Morphologie et remplissage sédimentaire des vallées martiennes : marqueurs des conditions climatiques pré-amazoniennes

Nowadays Mars has a cold and dry climate. At some locations its surface is more than 3.5 Gy old and exhibits valleys, channels, terraces and deltas attesting the occurrence of liquid water in the past. This fluid is currently unstable on the surface of the planet, suggesting these morphologies would involve different climatic conditions from the current ones. What did early Martian climate look like? It is one of the main issues regarding this planet. Among the geomorphic evidence classically used to reconstruct past climates, Martian valleys are numerous and heterogeneously thus particularly attractive to study. Their morphologies can inform on the climatic conditions prevailing at the time of their formation and/or later during their evolution. The objective of this thesis is to contribute to solve the enigma concerning Martian pre-Amazonian (> 3.5Gy) climatic conditions by focusing on the study of equatorial valleys and the equatorial canyon of Valles Marineris. Our study reveals that the morphometry and spatial organization of some Martian valleys differ significantly from those of common terrestrial fluvial and sapping valleys. By contrast, they share a lot of similarities with terrestrial subglacial tunnel valley. The subglacial or pro-glacialmelting of surface ice deposits is plausible as an origin of some Martian valleys under cold climatic conditions. Moreover, regional geomorphological reconstitutions of Valles Marineris reveal that this valley system had witnessed glacial processes since its formation. Current glacial relicts of this former filling, probably several billion years old, still, subsist protected under a debris cover. This study suggests the possibility that pre-Amazonian atmospheric conditions on Mars do not require to be significantly different from the current ones.Mars présente à l’heure actuelle un climat sec et froid. Certains endroits de sa surface sont âgés de plus de 3,5 Ga et présentent des vallées, des chenaux, des terrasses ou encore des deltas qui attestent de la présence d’eau liquide dans le passé. Cette dernière étant actuellement instable à la surface de la planète, l’existence de ces morphologies impliquerait des conditions climatiques passées différentes de celles actuelles. À quoi pouvait ressembler le climat de Mars à cette époque ? Il s’agit de l’une des principales interrogations concernant cette planète. Parmi les marqueurs morphologiques classiquement utilisés dans les reconstitutions climatiques, les vallées sont nombreuses et réparties de façon hétérogène sur Mars ce qui les rend particulièrement intéressantes à étudier. Ces objets peuvent nous renseigner à travers leur morphologie sur les conditions climatiques qui régnaient au moment même de leur formation et/ou plus tard au cours de leur évolution. L’objectif de cette thèse est de contribuer à la reconstitution du climat pré-amazonien (>3,5 Ga) en se concentrant sur l’étude des vallées martiennes équatoriales et du canyon équatorial de Valles Marineris. L’étude des vallées révèle que leur morphométrie et leur organisation spatiale ne ressemblent que très rarement à celles des vallées fluviatiles et de sapement fréquemment rencontrées sur Terre. Elles présentent, en revanche, de nombreuses similitudes avec les vallées tunnels d’origine sous-glaciaires terrestres. Il est donc plausible que certaines de ces vallées martiennes pré-amazoniennes aient pu voir le jour sous un climat froid par la fonte sous- et/ou pro- glaciaire de dépôts de glace de surface. D’autre part, les reconstitutions géomorphologiques régionales de Valles Marineris montrent que ce système de vallées a été le siège d(e)’une glaciation(s) postérieure(s) à sa formation dont il subsiste aujourd’hui des reliques datant probablement de plusieurs milliards d’années et qui sont protégées par une couverture de débris. Cette étude suggère la possibilité que les conditions atmosphériques pré-amazoniennes de Mars ne furent pas nécessairement très différentes de celles qui prévalent actuellement

One million cubic kilometers of fossil ice in Valles Marineris: Relicts of a 3.5 Gy old glacial landsystem along the Martian equator

International audienceSelf-consistent landform assemblages suggest that Valles Marineris, the giant valley system that stretches along the Martian equator, was entirely glaciated during Late Noachian to Early Hesperian times and still contains huge volumes of fossil ice. Some of these glacial landform assemblages are illustrated here, with representative examples selected in three regions: Ius Chasma, Central Candor Chasma and the junction between Coprates Chasma and Capri Chasma. A morphological boundary separating an upper spur-and-gully morphology from a smooth basal escarpment has been spectacularly preserved along valley walls throughout Valles Marineris. The boundary winds around topographic obstacles and displays long-wavelength variations in elevation. It is associated with lateral benches, hanging valleys and truncated spurs. Comparisons with terrestrial analogs indicate that it is most reasonably interpreted as a glacial trimline. Chasma floors are covered by various kinds of terrains, including hummocky terrains, platy terrains, lateral banks, layered benches and a draping mantle. Landforms in these terrains and their spatial relationship with the interpreted trimline suggest that they correspond to various disintegration stages of an ancient glacial fill, currently protected by a superficial cover of ablation till. Altogether, these land-forms and terrains compose a full glacial landsystem with wet-based glaciers that were able to flow and slide over their beds. It was most probably fed by ice accumulating at low elevations directly from the atmosphere onto valley floors and walls, with only minor contributions from tributary glaciers flowing down from higher elevations. Similar fossil glacial landsystems dating back from the early Martian history are to be expected in many other low-latitude troughs such as chasmata, chaos, valleys, impact craters and other basins

One million cubic kilometers of fossil ice in Valles Marineris: Relicts of a 3.5 Gy old glacial landsystem along the Martian equator

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