Morphometric evidence of 3.6 Ga glacial valleys and glacial cirques in martian highlands: South of Terra Sabaea

The climate of early Mars remains unclear and the debate is topical. Recently, climatic models have suggested a cold climate during the Noachian/Early Hesperian on Mars, which goes against the wet and warm climate often put forward. The purpose of this study is to seek geomorphologic evidence of this early cold climate. To achieve this, a step-by-step study of the geometry and morphology of several martian valleys has been done in the southern highlands. The analysis highlights the morphometric properties which enable the identification of martian glacial landscapes. We identified 100 cirques and 83 glacial valleys in two craters and on one mountain in the southern part of Terra Sabaea. The studied morphologies have the same morphometric characteristics and trends as terrestrial and martian glacial valleys and glacial cirques. In contrary, these trends are very different from those observed in fluvial valleys on Earth and on Mars. (1) The martian glacial valleys are U-shaped with a large flat floor and a V-index >0.20. While martian and terrestrial fluvial valleys are V-shaped with a V-index 1 while for fluvial valleys this ratio is ≫1. (3) Finally, the cross-sectional area and the elevation are higher for the glacial valleys compared to the fluvial ones. These glacial valleys often originate with a topographic hollow which have the same properties than terrestrial glacial cirque. The glacial landscapes identified in southern Terra Sabaea are restricted to elevations >1000 m and are dated by crater counting to be 3.6 Ga. This study strongly supports glacial processes as the origin of these valleys and cirques, and is the first morphometric evidence of glacial valleys associated with glacial cirques in the southern highlands in agreement with the climatic models. We propose that the martian climate during the late Noachian/early Hesperian was characterized by glaciated highlands at elevations >1500 m and at lower elevations where fluvial valley network has been identified ( 1500 m and temperate lowlands < 1500 m.