Glacial landscape and paleoglaciation in Terra Sabaea: Evidence for a 3.6 Ga polythermal plateau ice cap

International audienceIn a previous study, we demonstrated with a comparative morphometrical analysis the first morphometric evidence of a glacial landscape composed of glacial cirques and glacial valleys in the south of Terra Sabaea at an elevation > 1000 m in two impact craters and one mountain. The purpose of this study is to use the same method to seek other geomorphologic evidence of glacial landscapes elsewhere in Terra Sabaea. Based on a comparison between current and old glacial landscapes on Earth and Mars, we identified 81 glacial valleys and possible evidence for a former plateau ice cap dated at 3.6 Ga at the highest elevation in Terra Sabaea. The identified glacial valleys have the same morphometric properties as terrestrial and martian glacial valleys with U-shaped cross-sectional profiles, a V-index >0.2, a length to with ratio >1 and a cross-sectional area to drainage area ratio four times higher than the fluvial ones. Moreover, these properties are different from terrestrial and martian fluvial valleys. We did not find well preserved glacial cirques in this area, this absence questions the origin of glacial valleys. However, the presence of an extensive flat plateau, from which the long valleys radiate, could have hosted an ancient plateau ice cap which was the source of these glacial valleys. A comparison with the Cantal and the Shaluli Shan in the southeastern Tibetan plateau on Earth reveals morphometrical similarities with our study area. In fact, long glacial valleys, originating radially from a plateau at higher elevation are characteristics of an ancient plateau ice cap. This analysis allowed us to propose a polythermal regime for martian glacial landscape, namely a cold-based ice cap except at the margin where the regime is warmed-based due to the steeper topography. This topography created shear stress which increased the heat at the base of the ice and created the outlet glacial valleys. Near the plateau, the radial valleys are U-shaped with a V-index >0.2 but downstream, to the low elevation area, these valleys become more V-shaped with a V-index around 0.1. This hypothesis is supported by the presence of an open-basin paleolake making the transition between inlet glacial valleys upstream and an outlet V-shaped valley downstream. So the morphometry of the radiating valleys suggests that liquid water played a role in the formation of this landscape