An equatorial periglacial landscape on Mars

We present evidence, drawn from new 25 cm/pixel HiRISE images of Mars, of landforms indicative of geologically-recent thaw degradation of ice-rich terrain near the Martian equator. We have focused our study on the head-region of Athabasca Vallis, a catastrophic flood-carved outflow channel thought to have been active within the last 2-8 Ma. The geomorphology of the study area is characterised by a series of polygonally-patterned surfaces that are bounded by erosional scarps. The scarps are indented with cirque-shaped niches that appear to be controlled in scale and form by the pre-existing polygonal sculpture. Shallow basins, defined by niche-indented scarps, are also seen and contain pitted mound and cone structures that are similar in form to terrestrial pingos. Associated with many of these scarps are dendritic channels and gullies, blocky debris and hummocky terminal deposits. Together, these correspond morphologically to terrestrial retrogressive thaw slumps which are landforms typical of thermokarst environments. We have also observed an evolutionary sequence of morphologies beginning with subsidence of polygonised surfaces and ending with scarp bounded basins fed by inter-polygon channels. This is consistent with the former presence of surficial liquid water released by ground-ice thaw and is again indicative of thermokarst. Moreover, the observations of epigenetic polygons and channels, and of polycyclic retrogressive thaw slumps, suggest that thaw and standing water persisted for many seasonal cycles. The assemblage of landforms, the occurrence of polygons and pingo-like forms within erosional basins, and observations of fluvial-like channels cross-cutting pre-existing surface features are consistent with a surface that has evolved significantly since it was originally emplaced. This precludes this terrain being a volcanic (i.e., primary) landscape, as has been suggested in the past. The formation of this landform assemblage requires that there was thaw of ground ice and persistent liquid water at the surface. Given the geologically young flood deposits in which this assemblage occurs, our observations point to a recent period in which Mars' climate supported thaw conditions in this region. (C) 2009 Elsevier B.V. All rights reserved.