Le relief de la planète Mars, comparaison avec celui de la Terre

• Published in: Annales de Géographie
• Format: Article in Journal/Newspaper
• Language: French
• Published: PERSEE 1986
• Subjects: Tempêtes; Ice; permafrost; pergélisol
• Online Access: http://www.persee.fr/doc/geo_0003-4010_1986_num_95_530_20433; https://doi.org/10.3406/geo.1986.20433

Mars présente certaines différences importantes avec la Terre au plan de sa composition, de son atmosphère, de son climat (Tableau 1). L'absence de tectonique de plaques explique le gigantisme des volcans, atteignant 20-28 km de haut, avec des diamètres de centaines de km et des coulées s'allongeant sur un millier de km. Aucun n'est plus actif. Les fortes variations de température et la présence de H2O, qui sous les conditions actuelles ne peut revêtir que les états solide et gazeux permettent une certaine météorisation (poussière très fine). Le vent est le principal agent actuel, mais son efficacité semble faible : surtout un saupoudrage de poussières après les tempêtes. L'essentiel du relief de Mars est formé de reliefs anciens, qui persistent depuis très longtemps du fait de la lenteur et de la faiblesse de la morphogenèse. Les impacts de grands corps célestes ont donné, il y a entre 4,6 et 3,5 milliards d'années, de grandes dépressions et des cratères atteignant 100-200 km de diamètre. Ces reliefs ont été incisés par des vallées en V, formant des réseaux ramifiés, ce qui implique une période de climat très différent de l'actuel, moins froid et une pression atmosphérique plus élevée : la Période Pluviale. Deux remblaiements, discordants entre eux se sont édifiés alors dans les dépressions des vieux reliefs. Un refroidissement a permis la formation de glaciers, quelques-uns de type terrestre, d'autres, plus importants, formés à partir de venues d'eaux souterraines contenues dans les remblaiements. Leur fusion implique un nouveau réchauffement. Ensuite, un pergélisol s'est établi en surface, recouvrant des couches profondes contenant un aquifère non gelé (cryohydrosphère). Il s'est formé des dépressions cyrokarstiques, des rides dues à des ségrégrations de glace lorsque le pergélisol était peu épais, des astroblèmes dont le cratère est ceinturé de langues boueuses, l'impact ayant chassé jusqu'en surface le matériel profond gorgé d'eau et provoqué la fonte locale du pergélisol. Mars and terrestrian landforms compared. Viking frames and mosaics (scales 1/2 000 000 and, for limited areas, 1/500 000) give an excellent opportunity for the study of Martian landforms. The Authors combine information gathered in the litterature with their own research results. First of all, Mars is smaller than Earth and has too, a lower bulk density. It results in a lower gravitational attraction and in the loss of most of its atmosphere (pressure around 7 mb, CO2 content 95 %). Presently, H2O cannot be under the form of water, only solid or gazeous. Weathering results from very big temperature amplitude and from H2O exhange between soil and atmosphere. Viking Landers have identified sulphates cementing dust a few centimeters below surface. Dust is abundant, sand scarce as a result of the basic nature of lavas which form all the solid rock. Presently, it is reworked by wind, great gales occurring nearly every year, as just when Mariner 9 arrive in orbit around Mars. But, wind efficiency is low and landforms persist during enormous durations : hundreds of million and, even, billion years. Most of them are inherited from former periods with a different climate, probably a denser atmosphere, during which water was surficially available on Mars. It is the Pluvial Period. Valley networks have been cut in resistant rocks, some of them hundreds of km long. Low-lying areas between old landforms rugged topography have been filled with probably volcanic ashes reworked by water. Two fills can be eventually identified, the elder one having been tilted and very gently folded, the younger one remained tabular and is laid uncomformably upon. Ground water occurs in these fills and has seeped into the valleys. It has resulted in box-like valleys, with an abrupt end above the terminal spring. Later, under a colder climate, a surficial permafrost layer has formed. Some valleys have been excavated by glaciers, most of them resulting of ground water seepage, some others of terrestrial type, fed by snow. Both types have melted as a result of an increase of temperatures. Ground-ice melting resulted in cryokarst forms, blind valleys, chaotic and fretted " terrain " as named by US scientists. Surficial permafrost is underlain by ground water when rock nature is convenient. This cryohydrosphere, when affected by meteoritic impacts, is blown up under viscous conditions and mudflows spill around the crater of the astroblem. Mars, after having been formed by accretion around 4,6 billion years ago, has suffered during 1 billion years or a most intense " bombing " par big meteorites, whose basins and craters have not been obliteraded as on the Earth, where subsequent landforming processes have been much more efficient. These old basins and craters have suffered disection by running water during the Pluvial Period. The very faint magnetic field of Mars reveals a much less differenciated interior than the one of Earth. There is no evidence of plate-tectonics, and this explains the gigantism of volcanoes (Tharsis, Elysium), which are 20-28 km high with lava gushes as long as 1 000 km. Gases resulting from rock crushing and vaporization by impacts and from degasification from lavas can have resulted in the denser and different composition of the atmosphere in the past.