basin, at an altitude of about −3,300 m above baseline. The average atmospheric pressure has been shown to change throughout the year according to the seasons, with approximate values, in round numbers, of 850 Pa in spring,680 Pa in summer,900 Pa in autumn and 800 Pa in winter: these variations can be explained well if we consider that the southern winter ice cap condenses a mass of dry ice greater than that of the boreal winter ice cap, while in the autumn of the northern hemisphere, most of the southern cap has sublimated while the boreal cap is just beginning to condense.

Polar caps

The polar caps of Mars were first observed in the mid-seventeenth century by Jean-Dominique Cassini and Christian Huygens. Their size varies considerably over the seasons by exchanging carbon dioxide and water with the atmosphere. Thus, in both hemispheres, there is a polar cap called “residual” or “summer” which is maintained throughout the summer, and a polar cap called “seasonal” or “winter” which comes to cover it from autumn.

Because the austral winter is longer and colder than the boreal winter, the southern seasonal cap is larger than the boreal seasonal cap. During the austral winter, the CO₂ contained in the atmosphere condenses into dry ice above 55° S while it is rather above 65° N that it condenses during the boreal winter. It is a very pure and almost transparent carbon dioxide (CO₂) ice, with a thickness not exceeding a few meters, which shows the ground plumb on the images taken by space probes orbiting over the polar regions.

With its 300 km diameter, the southern residual cap is three times smaller than the boreal residual cap (1,000 km in diameter). They are very different in nature from seasonal ice caps, containing a high proportion of water ice mixed with the earth with a stratified structure revealed by the THEMIS instrument of the 2001 Mars Odyssey probe, with a thickness locally reaching several kilometers. Their surfaces are notched by deep valleys, called chasmata (plural of the Latin chasma designating steep valleys), which form spirals whose direction of rotation is conditioned by the Coriolis force. Thus, the valleys wrap around the South Pole clockwise while they wrap around the North Pole in the opposite direction.

The residual boreal cap does not contain dry ice, but the residual southern ice cap is almost entirely covered with a crust about ten meters thick whose honeycomb surface recalls that of a slice of Gruyère; observations by the Mars Global Surveyor probe showed that the average diameter of the alveoli increased with the seasons, suggesting global warming in the southern hemisphere (see next paragraph).

Polar caps have a significant impact on the global atmospheric composition of the planet. The cycle of CO2 condensations and sublimations varies atmospheric pressure by almost a third, and during the boreal summer, the water ice that makes up the northern residual polar cap sublimates, injecting large amounts of water vapor into the atmosphere. If all the water vapor in the atmosphere were to precipitate, it would form a layer less than 10 μm thick during the winter and more than 40 μm in the middle of summer.

Climatic variations observed on the southern residual cap

A comparison of the photographs of the southern residual cap taken by Mars Global Surveyor in 1999 and 2001 revealed a general trend towards regression of the surface dry ice crust of this region. This would result from the gradual sublimation of CO₂ constituting the surface crust of the southern residual cap to reveal the deeper layers, consisting essentially of water ice mixed with dust.

This phenomenon seems to have been quite rapid, the edge of the cavities observed in the dry ice crust then progressing by 3 m per Martian year. Unambiguously observed over three consecutive Martian years, this trend towards sublimation of the southern residual cap has been added to various observations elsewhere on the planet, such as the appearance of gullies on the edges of craters or depressions, indicating that the Martian surface is subject to more transformations than previously thought.

These data, interpreted by scientists as a sign that Mars could currently experience a transition between an ice age and an interglacial period similar to that experienced by Earth nearly 12,000 years ago, have sometimes been understood by the general public as indicative of a “Martian global warming”, necessarily of non-human origin, and contradicting, therefore, the conclusions of the IPCC Fourth Report on the human origin of global warming. Discussions on the issue were particularly acute in the autumn of 2007, following the publication of this report.

In hindsight, however, it appears that Martian observations have never indicated anything other than global warming localized to the southern residual cap, and not global warming. In addition, the Martian climate is largely conditioned by dust storms and the resulting albedo variations, more than by solar radiation — unlike Earth’s climate — which limits the relevance of reasoning drawing parallels between the two planets. And, above all, the most recent observations, in particular those of the 2001 Mars Odyssey probe, which is still in operation in 2018, do not confirm the long-term trend towards sublimation of the polar caps, but would instead indicate annual variations around a stable value.

Radiation

The absence of a magnetosphere around Mars has the consequence of directly exposing the surface of the planet to cosmic rays and puffs of solar protons, causing an ambient radioactivity much higher on Mars than that recorded on the surface of the Earth.

The MARIE instrument — Mars Radiation Environment — of the 2001 Mars Odyssey probe made it possible, in the years 2002-2003, to evaluate the effective dose in Martian orbit between 400 and 500 mSv/year, at least four times that received in the International Space Station (50 to 100 mSv/year, while on the ground, at the Martian reference level, the doses received would be two to three times lower — just under 200 mSv/year — due to the absorption of some of the solar and galactic radiation by the atmosphere of Mars.

By way of comparison, the average radioactivity on Earth in France is about 3.5 mSv/year and the cumulative dose allowed for an astronaut over his entire career, regardless of sex and age, does not exceed 1,000 mSv for several space agencies (European, Russian and Japanese).

The MARIE instrument also revealed that this radioactivity is very unevenly distributed over time, with a background noise of about 220 μGy/day on which are inscribed peaks sometimes 150 times more intense, corresponding to the puffs of energetic protons – several tens of mega electron-volts – emitted during a solar flare or by the shock wave of a coronal mass ejection.

Added to this is the radiation due to neutrons emitted by the spallation of atoms on the surface of Mars under the impact of cosmic radiation. This contribution is estimated using data from Curiosity and 2001 Mars Odyssey at up to 45 ± 7 μSv per day, or about 7% of the total radiation at the surface.

Geology of Mars

Martian geological time scale

Martian geology is marked by the crustal dichotomy between the low low low-cratered plains of the northern hemisphere and the highly cratered highlands of the southern hemisphere, with, between these two main domains, two well-differentiated volcanic regions. By virtue of the empirical principle that the age of a region is an increasing function of its craterization rate, these three major types of Martian terrain were very early linked to three characteristic epochs of the planet’s geological history, named after regions typical of these periods:

Noachian

The Noachian (named after Noachis Terra) corresponds to the oldest terrains, from the formation of the planet 4.6 billion years ago, up to 3.7 billion years according to the Hartmann Neukum scale (but 3.5 billion years according to the standard Hartmann scale), highly cratered and located mainly in the southern hemisphere.

Mars probably had a thick atmosphere at that time, whose pressure and greenhouse effect certainly allowed the existence of a hydrosphere thanks to large amounts of liquid water. The end of this period would have been marked by the asteroid impacts of the great late bombardment, dated to around 4.1 to 3.8 billion years ago, as well as by the beginning of intense volcanic activity, especially in the region of the Tharsis bulge.

Hesperian

The Hesperian (named Hesperia Planum) corresponds to terrains 3.7 to 3.2 billion years ago on the Hartmann Neukum scale (but 3.5 to 1.8 billion years ago on the standard Hartmann scale), marked by an episode of major volcanic activity resulting in lava flows and sulfur deposits. The global magnetic field would have disappeared as soon as the Noachian ended, allowing the solar wind to erode the atmosphere of Mars, whose temperature and ground pressure would have begun to drop significantly, so that liquid water would have ceased to exist permanently on the surface of the planet.

Amazonian

The Amazonian (named After Amazonis Planitia) corresponds to soils less than 3.2 billion years old according to the Hartmann Neukum scale (but only 1.8 billion years old according to the standard Hartmann scale), very little cratered and located mostly in the northern hemisphere, at an altitude below the reference level of the planet.

Volcanic activity would have continued, losing its intensity throughout this period, in four major episodes, the last occurring about a hundred million years ago, some volcanic terrains even seeming to be only a few million years old. The erosion of the atmosphere by the solar wind would have lasted for billions of years until the pressure stabilized in the vicinity of the triple point of pure water, whose pressure is 611.73 Pa. Amazonian geological structures are marked by the extreme aridity of the Martian environment, then totally devoid of a hydrosphere — which does not prevent the discontinuous and episodic existence of liquid water at certain points on the surface.

This chronology in three epochs is now well accepted — the dating of each of these epochs remains, on the other hand, very uncertain — and makes it possible to account for the phenomena observed on the surface of Mars by the various probes active around this planet, in particular the simultaneous presence of minerals, formed at different times, assuming for some a very humid environment and for others on the contrary the total absence of liquid water. The proposed dates for these three geological epochs — or eons — according to the standard Hartmann scale and the Hartmann Neukum scale, are as follows (ages in millions of years):

Chemical composition

Between the 1970s and 2010s, models of the composition of Mars were based on that of carbonaceous chondrites of type CI, considered representative of the condensable part of the protosolar nebula, and on the condensation models of the nebula, given the distance from Mars to the Sun. They essentially accepted that the relative proportions of elements as refractory or more refractory than manganese were those of ICs, and that those of less refractory elements were given by their correlations with refractory elements, observed or deduced from condensation models.

At the beginning of the twenty-first century, discrepancies arose between spectroscopic data on the composition of the solar photosphere and other approaches to the composition of the Sun (helioseismology, solar neutrino flux, composition of the solar wind and experimental data on the opacity of metals in high-temperature plasmas), which called into question the representativeness of ICs.

The isotopic compositions (in particular elements O, Ni, Cr, Ti, Mo and W) and the trace element contents have also led to the consideration of carbonaceous chondrites separately from other chondrites (essentially ordinary chondrites and enstatite chondrites). ), the former remain representative of bodies accreted far from the Sun but the latter are now considered more representative of condensed matter in the inner areas of the Solar System (including Earth and Mars).

A new composition model, based on analysis of Martian meteorites, measurements from Martian probes and correlations observed in non-carbonaceous chondrites, implies refractory element contents 2.26 times higher than those of ICs, and lithophilic element contents. moderately volatile systematically lower (the ratio depending on the condensation temperature of each element). One of the consequences of this model is that the Martian core would have less than 7% pds of sulfur (against more than 10% according to previous models) but on the other hand a little oxygen and hydrogen.

Estimates prior to the Insight mission

In the absence of actionable seismic data — the seismometers of the Viking probes were too sensitive to wind to make reliable measurements — it was long not possible to directly determine the internal structure of the planet. A standard model was therefore developed from the indirect data collected by the various probes that explored the planet, making it possible to specify in particular the structure of its gravitational field, its moment of inertia and the density of its different layers of materials.

The most striking result is that the core of Mars, whose temperature would be of the order of 2,000 K, is most certainly liquid, at least for the most part, due to a high charge — precisely a weight fraction of at least 14.2% — of light elements, especially sulfur, which lower the melting point of the mixture of iron and nickel. supposed to constitute the bulk of the core.

This core would have a radius of between 1,300 and 2,000 km (between 38% and 59% of the radius of the planet), perhaps more precisely between 1,520 and 1,840 km (or between 45% and 54% of the radius of Mars), uncertainty due in part to the unknown concerning the fraction of mantle that could be liquid and would therefore reduce the size of the core; we find quite often cited the value 1,480 km as the radius of the core of Mars, or 43.7% of the average radius of the planet itself (3,389.5 km).

The physical characteristics (size, density) of the core can be approximated qualitatively by the moment of inertia of the planet, which can be evaluated by analyzing the precession of its axis of rotation as well as the variations in its speed of rotation through the Doppler modulations of the radio signals emitted by the probes placed on its surface; the data from Mars Pathfinder have thus made it possible to refine those previously collected with the Viking probes and to establish that the mass of Mars is rather concentrated in its center, which argues for a dense and not too large core.

The mantle of Mars would be very similar to that of Earth, consisting of solid phases dominated by iron-rich silicates, the latter representing a weight fraction of 11 to 15.5% of the mantle.

The Martian crust seems, in coherence with the topography, much thicker in the southern hemisphere than in the northern hemisphere: a simple model with a uniform density of 2,900 kg/m leads to an average thickness of about 50 km, or 4.4% of the volume of the planet, with extreme values of 92 km in the Syria Planum region and barely 3 km under the Impact Basin of Isidis Planitia, while the crust would be less than 10 km under the entire region of Utopia Planitia.

Results of the Insight mission

The InSight lander was built with the aim of studying the internal structure of Mars using the SEIS seismometer. On April 6, 2019, it provides the first recording of a Martian earthquake.

In 2021, the seismic data collected make it possible for the first time to determine with certainty the radius of the Martian core: between 1,810 and 1,860 km, or about half that of the Earth’s core. This result, significantly higher than estimates based on mass and moment of inertia, implies that the Martian core contains light elements, possibly oxygen, in addition to iron-nickel and sulfur.

Magnetic field

Mars does not have a magnetosphere. However, the MAG/ER magnetometer and electron reflectometer of the Mars Global Surveyor probe showed as early as 1997 a persistent magnetism, up to 30 times greater than that of the Earth’s crust, over certain geologically ancient regions of the southern hemisphere, and in particular in the region of Terra Cimmeria and Terra Sirenum.

Measurements show a magnetic field reaching 1.5 μT at an altitude of 100 km, which requires the magnetization of a significant volume of Martian bark, at least 10 km. For nine years, Mars Global Surveyor has measured magnetic parameters above the Martian surface, with the MGS MAG (MGS Magnetometer) instrument collecting vector data from a typical altitude of 400 km, sometimes approaching 90 km from the surface, and MGS ER (MGS Electron Reflectometer) measuring total magnetism from an altitude of 185 km on average. There is therefore currently no magnetic map of the Martian surface itself, just as the exact nature of magnetized minerals can only be assumed in the current state of our knowledge.

Geography of Martian paleomagnetism and minerals involved

The study of the meteorites of Mars suggests that this paleomagnetism results, as on Earth, from the magnetization of ferromagnetic minerals such as magnetite Fe₃O₄ and pyrrhotite Fe_1-δS whose atoms align their magnetic moment with the global magnetic field and freeze this configuration by passing below the Curie temperature of the mineral, i. e.858 K (585 °C) for magnetite, but only 593 K (320 °C) for pyrrhotite.

Other candidate minerals as vectors of paleomagnetism of the Martian bark are ilmenite FeTiO₃ in solid solution with hematite Fe₂O₃, of the same structure, to form titanohematites, and to a lesser extent titanomagnetite Fe₂TiO₄, whose magnetization and Curie temperature are however lower.

The absence of such paleomagnetism over southern hemisphere impact basins such as Hellas and Argyre is generally interpreted as an indication that Mars no longer possessed a global magnetic field at the time of these impacts, although it is also possible that the cooling of the materials at the impact site was too rapid to allow their eventual magnetization to align with the global magnetic field.

On the other hand, significant paleomagnetism, and sometimes even quite high, has been noted above the 14 oldest basins identified on the planet. Similarly, no significant magnetic field was detected over the major volcanic regions of Elysium Planitia and the Tharsis bulge, but weaker but higher intensity magnetism was found over the smaller, older volcanic provinces of the southern highlands.

The analysis of the three-dimensional components of the magnetic field recorded at a few dozen significant points on the Martian surface allowed several teams to extrapolate the position of the paleomagnetic pole of Mars. These simulations – which must nevertheless be taken with some hindsight – are quite consistent with each other and lead to the location of one of the Martian paleomagnetic poles between 150° E and 330° E on the one hand and 30° S and 90° N on the other hand, that is to say approximately within a radius of 3,600 km around a point halfway between Alba Mons and Olympus Mons.

Reversals of polarity and disappearance of global magnetism

Remarkably, the magnetization measured by Mars Global Surveyor is structured in parallel bands of opposite polarity, reminiscent of those of the seafloor on Earth (see diagram opposite): it crystallizes on both sides of the ridges as the plates move apart by “memorizing” the orientation of the Earth’s magnetic field at the time of solidification; each reversal of the Earth’s magnetic field is therefore “recorded” in the rocks thus formed, whose magnetization is therefore symmetrical on each side of each ridge.

Such symmetry, however, has never been found on Mars, so there is currently no evidence to suggest the past existence of any plate tectonics on the red planet. Only a comment at higher resolutions would bring the debate to a close.

When global, a planet’s magnetic field is mainly of internal origin. It is assumed that it is caused by the convection of conductive fluids (i. e. liquid metals) composing the outer part of the core. This process is known as the dynamo effect. These convection movements imply the existence of a sufficient thermal gradient from the core to the mantle; in the absence of such a gradient, the dynamo effect could not be maintained.

This fact would be at the origin of the disappearance of the global magnetic field of Mars, probably at least four billion years ago: the asteroid impacts of the great late bombardment would have injected enough thermal energy into the mantle of Mars by converting into heat the kinetic energy of the impactors, which would have stopped the dynamo effect by canceling the thermal gradient necessary to maintain it.

Origin of the magnetic dichotomy between the Northern and Southern hemispheres

The attribution of the disappearance of the Martian global magnetic field to a cosmic impact has been taken up in an alternative theory involving this time a residual protoplanet the size of the Moon hitting Mars well before the great late bombardment, that is to say, only a few tens of millions of years after the formation of the planet (similar to the hypothetical impact of Theia with the proto-Earth), in the vicinity of the current North Pole and from a rather low angle of incidence.

This impact would be at the origin on the one hand of the crustal dichotomy (the idea is not new, overlapping the theory, quite discussed, of the boreal basin) and on the other hand of the absence of paleomagnetism in the bark of the northern hemisphere, due to the disappearance of the thermal gradient between the core and the mantle in the northern hemisphere only, leaving a dynamo effect concentrated in the southern hemisphere.

Mars would thus have transiently experienced a magnetism not global, but “hemispherical” and off-center towards the south pole, which would explain the exceptional intensity of residual magnetism in certain parts of the bark of the southern hemisphere, as well as the absence of significant paleomagnetism in the northern hemisphere.

This theory is not the only one proposed to account for the superposition of a “magnetic dichotomy” to the Martian crustal dichotomy: the difference in thickness and structure of the Martian bark between the two hemispheres, the partial melting of the bark of the northern hemisphere at the origin of the remodeling of its surface, and the serpentinization of the Martian bark in Noachian, are the most commonly advanced explanations.

Northern

Auroras can occur above magnetic anomalies in the Martian crust. In all likelihood, however, they cannot be perceived by the human eye, as they emit mainly in the ultraviolet.

Volcanism of Mars

Martian volcanism is said to have begun nearly four billion years ago, at the end of the Noachian after the great late bombardment. It would have experienced its maximum intensity in the Hesperian – between 3.7 and 3.2 Ga according to the Hartmann Neukum scale – and then gradually weakened throughout the Amazonian.

It produced huge shield volcanoes that are the largest known volcanic edifices in the Solar System: the widest of them, Alba Mons, has a diameter of about 1,600 km at the base, while the largest is Olympus Mons, on the western margin of the Tharsis bulge, which reaches 22.5 km high from base to summit. It has also produced many stratovolcanoes, much smaller, several hundred small volcanoes a few hundred meters wide (for example on Syria Planum) as well as lava plains, similar to the volcanic expanses identified on the Moon, Venus or Mercury.

Lava plains

The oldest form of Martian volcanism, dating back to the end of the Noachian and lasting until the beginning of the Hesperian, would be that of the basaltic expanses that cover the bottom of the impact basins of Argyre Planitia and Hellas Planitia, as well as some flat and smooth expanses located between these two basins and that of Isidis, reminiscent of the smooth volcanic terrain identified on Mercury (for example Borealis Planitia), on Venus (typically Guinevere Planitia) and on the Moon ( the lunar “seas”), mostly correlated with cosmic impacts.

On Mars, these Noachian lava plains constitute the regions of Malea Planum, Hesperia Planum and Syrtis Major Planum, which present themselves as basaltic plateaus whose surface, typical of the Hesperian, is geologically more recent. The dynamics underlying this type of volcanism, between crack and hot spot, is not really understood; in particular, it is not fully explained that the volcanoes of Malea, Hesperia and Elysium are more or less aligned on more than a third of Martian circumference.

Typology and distribution of Martian volcanoes

Martian volcanism is best known for its shield volcanoes, the largest in the Solar System. This type of volcano is characterized by the very slight slope of its flanks. On Earth, such a volcano results from effusions of lava poor in silica, very fluid, which flow easily over large distances, forming flattened structures spreading over very large surfaces, unlike, for example, stratovolcanoes, whose cone, well-formed, has a much smaller base. The very type of shield volcano is, on Earth, Mauna Loa, in Hawaii; the Piton de la Fournaise, in Reunion, is another, smaller but very active.

The most emblematic of the Martian shield volcanoes, Olympus Mons, is some 22.5 km high and 648 km wide and has a summit caldera of 85 × 60 × 3 km resulting from the coalescence of six separate craters. Mars has the five highest known volcanoes in the Solar System (altitudes given relative to the Martian reference level):

1. Olympus Mons (21,229 m), on the western margin of the Tharsis bulge;
2. Ascraeus Mons (18,225 m), northern volcano of Tharsis Montes;
3. Arsia Mons (17,761 m), southern volcano of Tharsis Montes;
4. Pavonis Mons (14,058 m), central volcano of Tharsis Montes;
5. Elysium Mons (14,028 m), main volcano of Elysium Planitia.

For comparison, the highest Venusian volcano, Maat Mons, rises only about 8,000 m above the average radius of Venus, which serves as the reference level on this planet.

On Mars is also the widest of the volcanoes in the Solar System, Alba Mons, whose altitude does not exceed 6,600 m but which extends about 1,600 km wide.

Martian shield volcanoes reach gigantic sizes compared to their terrestrial counterparts due to the lack of plate tectonics on Mars: the Martian bark remains motionless in relation to hot spots, which can thus pierce it in the same place for very long periods of time to give rise to volcanic edifices resulting from the accumulation of lava for sometimes several billion years, whereas, on Earth, the displacement of lithospheric plates above these hot spots leads to the formation of a string of sometimes several dozen volcanoes, each remaining active for only a few million years, which is far too brief to allow the formation of structures as imposing as on Mars.

The Hawaiian archipelago is the best terrestrial example of the displacement of a tectonic plate over a hot spot, in this case the Pacific plate above the Hawaii hot spot; similarly, the Mascarene archipelago results from the displacement of the Somali plate over the reunion hot spot.

The six Martian shield volcanoes are geographically divided into two neighboring volcanic regions of unequal importance:

• the region of Elysium Planitia, west of Amazonis Planitia, where Elysium Mons is located, which appears to be of a different nature (less “red” and more “grey”) from other volcanoes, and three other smaller volcanoes;
• the Tharsis Bulge, a huge crustal uplift 5,500 km in diameter southeast of Amazonis, where the other five large Martian shield volcanoes are located as well as countless smaller volcanoes, of which only five have been given a name.

These smaller volcanoes are often anonymous shield volcanoes, like those of Syria Planum, but some of the intermediate size are more reminiscent of stratovolcanoes, which result from the accumulation of lava deposits mixed with volcanic ash. These are the tholi (Latin plural of tholus), buildings of more modest size than the shield volcanoes, with steeper slopes, especially near the crater, as well as the paterae, which are sometimes reduced to their caldera. All these types of volcanoes are present in the regions of the bulge of Tharsis and Elysium Planitia, the general trend being however to observe the shield volcanoes rather in the region of Tharsis while the volcanoes of Elysium are more similar to stratovolcanoes.

Origin and chronology of Martian volcanism

The discontinuity between Phyllosian and Theiikien, which would more or less coincide with the beginnings of the hypothetical “great late bombardment” (LHB in English), would materialize the epoch of maximum volcanic activity, which would extend to the Theiikien and the Siderikian — and thus to the Hesperian and the Amazonian — by gradually disappearing as the planet has lost most of its internal activity. A correlation between the volcanism of the Hesperian and the cosmic impacts of the Noachian is not to be excluded. This volcanism would have reached its maximum as a result of the massive cosmic impacts at the end of the previous eon, and each of the five volcanic regions of the planet directly adjoins an impact basin:

• the bulge of Tharsis, the largest Martian volcanic formation, on the edge of the hypothetical boreal basin, the largest impact basin on the planet (and the Solar System), the shield of Alba Mons being, moreover, located exactly at the antipodes of Hellas Planitia;
• the region of Elysium Mons, bordering Utopia Planitia and neighboring the antipodes of Argyre Planitia;
• Malea Planum, on the southwestern edge of Hellas Planitia, and Hesperia Planumon the northeast edge, the latter region also being close to the antipodes of Chryse Planitia;
• Syrtis Major Planum, on the edge of Isidis Planitia.

The area and mass of Mars being respectively 3.5 and 10 times smaller than those of the Earth, this planet has cooled faster than ours and its internal activity has therefore also decreased faster: while volcanism and, more generally, tectonics (orogeny, earthquakes, plate tectonics), etc. ) are still very active on Earth, they no longer seem to be noticeable on Mars, where no plate tectonics, even past, could ever be highlighted.

Martian volcanism also seems to have ceased to be active, although the age, it seems very recent, of some lava flows suggests, for some volcanoes, an activity currently very reduced, but perhaps not rigorously zero, especially since Mars, unlike the Moon, has not finished cooling, and its interior, far from being entirely frozen, actually contains a perhaps entirely liquid core. In general, the analysis of the data collected by Mars Express led a team of ESA planetary scientists led by the German Gerhard Neukum to propose a sequence in five volcanic episodes:

• 1: major volcanic episode of the Hesperian about 3.5 billion years ago;
• 2 and 3: revival of volcanism about 1.5 billion years ago, then between 800 and 400 million years before the present;
• 4 and 5: Recent volcanic episodes of rapidly decreasing intensity about 200 and 100 million years ago.

These dates are based on the evaluation of the craterization rate of the corresponding lava flows, which seems to be cross-checked by indirect observations over the medium term but contradicted by direct short-term observations deduced from the frequency of recent impacts observed over more than ten years by satellite probes around Mars, the main difficulty of this type of dating being to evaluate the statistical biases introduced by the significant difference in orders of magnitude between ancient surfaces (more than 2 billion years old), which represent a significant fraction of Mars’ surface, and the most recent surfaces (less than 200 million years old), which are comparatively extremely small.

Moreover, if the frequency of recent impacts recorded by satellite probes around Mars seems to suggest a higher rate of craterization than that usually used to date Martian formations (which would lead to having to “rejuvenate” all these dates), it would rather seem that, in the long term, this rate of craterization has on the contrary been divided by three for 3 billion years, which would tend to “age” Martian dating, especially since they are related to recent phenomena.

Chemistry and Mineralogy on Mars

The mineralogy of the Martian surface could long only be approached through the study of a few dozen meteorites from Mars. Although few in number and restricted to limited geological epochs, these meteorites make it possible to assess the importance of basaltic rocks on Mars. They highlight the differences in chemical composition between Mars and Earth and testify to the presence of liquid water on the planet’s surface more than four billion years ago. “Orbiters”, whose spectrometers make it possible to determine the nature of the solid phases present on the surface, and landers, which can chemically analyze the composition of samples taken from rocks or the ground, have since allowed us to refine our knowledge of Martian minerals.

In situ analyses by landers

As early as the 1970s, the Viking 1 and Viking 2 probes analyzed the Martian soil, revealing a nature that could correspond to basalt erosion. These analyses showed a high abundance of silicon Si and iron Fe, as well as magnesium Mg, aluminum Al, sulfur S, calcium Ca and titanium Ti, with traces of strontium Sr, yttrium Y and possibly zirconium Zr. The level of sulfur was almost twice as high, and that of potassium five times lower, than the average of the Earth’s crust. The soil also contained sulfur and chlorine compounds resembling evaporite deposits, resulting on Earth from the evaporation of seawater.

The sulfur concentration was higher at the surface than at depth. Experiments to determine the presence of possible microorganisms in the Martian soil by measuring the release of oxygen after the addition of “nutrients” measured a significant release of O2 molecules, which, in the absence of other biological traces otherwise noted, was attributed to the presence of O2 superoxide ions. In the autumn of 1997, Mars Pathfinder’s APXS spectrometer carried out a set of measurements expressed as a percentage by weight of oxides that complemented these results with those of a different region of the Mars surface.

The reddish hue of the planet comes above all from the iron(III) oxide Fe₂O₃, omnipresent on its surface. This amorphous hematite (crystallized hematite, on the other hand, is gray in color) is very present on the surface of rocks as well as dust grains carried by the winds that continually sweep the surface of the planet, but does not seem to penetrate very deep into the ground, judging by the traces left since the winter of 2004 by the wheels of the roots of Mars Exploration Rover, which show that the rust color is that of the dust layers, thicker and covered with dark dust for Opportunity, while the rocks themselves are significantly darker.

In addition, the soil of Mars analyzed in situ by the Phoenix probe in autumn 2008 was found to be alkaline (pH ≈ 7.7 ± 0.5) and contain many salts, with a high abundance of potassium K, Cl chlorides, ClO₄ perchlorates and magnesium. Mg. The presence of perchlorates, in particular, has been extensively commented, because a priori is not compatible with the possibility of Martian life. These salts have the particularity of significantly lowering the melting temperature of water ice and could explain the “gullies” ( gullies in English – regularly observed by probes orbiting the planet, which would thus be the traces of brine flows on sloping ground.

Generally speaking, Martian rocks have been found to be predominantly of a tholeiitic basaltic nature.

In 2018, the SAM mini-laboratory aboard the Curiosity rover detects organic compounds (thiophenic, aromatic and aliphatic) in the soils of Mojave Crater and Confidence Hills.

Results collected by orbiters

American (notably 2001 Mars Odyssey and Mars Reconnaissance Orbiter) and European (Mars Express) probes have been studying the planet globally for several years (2002,2006 and 2003 respectively), broadening and refining our understanding of its nature and history. While they confirmed the predominance of basalts on the planet’s surface, these probes also collected some unexpected results.

Olivine and pyroxenes

Thus, ESA’s Mars Express probe has an instrument called OMEGA — “Observatory for Mineralogy, Water, Ice and Activity” — of essentially French realization, under the responsibility of Jean-Pierre Bibring, of the IAS in Orsay, which measures the infrared spectrum (in wavelengths). between 0.35 and 5.2 μm) of the sunlight reflected from the Martian surface in order to detect the absorption spectrum of the various minerals that compose it.

This experiment was able to confirm the abundance of igneous rocks on the surface of Mars, including olivines and pyroxenes, the latter having a lower calcium level in the cratered highlands of the southern hemisphere than in the rest of the planet, where it is found with olivine. ; thus, the oldest materials of the Martian bark would have formed from a mantle depleted of aluminum and calcium.

Olivines and pyroxenes are the main constituents of peridotites, plutonic rocks well known on Earth to be the main constituent of the mantle.

Phyllosilicates, aqueous weathering of igneous rocks

A breakthrough in understanding the history of Mars was OMEGA’s identification of phyllosilicates widely distributed in the oldest regions of the planet, revealing the prolonged interaction of igneous rocks with liquid water. The COMPACT Reconnaissance Imaging Spectrometer for Mars (CRISM ) instrument of the Mars Reconnaissance Orbiter probe has made it possible to clarify the nature of these minerals.

Hydrated chlorides and sulfates, markers of a wet past

OMEGA has also made it possible to detect, in many parts of the world, hydrated sulfates, such as, for example, mgSO₄•H₂ O kieserite in the Meridiani Planum region, or even, in the Valles Marineris region, even more hydrated sulfates whose mineralogical nature has not been possible to identify, as well as deposits of CaSO₄•2H₂O gypsum on kieserite at the bottom of a dry lake, indicating a change in the saline nature of this body of water during its drying, from magnesium sulfate to calcium sulfate.

Large tracts of hydrated calcium sulfate, presumably gypsum, have also been detected at the edge of the boreal polar cap. The presence of these hydrated minerals is a strong indication of the past presence of bodies of liquid water on the surface of Mars, water containing dissolved magnesium and calcium sulfates.

The 2001 Mars Odyssey probe also detected the presence of chlorides in the highlands of the southern hemisphere, resulting from the evaporation of saltwater bodies not exceeding 25 km in various places of these ancient terrains dating back to the Noachian or, for some, to the early Hesperian.

Methane and hydrothermalism in the Nili Fossae region

One of the most astonishing results of the Mars Reconnaissance Orbiter comes from the detailed study in 2008 of the Nili Fossae region, identified in early 2009 as a source of significant methane releases. Methane was detected as early as 2003 in the atmosphere of Mars, both by probes such as Mars Express and from Earth; these CH₄ emissions would be concentrated in three particular zones of the Syrtis Major Planum region.

However, methane is unstable in the Martian atmosphere, recent studies even suggesting that it is six hundred times less stable than initially estimated (its average lifespan was estimated at 300 years) because the methane level does not have time to standardize in the atmosphere and remains concentrated around its emission zones, which would correspond to a lifespan of a few hundred days; the corresponding methane source would also be 600 times more powerful than initially estimated, emitting this gas about sixty days a Martian year, at the end of the summer of the northern hemisphere.

Geological analyses conducted in 2008 by the Mars Reconnaissance Orbiter probe in the Nili Fossae region revealed the presence of ferromagnesian clays (smectites), olivine (ferromagnesian silicate (Mg, Fe)₂SiO₄, detected as early as 2003) and magnesite (magnesium carbonate MgCO₃), revealing the presence of clays rich in iron, magnesium, olivine and magnesium carbonate as well as serpentine.

The simultaneous presence of these minerals makes it possible to explain the formation of methane quite simply, because, on Earth, methane CH₄ is formed in the presence of carbonates — such as MgCO₃ detected in 2008 — and liquid water during the hydrothermal metamorphism of iron(III) oxide Fe₂O₃ or olivine (Mg, Fe)₂SiO₄ in serpentine(Mg, Fe)₃If₂O₅(OH)₄, especially when the level of magnesium in olivine is not too high and when the partial pressure of carbon dioxide CO₂ is insufficient to lead to the formation of talcMg₃ If₄O₁₀(OH)₂ but leads on the contrary to the formation of serpentine and magnetite Fe₃O₄, as in the reaction:

24 mg_1.5Fe_0.5SiO₄ + 26 H₂O + CO₂ → 12 mg₃Si₂O₅(OH)₄ + 4 Fe₃O₄ + CH₄.

The probability of this type of reaction in the Nili Fossae region is enhanced by the volcanic nature of Syrtis Major Planum and by the close correlation, observed as early as 2004, between the humidity level of a region and the concentration of methane in the atmosphere.

Olivine and jarosite, subsisting only in arid climate

Olivine, discovered in the Nili Fossae region as well as other Martian regions by the Thermal Emission Spectrometer (TES) of Mars Global Surveyor, is an unstable mineral in an aqueous medium, easily yielding other minerals such as iddingsite, goethite, serpentine, chlorites, smectites, maghemite and hematite; the presence of olivine on Mars, therefore, indicates surfaces that have not been exposed to liquid water since the formation of these minerals, which dates back billions of years, to the Noachian for the oldest lands.

It is, therefore, a strong indication of the extreme aridity of the Martian climate during the Amazonian, aridity that had, it seems, already begun, at least locally, at the end of the Hesperian.

In addition, the discovery, by the Mars rover Opportunity on Meridiani Planumin 2004, of jarosite, a ferric sulfate hydrated sodium (on Earth, sodium is replaced by potassium) with the formula NaFe₃(OH)₆(SO₄)₂, made it possible to further clarify the sequence of climatic episodes on Mars. This mineral is formed, on Earth, by the alteration of volcanic rocks in an acidic oxidizing aqueous medium, so its detection on Mars implies the existence of a period of humid climate allowing the existence of acidic liquid water.

But this mineral is also quite quickly degraded by moisture, to form ferric oxyhydroxides such as goethite α-FeO(OH), which has also been found in other places on the planet (including by the Spirit rover in the Gusev crater). ). Therefore, the formation of jarosite in a humid climate had to be quickly followed to the present day by an arid climate in order to preserve this mineral, a new indication that liquid water had ceased to exist in the Amazonian but had been present in earlier times in the history of Mars.

Recent developments

On September 28, 2015, NASA announced that it had detected flows of “brines of different compositions, made of magnesium chlorate and perchlorate and sodium perchlorate, mixed with a little water. ” According to the analyses, there would be liquid or icy water in the Martian subsoil.

In 2021, NASA announces the discovery of traces of benzoic acid on the planet.

Geological history of Mars

The following scenario is a plausible synthesis deduced from the current knowledge from the various Mars exploration campaigns over the past forty years and whose results are summarized in the article Geology of Mars.

Training and differentiation

Like the other planets in the Solar System, Mars would have formed about 4.6 billion years ago by gravitational accretion of planetesimals resulting from the condensation of the solar nebula. Being located below the limit of the 4 AU of the Sun, beyond which can condense volatile compounds such as water H₂O, methane CH₄ or ammoniaNH₃, Mars was formed from planetesimals of essentially siderophilic nature (rich in iron) and lithophilic (made of silicates), but with an increased content of chalcophilic elements, starting with sulfur which seems much more abundant on Mars than on Earth, as revealed by measurements made by Mars Global Surveyor.

This high sulfur content would have had the effect of favoring the differentiation of the Martian globe, on the one hand by lowering the melting temperature of the materials that constitute it, and on the other hand by forming iron sulfides that chemically separated iron from silicates and accelerated its concentration in the center of the planet to form a core of siderophilic elements, richer in chalcophilic elements than the Earth’s core; the study of the radiogenic isotopes of the meteorites of Mars, and in particular of the Hf/W system, has revealed that the core of Mars would have formed in just 30 million years, against more than 50 million years for the Earth.

This rate of light elements would explain both why the core of Mars is still liquid, and why the oldest lava effusions identified on the surface of the planet seem to have been particularly fluid, until flowing for nearly a thousand kilometers around Alba Patera for example.

The nature of the planetesimals that led to the formation of the planet determined the nature of Mars’ primordial atmosphere, by gradually degassing molten materials into the mass of the planet being differentiated. In the current state of knowledge, this atmosphere must have been much denser than today, consisting mainly of water vapor H₂O as well as carbon dioxide CO₂, nitrogen N₂, sulfur dioxide SO₂, and perhaps quite large amounts of methane CH₄.

At the beginning of its existence, Mars must have lost, faster than the Earth, a significant fraction of the heat from the kinetic energy of planetesimals that crashed on each other to lead to its formation: its mass is indeed 10 times less than that of the Earth, while its surface is only 3.5 times smaller, which means that the surface-to-mass ratio of the red planet is almost three times higher than that of our planet. A crust must therefore have solidified on its surface in a hundred million years, and it is possible that the crustal dichotomy observed today between the northern and southern hemispheres dates back to the few hundred million years that followed the formation of the planet.

Once sufficiently cooled, about 4.5 to 4.4 billion years ago, the solid surface of the planet had to receive in rain the condensed atmospheric water vapor, which reacts with the iron contained in the heated minerals to oxidize it by releasing hydrogen H₂, which, too light to accumulate in the atmosphere, escaped into space. This would have led to a primitive atmosphere where only CO₂, N₂ and SO₂ remained as the majority constituents of the early Martian atmosphere, with a total atmospheric pressure then several hundred times higher than it is today; the current standard pressure at the Martian reference level. is, by definition,610 Pa.

Global magnetic field and humid temperate climate

The martian environment in Noachian

During the geological epoch called Noachien that ended about 3.7 to 3.5 billion years ago, Mars seems to have offered conditions very different from those of today and quite similar to those of the Earth at that time, with a global magnetic field protecting a thick and possibly temperate atmosphere allowing the existence of a hydrosphere centered around a boreal ocean. occupying the current expanse of Vastitas Borealis.

The past existence of a global magnetic field around Mars was discovered through the observation, carried out in 1998 by Mars Global Surveyor, of paleomagnetism over the oldest terrains of the southern hemisphere, particularly in the region of Terra Cimmeria and Terra Sirenum. The magnetosphere produced by this global magnetic field was to act, like the Earth’s magnetosphere today, by protecting the atmosphere of Mars from erosion by the solar wind, which tends to eject into space the atoms of the upper atmosphere by transferring the energy necessary to reach the speed of release.

A greenhouse effect would have been at work to temper the Martian atmosphere, which otherwise would have been colder than today due to the weaker radiation emitted by the Sun, then still young and in the process of stabilization. The simulations show that a partial pressure of 150 kPa of C02 would have made it possible to have an average ground temperature equal to that of today, i.e.210 K (just under −60 °C). Reinforcement of this greenhouse effect beyond this temperature could have come from several complementary factors:

• the condensation of_CO2 into reflective clouds in the infrared range would have contributed to returning to the ground the thermal radiation it emits, even more efficiently than terrestrial clouds, made up of water,
• the presence at high altitude of highly absorbent SO 2 in the ultraviolet range would have contributed to warming the upper atmosphere, as does the ozone layer on Earth by a similar mechanism,
• the role of water and methane (CH₄ generates a greenhouse effect twenty times more powerful than that of CO₂) is perhaps not to be neglected either.

Clues to a Martian hydrosphere in noachien

We know that liquid water was then abundant on Mars because the mineralogical study of the surface of the planet revealed the significant presence of phyllosilicates in the terrain dating back to that time. However, phyllosilicates are good indicators of the alteration of igneous rocks in wetlands. The abundance of these minerals in soils about 4.2 billion years ago led the esa team of planetary scientists responsible for the OMEGA instrument and led by Jean-Pierre Bibring to propose the name Phyllosian for the corresponding stratigraphic eon: it is apparently the wettest epoch on Mars.

More detailed studies carried out in situ by the two Mars Exploration Rovers, Spirit and Opportunity, respectively in the Gusev crater, south of Apollinaris Patera, and on Meridiani Planum, even suggest the past existence of a hydrosphere large enough to have been able to homogenize the phosphorus level minerals analyzed at these two sites located on both sides of the planet. A different approach, based on the mapping of the abundance of thorium, potassium and chlorine on the surface of Mars by the gamma spectrometer (GRS) of the Mars Odyssey probe, leads to the same result.

In addition, the detailed study of the traces left in the Martian landscape by supposed rivers and liquid expanses has led to the existence of a real ocean covering nearly a third of the surface of the planet at the level of the current Vastitas Borealis. In a 1991 paper that became classic, Baker et al. went so far as to identify certain structures with traces of an ancient shoreline.

The coastal lines thus identified were also to correspond to the constant altitude curves corrected for subsequent deformations deduced from volcanism and estimates of the change in the axis of rotation of the planet. These projections, sometimes quite bold, have not convinced everyone, however, and other theories have also been proposed to account for these observations, in particular on the basis of the possible volcanic origin of the structures thus interpreted.

The idea of a boreal ocean at the heart of an extended hydrosphere remains as attractive as ever, and many teams are working to analyze, with ever more efficient tools, the topographic data continuously enriched with information collected by the probes currently operating around Mars, in the hope of establishing the geographical distribution of the Martian hydrosphere in noachien.

In the same vein, the existence of Lake Eridania in the heart of the Terra Cimmeria highlands has been suggested to explain in particular the genesis of Ma’adim Vallis from the observation of certain topographical formations interpreted as ancient fossilized shores.

Possibility of a noachian abiogenesis

The Martian conditions of the Noachian could perhaps have allowed the emergence of life forms on Mars as happened on Earth: in addition to the presence of liquid water and the greenhouse effect that could have maintained a sufficiently high temperature, the abundance of clays makes it possible to envisage scenarios of the appearance of life elaborated within the framework of some of the (numerous) theories of abiogenesis, while other theories (e.g. the one conceived at the end of the twentieth century by Günter Wächtershäuser) envisage terrestrial abiogenesis in hydrothermal springs rich in iron(II) Sulphide FeS, an environment also likely to have existed on Mars in the Noachian.

These conditions, however, would have quickly become much less favorable to the next eon, the Hesperian, which would have begun no later than 3.5 billion years ago: dominated by sulfur chemistry, it certainly resulted in a significant lowering of the pH of Martian water under the effect of sulfuric acid rains H ₂SO₄, which would have had the incidental consequence of allowing the existence of liquid water at temperatures appreciably below 0 °C.

However, the oldest traces of “life” detected on our planet do not go back more than 3.85 billion years for the most remote of all published dates (around the conventional limit between the Hadean and the Archean), or 700 million years after the formation of the Earth, that is to say almost as much as the total duration of the first Martian eon in the most favorable hypothesis, as the following chronology of terrestrial eons compared to the standard Hartmann scale and the Hartmann Neukum scale reminds us:

Under these conditions, if a process of abiogenesis could have led to Mars in the Noachian, it would have led to life forms that would have had very little time to evolve before the upheavals of the Hesperian, at a time — around 4 to 3.8 billion years before the present — marked by the asteroid impacts of the great late bombardment.

For comparison, photosynthesis would not have appeared on Earth for 3 billion years, or even only 2.8 billion years, while the oldest eukaryotic cells would not date back more than 2.1 billion years, and sexual reproduction would not date back more than 1.2 billion years.

First volcanic effusions and great late bombardment

While the Phyllosian seems to have been rather devoid of volcanic activity, the detailed analysis of the data collected by the OMEGA instrument of Mars Express, designed for the mineralogical analysis of the Martian surface, led to the identification, at the end of this eon, a period of transition, extending from about 4.2 to 4.0 billion years before the present, marked by the appearance of significant volcanic activity while the planet was probably still experiencing temperate and humid conditions under a rather thick atmosphere.

In addition, the exploration by probes of the surface of terrestrial planets – starting with the Moon – at the end of the XX century led to the postulate of an episode called “great late bombardment” (called Late Heavy Bombardment by the Anglo-Saxons) extending over a period dated approximately 4.0 to 3.8 billion years before the present, to the nearest 50 million years. It is during this episode that the large impact basins now visible on Mars would have formed, such as Hellas, Argyre or Utopia.

Occurring both on Earth and on Mars, this cataclysm may also be at the origin of the difference in iron oxide concentration (more than single to double) observed between the mantle of the Earth and that of Mars. The cosmic impacts would have liquefied the Earth’s mantle on perhaps 1,200 to 2,000 km thick, bringing the temperature of this material up to 3,200 ° C, a temperature sufficient to reduce the FeO in iron and oxygen.

The Earth’s core would thus have experienced an additional supply of iron from the reduction of the mantle at the end of this meteorite bombardment, which would explain the residual weight content of about 8% of FeO in the Earth’s mantle. On Mars, on the contrary, the temperature of the molten mantle would never have exceeded 2,200 °C, a temperature insufficient to reduce iron(II) oxide and thus leaving unchanged the FeO content of the Martian mantle at about 18%. This would explain why Mars today is outwardly more than twice as rich in iron oxides as Earth when these two planets are supposed to have been originally similar.

As a result of these giant impacts, conditions on the planet’s surface have likely been significantly altered. In the first place, Mars would have lost a significant fraction of its atmosphere, scattered in space under the effect of these collisions. The general climate of the planet would have been upset by the dust and gases injected into the atmosphere during these collisions, as well as by a possible change in obliquity during such impacts.

But it is also possible that the kinetic energy of the impactors, by injecting thermal energy into the Martian mantle, has modified the thermal gradient which is supposed to maintain, in the planetary core, the convection movements at the origin of the dynamo effect generating the global magnetic field, which would have made the disappearance of the Martian magnetosphere from the end of the Noachian.

Formation of large Martian volcanic structures

The impacts at the origin of the great Martian basins may have initiated the largest volcanic episode in the history of the planet, defining the era called the Hesperian. This is characterized, from a petrological point of view, by the abundance of minerals containing sulfur, and in particular hydrated sulfates such as kieserite MgSO₄•H₂O and gypsum CaSO₄•2H₂O.

The main Martian volcanic formations would have appeared in the Hesperian, perhaps even, for some, as early as the end of the Noachian; this is particularly the case of lava plains such as Malea Planum, Hesperia Planum and Syrtis Major Planum. Alba Mons may also have started its activity at this time, following the impact at the origin of the Hellas Planitia basin located at the antipodes.

The bulge of Tharsis and the volcanoes of Elysium Planitia, on the other hand, date back to the middle of the Hesperian, around 3.5 billion years before the present, a date that would correspond to the period of maximum volcanic activity on the red planet; Alba Mons would thus have known its greatest activity in the second half of the Hesperian until the beginning of the Amazonian.

This volcanism would have released into the atmosphere of Mars large quantities of sulfur dioxide SO₂ which, reacting with the water in the clouds, would have formed sulfur trioxide SO₃ giving, in solution in the water, sulfuric acid H₂SO₄. This reaction would probably have been favored on Mars by the photolysis at high altitude of water molecules, under the action of ultraviolet radiation from the Sun, which releases hydroxyl radicals HO and produces hydrogen peroxide H₂O₂, an oxidant. The comparison with the atmosphere of Venus, which has clouds of sulfuric acid in an atmosphere of carbon dioxide, also highlights the role of photochemical dissociation of carbon dioxide (CO₂) by ultraviolet less than 169 nm to initiate the oxidation of sulfur dioxide. :

CO₂ + hν → CO + O SO₂ + O → SO₃ SO₃ + H₂O → H₂SO₄

Martian water would therefore have been loaded with sulfuric acid in the Hesperian, which would have the effect of significantly lowering its freezing point — the eutectic of the mixture H₂SO₄•2H₂O – H₂SO₄•3H₂O thus freezes a little below −20 °C, and that of the mixture H₂SO₄ •6.5H₂O – H₂O freezes around 210 K, temperature slightly below −60 °C, which is the current average temperature on Mars — and lead to the formation of sulfates rather than carbonates.

This would explain why, while Mars had a priori an atmosphere of CO₂ and large bodies of liquid water, there are almost no carbonates, while sulfates seem, on the contrary, particularly abundant: the formation of carbonates is inhibited by acidity — which the presence of sulfates suggests (FeCO₃ siderite, a priori the least soluble carbonate, precipitates only at pH greater than 5) — and the continuous release of SO₂ by volcanic activity in the Hesperian would have displaced the CO2 of the carbonates that could have formed in the Noachian to replace them with sulfates, as happens for example at low pH with magnesium:

MgCO₃ + H₂SO₄ → MgSO₄ + H₂O + CO₂

The mineralogical chronostratigraphy proposed by the team of planetary scientists responsible for the OMEGA instrument of the Mars Express probe matches, to the Hesperian, the stratigraphic eon called “Theiikien”, a term forged via English from ancient Greek τὸ θεΐον meaning “sulfur” — the exact root would be the adjective *θειικον in the sense of “sulfuric”. This eon would however be dated 4.0 to 3.6 billion years before the present, that is to say with a shift of 300 to 400 million years to the past compared to the scale of Hartmann Neukum.

Slowing of volcanism and desiccation of the atmosphere

Once past the major volcanic episode of the Hesperian, Mars would have gradually seen its internal activity reduce to the present day, where it seems to have become imperceptible, or perhaps even zero. Indeed, several volcanic episodes, of decreasing intensity, would have taken place during the Amazonian, especially at Olympus Mons, and some eruptions would even have occurred only 2 million years ago, but this activity remains episodic and, in any case, insignificant compared for example to the volcanism currently existing on Earth.

At the same time, the atmosphere of Mars would have undergone continuous erosion since the beginning of the Hesperian under the effect of the solar wind following the disappearance of the magnetosphere, probably from the end of the Noachian. Such erosion, even moderate, but continuous over several billion years, would have easily dispersed in space most of what remained of the gaseous envelope on the surface of Mars after the great late bombardment. This was followed by the gradual disappearance of the greenhouse effect due to Martian CARBON DIOXIDE CO₂, hence the continuous decline in the temperature and atmospheric pressure of the planet from the Hesperian and throughout the Amazonian.

The presence of liquid water on Mars has therefore gradually ceased to be continuous to be only sparse and episodic. Current Martian conditions allow the existence of liquid water in the lowest regions of the planet to the extent that this water is loaded with chlorides and/or sulfuric acid, which seems to be precisely the case on Mars given the result of the analyzes carried out in situ by the probes that chemically studied the soil of the red planet. Significant rainfall also seems to have occurred as far as the middle of the Amazonian, judging by the sinuous ridges identified for example east of Aeolis Mensae. But, during the Hesperian and Amazonian periods, global Martian conditions shifted from a thick, humid and temperate atmosphere to a tenuous, arid and cold atmosphere.

These particular conditions, exposing, for billions of years, the minerals of the Martian surface to a dry atmosphere loaded with oxidizing ions, favored the anhydrous oxidation of iron in the form of amorphous iron(III)Fe₂O₃ (hematite) oxide, which causes the rust color. characteristic of the planet. However, this oxidation remains limited to the surface, the materials located immediately below having most of the time remained in their previous state, with a darker color.

This predominance of ferric oxides is at the origin of the siderikian term designating the corresponding stratigraphic eon, forged by the planetary scientists responsible for the OMEGA instrument of the Mars Express probe at ESA, from the ancient Greek ὁ σίδηρος meaning “iron” — the exact root would rather be the adjective *σιδηρικος in the sense of “ferric” — and which would begin as early as 3.6 billion years before the present.

The transition between Hesperian and Amazonian would have been quite gradual, which explains the extreme variability of the dates defining the boundary between these two epochs: 3.2 billion years before the present according to the Hartmann Neukum scale, but only 1.8 billion years according to the standard Hartmann scale.

Water on Mars

Of the abundance of liquid water of the Noachian, there remain, today, only traces in the atmosphere of Mars and, no doubt, significant amounts of water frozen in the soil and polar caps of Mars, in the form of permafrost, or even mollisol. In 2005, the Mars Express probe detected a lake of water ice in a crater near the North Pole. In 2007, Mars Express’ MARSIS radar highlighted large amounts of water ice buried in the land bordering the southern residual ice cap. Thus, the volume of water ice contained in the South Pole is estimated at 1.6 million cubic kilometers, approximately the volume of water ice in the boreal residual ice cap.

The presence of water in the subsoil was also detected halfway between the equator and the North Pole. For example, in 2009, the Mars Reconnaissance Orbiter revealed that newly formed impact craters contained 99% pure ice.

The lasting presence of liquid water on the surface of Mars is considered unlikely. Indeed, given the pressure and temperature on the surface of Mars, water cannot exist in the liquid state and passes directly from the solid state to the gaseous state by sublimation. However, recent evidence suggests the temporary presence of liquid water under particular conditions. Experimentally, low-pressure water and brine flows were carried out to study their impact on the surface.

• In 2004, the scientific team of THEMIS, the Mars Odyssey instrument designed to detect the presence of water passed on Mars, discovered on one of the images of the probe a “structure that resembles a lake located in the center of the crater”.
• Very brief castings could still take place. Thus, Michael Malin and Kenneth Edgett (and co-authors), NASA researchers, announced in December 2006 that they now have evidence of active episodic granular flows. Analysis of high-resolution MOC images taken by the Mars Global Surveyor probe revealed the presence of new gullies whose placement could be linked to mud or debris flows. But subsequent analyses showed that these observations could just as easily be explained by dry flows. Analysis of these flows with HiRISE data shows that they are seasonal and occur in late winter as well as early spring.
• At the Hellas Planitia impact basin, the difference in altitude between the ledge and the bottom is about 9 km. The depth of the crater (7,152 meters below the reference topographic level) explains the atmospheric pressure below: 1,155 Pa (or 11.55 mbar or 0.01 atm). This is 89% higher than the pressure at zero (610 Pa, or 6.1 mbar) and above the triple point of water, suggesting that the liquid phase would be ephemeral (evaporation as ice melts) if the temperature exceeds 273.16 K (0.01 °C) in the case of pure water. A lower temperature would nevertheless be sufficient for salt water, which would be precisely the case for Martian water — liquid water exists on Earth up to very low temperatures, for example in the very salty Lake Don Juan in Antarctica, and some brines remain liquid at even lower temperatures, as do some sulfuric acid solutions. .

Seasonal traces of flows were also identified in the spring of 2011 by the HiRISE instrument of the Mars Reconnaissance Orbiter probe at several points on the Martian surface in the form of dark traces that lengthen and widen on slopes exposed to the sun, including on the edges of Newton Crater. These rather dark formations,0.5 to 5 meters wide, are preferably formed facing the equator on slopes inclined from 25 ° to 40 ° between 48 ° S and 32 ° S, with a maximum length in late summer and early local autumn, while the surface temperature is between 250 and 300 K.

Variations in brightness, latitude distribution and seasonality of these manifestations suggest that they are caused by a volatile substance, but this has not been directly detected. They are found in points too hot on the Martian surface for it to be frozen carbon dioxide, and usually too cold for it to also be pure frozen water. These observations therefore also argue in favor of brines, which seem to form punctually from time to time on the surface of the planet. On September 28, 2015, NASA announced that analyses of images from the Mars Reconnaissance Orbiter probe would confirm the presence of liquid on Mars in the form of hydrated salts.

In March 2014, following exploration by the Curiosity robot, NASA announced that a large lake would have filled the river-fed Gale Crater for millions of years.

A study published in March 2017 showed that the flows would eventually be dry. Indeed, the quantities of water needed to explain these water sources each year are not sufficient in the atmosphere. The underground source is also very unlikely because dark flows (Recurring Slope Lineae, RSL) sometimes form on peaks. The new hypothesis proposes the Knudsen pump effect as a trigger for flows, which would therefore be completely dry.

On 25 July 2018, the Mars Express space probe launched by the European Space Agency detected the presence of an underground lake of liquid water 20 km wide,1.5 km below the surface of Mars, at the southern polar cap. Although at a temperature below the freezing point of pure water, this lake would be liquid due to its high concentration of Martian salts and minerals.

Natural satellites of Mars

Mars has two small natural satellites, Phobos and Deimos, resembling carbonaceous chondrite or D-type asteroids, whose origin remains uncertain with several hypotheses raised:

• These could be incident asteroids captured by Mars, but the difficulty of this scenario is to explain how, in this case, these two satellites were able to acquire their current orbits, circular and slightly inclined — barely 1° — relative to the Martian equator: this would involve mechanisms of atmospheric braking and regulation by tidal effects, scenarios that present difficulties compared to the insufficiency of the atmosphere of Mars to achieve such braking in the case of Phobos, and to the insufficient time required to circularize the orbit of Deimos. Nevertheless, this capture mechanism could have been greatly facilitated in the case of double asteroids where one of the components would have been ejected while the other satellited around the red planet.
• The two satellites of Mars could also have formed at the same time as their mother planet, the difficulty being in this case to explain the difference in composition between Mars on the one hand and its two satellites on the other hand.
• Finally, a third hypothesis proposes that Phobos and Deimos are two bodies agglomerated from the satellite residues following one or more major impacts of planetesimals shortly after the formation of Mars, scenario joining the hypothesis “Theia” explaining the formation of the Moon by a similar mechanism intervened on the proto-Earth.

Phobos

Phobos, the natural satellite of Mars closest to its planet, is an irregular mass of 27 × 22 × 18 km that orbits at less than 6,000 km altitude, to the point of not being visible from the polar regions of the Martian surface, beyond 70.4° north or south latitude, where it is masked by the curvature of the planet. The Mars Global Surveyor probe has revealed that its surface, very cratered, is covered with a regolith a hundred meters thick probably from the myriad impacts that occurred on the surface of this object. Its average density is half that of Mars, at just under 1,890 kg/m, suggesting a porous nature resulting from a structure in agglomerated blocks whose overall cohesion would be quite weak.

It would be a D-type asteroid, that is to say made of materials dominated by anhydrous silicates with a significant proportion of carbon, organic compounds and, perhaps, water ice. It would have a composition close to a carbonaceous chondrite, explaining its albedo of barely 0.071. The mineralogical nature of the surface examined by the ISM infrared spectrometer of the Phobos 2 probe appears to correspond to olivine with locally concentrations of orthopyroxene. The presence of water on the surface of the satellite has clearly been ruled out by several studies but does not remain excluded in depth.

One of the characteristic features of Phobos is the presence of parallel furrows no more than 30 m deep,200 m wide and 20 km long, which seem to envelop the satellite radially around the Stickney crater, and which could be traces of debris thrown into space during impacts on Mars that would have been swept into orbit by Phobos: the furrows actually seem to “flow” on the surface of the satellite from its “front” point — in the direction of its synchronous revolution around Mars — more than from the Stickney crater itself, located near the front point. These furrows are more precisely catenae, which result from chains of aligned craters.

Orbiting inside the synchronous orbit of Mars, located at an altitude of 17,000 km, Phobos is slowed down by the tidal forces exerted by the Martian globe, which causes it to lose altitude at a rate of about 18 cm per year: at this speed, it will reach its Roche limit in about 11 million years and will disintegrate at about 4,000 km above the Martian surface where it should gradually form a ring.

Deimos

Mars’ second satellite, Deimos, is even smaller than the first, with dimensions of 15 × 12.2 × 10.4 km. It orbits at an altitude of just over 23,000 km, in an almost circular orbit inclined less than one degree from the Martian equator. It seems to be of the same nature as Phobos — a D-type asteroid with a composition close to a carbonaceous chondrite — but its surface, a priori just as cratered as that of Phobos, would be much softened by a layer of regolith thick enough to fill most craters. The density of this regolith has been estimated on radar at about 1,100 kg/m, while that of the satellite as a whole is of the order of 1,470 kg/m.

The views taken by Mars Reconnaissance Orbiter showed a surface of varying color depending on the region, the regolith having a dark red hue more pronounced than the surfaces apparently more recent, located around some craters and on the edges of the edges. The catenae forming the characteristic furrows of the surface of Phobos have not been observed on Deimos.

Property	Phobos	Deimos
Size	26.8 × 22.4 × 18.4 km	15.0 × 12.2 × 10.4 km
Mass	1,072 × 10 kg	1.48 × 10 kg
Average density	1,887 kg/m	1,471 kg/m
Surface gravity	1.9 to 8.4 mm/s	approx.3.9 mm/s
Release speed	11.3 m/s	5.6 m/s
Albedo	0,071	0,068
Semi-major axis of the orbit	9,377.2 km	23,460 km
Orbital eccentricity	0,0151	0,0002
Axis tilt	1,075°	0,93°
Orbital period	0.310 841 8 soils ≈ 0.318 910 23 d	1,230 5 soils ≈ 1,262 44 d

Discovery and naming

The two satellites were discovered during the August 1877 opposition by Asaph Hall using a 26-inch telescope from the United States Naval Observatory in Washington.

They were originally named Phobus and Deimus after a suggestion by Henry Madan professor at Eton College after line 119 of song XV of the Iliad:

Ὣς φάτο, καί ῥ’ ἵππους κέλετο Δεῖμόν τε Φόβον τε ζευγνύμεν, αὐτὸς δ’ ἔντε’ ἐδύσετο παμφανόωντα.

— translation from ancient Greek by Leconte de Lisle, The Iliad – Song XV none

“He spoke thus, and he commanded terror and fear to harness his horses, and he covered himself with his splendid armor. “

— The Iliad – Song XV none

In Greek mythology, Phobos and Deimos are the sons of the god Ares, in ancient Greek Φόβος / Phóbos means “fear” and Δεῖμος / Deĩmos “terror”. This name is a play on words on the polysemy of the word satellite which can designate both a celestial body (the satellites of the planet) or a person, a bodyguard (the satellites of the god).

Trojan asteroids and cruisers from Mars

Currently are known four Trojans in the wake of Mars. The first, discovered in 1990, and the best known of them, is (5261) Eureka, located at the Point of Lagrange L₅. The other three are 1998 VF31 (at L(₄),1999 UJ7 (at L(₅)), and 2007 NS2 (at L(₅). )

Mars also has a coorbital asteroid: (26677) 2001 EJ18.

Six other asteroids are also closely related to Mars, but do not appear to be Trojans: 2001 FR127,2001 FG24,2001 DH47,1999 ND43,1998 QH56 and 1998 SD4.

2007 WD₅ is a 50 m-long near-Earth asteroid discovered on November 20, 2007 by Andrea Boattini of the Catalina Sky Survey. According to NASA’s Near Earth Object Program, there was a one in 10,000 chance (or 0.01%) of impacting Mars on January 30, 2008, an impact that ultimately did not occur.

History of Mars observations of the planet

Ancient observations

Mars being one of the five planets visible to the naked eye (along with Mercury, Venus, Jupiter, and Saturn), it has been observed since humans looked at the night sky. During its oppositions, it is the brightest planet after Venus (its apparent magnitude can then reach -2.9, the rest of the time, the second brightest planet is Jupiter).

The characteristic red color of Mars earned it in antiquity the rapprochement with the Greek god of war Ares and then with its Roman equivalent Mars, the red evoking the blood of the battlefields.

The Babylonians named her Nirgal or Nergal, the god of death, destruction and fire.

The Egyptians called it “Red Horus” (ḥr Dšr, Hor-desher) and knew its “backward movement” (currently known as the retrograde movement).

In Hindu mythology, Mars is named Mangala (मंगल) after the god of war. In the quadrangle of Memnonia, Mangala Valles is named in his honor.

In Hebrew, she is called Ma’adim (מאדים): “He who blushes”). Ma’adim Vallis uses this term.

In East Asia (China, Japan, Korea and Vietnam) Mars is 火星, literally the star (星) fire (火). In Mandarin and Cantonese, it is commonly called huoxing (火星, huǒxīng in pinyin) and traditionally Yinghuo (荧惑, yínghuòen pinyin, litt. “flamboyant confused”). In Japanese, 火星 in kanji, かせい in hiragana, or kasei in rōmaji (which gave its name to Kasei Vallis). In Korean, 火星en hanja and 화성 in Hangeul, transcribed in Hwaseong.

Mars is still known today as the “Red Planet”.

Observations of pre-telescopic astronomy, there are few documents left, and these are tinged with religion or astrology (like the zodiac of Dendera in Upper Egypt). In addition, observations with the naked eye do not make it possible to observe the planet itself but rather its trajectory in the sky.

Telescopic observations

In 1600 in Prague, Johannes Kepler became the assistant of Tycho Brahe (died 1601) for whom he had to calculate the precise orbit of Mars. It takes him six years to do the calculation and discovers that the orbits of the planets are ellipses and not circles. This is Kepler’s first law that he published in 1609 in his work Astronomia Nova.

Belief in the existence of Martian canals lasted from the late nineteenth century to the early twentieth century and marked the popular imagination, contributing to the myth of the existence of intelligent life on the fourth planet of the Solar System. Their observation, which was never unanimous, came from an optical illusion, a frequent phenomenon in the observation conditions of the time (pareidolia).

In the twentieth century, the use of large telescopes made it possible to obtain the most accurate maps before sending the probes. At the Meudon Observatory, Eugène Antoniadi’s observations in 1909 led to the publication of The Planet Mars in 1930. At the Pic du Midi Observatory, observations were made by Bernard Lyot, Henri Camichel, Audouin Dollfus, and Jean-Henri Focas.

Exploration

The exploration of Mars is done using space probes, including artificial satellites and rovers.

It holds an important place in the space exploration programs of Russia (and before it by the USSR), the United States, the European Union, and Japan, and begins to materialize in the space program of the People’s Republic of China. About forty orbital probes and landers have been launched to Mars since the 1960s.

N. B.: the dates below are those of the launch and end of the missions; the intermediate date is that of the insertion of a satellite into Martian orbit (orbiter) or the landing of a lander (lander).

Failed missions

• Soviet probes:
  • Mars 1960A
  • Mars 1960B
  • Mars 1962A
  • Mars 1962B
  • Mars 1 (November 1, 1962 – March 21, 1963)
  • Mars 4
  • Mars 7
  • Zond 2
  • Phobos 1
  • Phobos 2
• U.S. probes:
  • Mariner 3
  • Mariner 8
  • Deep Space 2
  • Mars Observer (September 25, 1992 – lost contact on August 21, 1993)
  • Mars Climate Orbiter (December 11, 1998 – September 23, 1999)
  • Mars Polar Lander (January 3, 1999 – December 3, 1999)
• Russian probes:
  • Mars 96
  • Phobos-Grunt was a Russian-led mission launched on November 8, 2011, but could not place the probe in its transit orbit to Mars, so the craft crashed to Earth on January 15, 2012, in the South Pacific. The goal was to bring back soil samples from Phobos.
• Yinghuo 1 was a Chinese mission consisting of a small module to be placed in Martian orbit by the Russian Phobos-Grunt spacecraft to study the immediate environment of the Red Planet; the failure of the Phobos-Grunt mission led to that of Yinghuo 1.
• The European probe Beagle 2 (2 June 2003 – 25 December 2003). The landing appears to have gone well but contact with the probe was lost. In January 2015, it was found in photos of the surface of Mars taken by the Mars Reconnaissance Orbiter.
• The Japanese probe Nozomi (のぞみ) (July 3, 1998 – December 9, 2003) has since remained in heliocentric orbit.
• The European probe Schiaparelli (14 March 2016 – 19 October 2016) lost due to a premature ejection of the parachute during atmospheric descent, thus leading to a crash.

Missions accomplished

• American probes (simple flybys):
  • Mariner 4 (28 November 1964 – overflight 14 July 1965 – mission ended 21 December 1967)
  • Mariner 6 (February 24, 1969 – overflight July 31, 1969)
  • Mariner 7 (March 27, 1969 – overflight August 5, 1969)
• U. S. satellites:
  • Mariner 9 (30 May 1971 – 13 November 1971 – 27 October 1972)
  • Mars Global Surveyor (November 7, 1996 – September 11, 1997 – November 5, 2006)
• Soviet orbiters:
  • Mars 5 (July 25, 1973 – February 12, 1974 – March 5, 1974)
• Soviet landers:
  • Mars 2 (May 19, 1971 – November 27, 1971 – August 22, 1972)
  • Mars 3 (28 May 1971 – 2 December 1971 – 22 August 1972)
  • Mars 6 (August 5, 1973 – March 12, 1974 – March 12, 1974) data transmitted only during the descent
• Viking program, with lander and orbiter:
  • Viking 1 (August 20, 1975 – November 11, 1982)
  • Viking 2 (September 9, 1975 – April 11, 1980)
• Mars Pathfinder lander and rover (December 4, 1996 – July 4, 1997 – September 27, 1997)
• Phoenix Lander (August 4, 2007 – May 26, 2008 – November 10, 2008)
• Mars Exploration Rover:
  • Spirit (June 10, 2003 – January 3, 2004 – March 22, 2010, date of the last contact with this rover now bogged down. )
  • Opportunity (July 8, 2003 – January 24, 2004 – February 13, 2019, date of last contact)

Ongoing orbital missions

• 2001 Mars Odyssey (April 7, 2001 – October 24, 2001 – Planned end?)
• Mars Express (2 June 2003 – 26 December 2003 – extended many times, scheduled shutdown at the end of 2022)
• Mars Reconnaissance Orbiter (August 12, 2005 – March 10, 2006 – Expected end?)
• Mars Orbiter Mission (November 5, 2013 – September 24, 2014 – October 2016 is the planned end of the mission but the satellite is still active in 2022. )
• MAVEN (18 November 2013 – 22 September 2014 – expected end?)
• Trace Gas Orbiter (March 14, 2016 – October 19, 2016 – expected completion in 2022)
• The Emirates’ Mars mission (also called Hop) is a probe of the Emirati space agency that takes off on July 19, 2020. Hope consists of an orbiter equipped with three scientific instruments dedicated to the study of the atmosphere of Mars. It is the UAE’s first space probe and the Arab world’s first space probe. It orbited Mars on February 9, 2021.

Ongoing ground missions

• Mars Science Laboratory is a mission developed by NASA and launched on November 26, 2011. It is equipped with a rover named Curiosity much more efficient than Spirit and Opportunity in order to search for traces of past life through various geological analyses. Curiosity landed in the Bradbury landing zone in Gale Crater on August 6, 2012.
• InSight, embarks European scientific instruments (seismometer, heat flow sensor and weather station). successful landing on November 26, 2018, at a latitude of 4.5 ° N and longitude of 135.9 ° E. The end of the mission is scheduled for 2020.
• Mars 2020 is a NASA mission that took off on July 30, 2020, with on board the Perseverance rover (its construction is largely based on Curiosity, while featuring more advanced instruments), as well as a mini helicopter (drone type), the Mars Helicopter Scout Ingenuity. The rover landed on February 18, 2021, in the Jezero crater.

Mission in orbit awaiting landing

• Tianwen-1 is a probe of the Chinese Space Agency (CNSA) that takes off on July 23, 2020. It includes an orbiter and a lander that must deposit a rover on the surface of the planet. This is the first Mars mission conducted independently by China. It orbited Mars on February 10, 2021, while the lander and rover are expected to land on its surface in May or June 2021.

Planned programs

• The European Space Agency’s (ESA) Aurora program includes several components, including the ExoMars mission in collaboration with the Russian space agency Roscosmos and the Mars Sample Return project in collaboration with NASA.

Canceled programs

• NASA’s Constellation program proposed sending humans back to the moon by 2020 to prepare for future manned missions to Mars. Deemed too expensive for outdated technological options, this program initiated by the Bush administration, which was already experiencing significant delays, was abandoned on February 1, 2010 by the Obama administration.

Artificial satellites around Mars

The various Martian missions have set up artificial satellites around the planet. They serve as relays for telecommunications with the modules placed on the ground, and carry out global measurements on the environment and the surface of Mars.

Ten artificial satellites currently orbit Mars, eight of which are still in operation, more craft than for any other object in the Solar System except Earth.

Mission	Launch	Putting into orbit	Status
Mariner 9	May 30, 1971	November 14, 1971	Mission completed on 27 October 1972
Mars Global Surveyor	November 7, 1996	September 11, 1997	Contact lost on November 2, 2006
2001 Mars Odyssey	April 7, 2001	October 24, 2001	In operation
Mars Express	June 2, 2003	December 25, 2003	In operation
Mars Reconnaissance Orbiter	August 12, 2005	March 10, 2006	In operation
Mars Orbiter Mission	November 5, 2013	September 24, 2014	In operation
MAVEN	November 12, 2013	September 21, 2014	In operation
Trace Gas Orbiter	March 14, 2016	October 19, 2016	In operation
EMM (Mars Hope)	July 19, 2020	February 9, 2021	In operation
Tianwen-1 (orbiter)	July 23, 2020	February 10, 2021	In operation

Mars in culture

Symbolization and symbolism

The astronomical symbol of Mars is a circle with an arrow pointing northeast. In alchemy, this symbol is associated with iron (whose oxide is red) and sometimes indicates an iron mine on maps.

Mars takes a little less than two years to circle the Sun, its symbol was used by Carl von Linnaeus to represent biennial plants in his book Species Plantarum.

This symbol is a stylized representation of the shield and spear of the god Mars. In biology, the same symbol is used as a bookmark for the male sex. Men Come from Mars, Women Come from Venus is a 1992 bestseller by John Gray.

Volvo has incorporated this symbol into its logo because of its association with iron, thus the steel industry.

The color red is associated with Mars. It is also associated with violence, anger, war: all the usual attributes of the god Mars.

The hypothetical correlation between the position of the planet Mars in relation to the horizon at the time of birth and the destiny of some athletes is called the Mars effect.

In the photos taken by Viking 1, on July 25, 1976, during its 35th orbit, we can see in Cydonia Mensae structures that seem artificial including a gigantic face and pyramids. This legend is featured in the 2000 American science fiction film Mission to Mars directed by Brian De Palma.

Music

“Mars, the One Who Brings War” is the first movement of the large orchestral work Les Planètes, composed and written by Gustav Holst between 1914 and 1916.

A song by British singer-songwriter David Bowie, Life on Mars? published in 1971, asks in its chorus the question: “Is there life on Mars? (“Is there life on Mars?”)

Fiction

Literature

• Herbert George Wells, The War of the Worlds none (1 ed.1898). — text from The War of the Worlds on Wikisource;
• The Mars Cycle, by Edgar Rice Burroughs, from February 1912;
• Ray Bradbury, The Martian Chronicles (1 ed.1950);
• Leigh Brackett, The Book of Mars (1953-1967);
• Kim Stanley Robinson:
  • The Mars Trilogy
    • Red Mars (1 ed.1992);
    • Green Mars (1 ed.1993);
    • Blue Mars (1 ed.1996);
  • The Martians (1st ed.1999) — collection of short stories;
• Stephen Baxter, Voyage (1 ed.1996);
• Gustave Le Rouge, Le Prisonnier de la planète Mars (1908); The Vampire War (1909)
• Dan Simmons, Ilium (1st ed.2003);
• Dan Simmons, Olympos (1st ed.2005);
• Andy Weir, Alone on Mars [“The Martian”] (1st ed.2011);
• Lady Astronaute, a collection of short stories by Mary Robinette Kowal published in 2020.

References (sources)

« Seventh International Conference on Mars: Mars subsurface warming at low obliquity »; MSL Science Team, « Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover »; Mars (planète), Wikipedia; « Ice Clouds in Martian Arctic (Accelerated Movie) », University of Arizona, 2008;  « Ice clouds put Mars in the shade », ESA, 2008.