Satellites, planètes, Terre : géomorphologie comparée

Résumé En Fr

. - When comparing the Earth, Phobos, the Moon, Mercury, Venus and Mars relief features and agencies, a few similarities may be noticed, but many more differences, either in their nature or in their frequency. A first factor is the relative abundance of atoms present at the origin or soon after : Ca for lunar anorthosite, K for granite on the Earth. Whether water is present (Earth, Venus, Mars) or not is fundamental, and if present, in which phase : liquid (hence follows an oriented transportation from land towards the sea, and isostatic unbalance, perhaps one of the causes of folding, as advocated by Dutton) or solid (snow, glaciers, ground ice) or only gaseous. Phases for H2O, CO2, etc. are determined by pressure and temperature, which partly depend on the distance from the Sun : if the Earth were at the same distance as Mars, almost all its water would be concentrated in the polar glaciers or in ground ice. All those features can be clearly seen on the phase diagrams. The most typical characteristics of the Earth probably is its water. Phase changes are fundamental, not only for internal agencies (magma-melting and solidifying), but also for external ones (rains nourishing rivers, etc.). - Presently, internal factors are advocated as primary causes by most geologists and above all by geophysicists. Judging from satellites and planets, external factors (cosmic impacts, sun radiation, atmosphere, H2O, etc.) must not be underestimated. Their importance is at least equal, if not greater.

. - Sur Mercure, la Lune et Phobos, domine le système morphogénétique à cratères d'impact, sans transport dirigé, ni chaîne plissée, ni H2O, ni zonalité. Mars, planète du périglaciaire aride, présente en plus des cratères, une atmosphère, deux calottes glaciaires polaires, du nivéo-éolien, du thermokarst, des ergs, une zonalité, des volcans en bouclier ; pas d'eau aujourd'hui, mais il y en a eu un peu autrefois. La géomorphologie d'un corps planétaire est commandée avant tout par la distance au soleil (dont dépend en partie la température), la présence ou l'absence d'une atmosphère, celle d'H2O, l'état de celle-ci et les changements d'état de CO2, H2O et des magmas.