Post by Dr. Colman Gallagher.
Mars’s Athabasca Vallis is a 10 km wide, 300 km long channel carved by floods originating in the Cerberus Fossae. Recent images acquired by the HiRISE instrument aboard the Mars Reconnaissance Orbiter provides strong evidence that the head reaches of Athabasca Vallis experienced repeated cycles of freezing, with the development of ground ice and polygonised terrain, and warming, marked by ground ice thaw.
This palaeoclimate interpretation comes from the widespread existence in the region of polygonised ground compartmented into separate surfaces and isolated depressions by scarps indented by cirque-shaped niches and gullies (Imagess 1, 2 and 3). These landforms have been interpreted as retrogressive thaw slumps (RTSs) characteristic of thaw-induced thermokarst degradation of ground ice (Balme and Gallagher, 2009). RTSs occur in the permafrost regions of Earth and are diagnostic of degrading permafrost (Harris, 1981). So, the assemblage of landforms near the head of Athabasca Vallis represents former conditions on Mars typical of terrestrial thermokarst environments.
RTSs comprise three main elements (Lantuit and Pollard, 2008): a steep headwall incised into the active layer (the ice-rich layer that experiences seasonal melt), a headscarp indented by cirque-shaped niches that retreats by ablation of permafrost, and the slump floor, which consists of mudflow and debris flow deposits left as the permafrost ablates (Images 4 and 5). Terrestrial RTSs can be triggered by the formation of a depression induced by thaw consolidation and downwearing of the ground (Czudek and Demek, 1970) or by wave erosion of permafrost shorelines (Are, 1978; Lewkowicz, 1990b; Mackay, 1966), when thermoabrasion uncovers massive ground ice which then melts on exposure to the warmer atmosphere. In such cases, the slumped debris is often quickly washed away, leaving “clean” cirque-shaped niches. RTSs can also form as active layer detachments caused by deep seasonal ground thaw (Lewkowicz, 1987a; Lewkowicz, 1990a). The multiple surfaces and depressions separated by cuspate scarps in the head reaches of Athabasca Vallis indicate that the RTSs are polycyclic, the younger RTSs having formed within the margins of older ones from the epigenetic freezing and thawing of ice-rich material transported down-slope from the older flows. A terrestrial example of this behaviour is shown in Image 5 and described by Lantuit and Pollard (2008).
The similarity of the landforms in the head reaches of Athabasca Vallis to terrestrial thermokarst landforms points to the former presence of flowing liquid water (or at least a water-based muddy fluid) and many bodies of standing water (Balme and Gallagher, 2009). This environment must have functioned after the enormous floods that carved the outflow channel had abated and the ground had frozen to considerable depth. So, it is probable that in the geologically recent past there was standing water, generated by the thaw of ground ice, in the shallow basins and water flowing through the channels that led away from thawing ground ice exposed in the scarp headwalls.
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