Thermokarst lakes and ponds on Mars in the very recent (late Amazonian) past

The history of water is fundamental to understanding the geological evolution of Mars and to questions concerning the possible development of life on the Red Planet. Today, Mars is cold and dry; its regolith is permanently frozen and except under highly localised and transient conditions, liquid water is unstable at the surface. Intriguingly, we have identified geological features that could be markers of very late-Amazonian “wet” or ice-rich periglacial processes in Utopia and western Elysium Planitiae: 1. rimless, flat-floored and lobate, sometimes scalloped, depressions that are suggestive of terrestrial alases (evaporated/drained thermokarst lakes); 2. small-sized polygonal patterned-ground (perhaps formed by thermal-contraction cracking and possibly underlain by ice wedges); and, 3. circular/near-circular raised-rim depressions (consistent in morphology and scale with pingo-scars) that are nested in rimless depressions. In terrestrial cold-climate, non-glacial environments, landscape assemblages of this type occur only in the presence of ice-rich permafrost.Commenting upon the origin of the putative periglacial features on Mars, most workers have suggested that sublimation and not evaporation has been the dominant process. By contrast, we propose that two key characteristics of the rimless depressions – inner terraces and orthogonally-oriented polygons – are markers of stable, ponded water and its slow loss by evaporation or drainage. If the raised-rim landforms are pingo scars, then this also points to boundary conditions that are supportive of stable liquid water.With regard to the relative age of the features described above, previous work identified some lobate depressions superposed on crater-rim gullies in the region (Soare et al., 2007). Gullies could be amongst the youngest geological features on Mars; superposed depressions point to an origin that is more youthful than the gullies. In turn, as some raised-rim landforms are superposed on rimless depressions, this is indicative of an origin that is even more recent than that of the depressions. Together with the geological evidence showing that the rimless depressions could have been formed by ponded water, the stratigraphy of the putative periglacial-landscape in this region suggests that the very late Amazonian period could have been warmer and wetter than had been thought hitherto.