Possible ice-wedge polygons and recent landscape modification by wet periglacial processes in and around the Argyre impact basin, Mars

The Argyre basin and associated rim-materials in the southern hemisphere of Mars are ancient, having been formed by the impact of a large body similar to 4 Gya. This notwithstanding, the regional landscape continues to be altered by a multiplicity of geological and geomorphological processes. Three landforms, whose close spatial-association is identified in a new geological map of the Argyre region (similar to 290-360 degrees E; similar to 30-72 degrees S), feature prominently in the list of very Late Amazonian Epoch alterations: (a) Small-sized (<= similar to 20m in diameter) and unsorted polygons that exhibit metre to sub-metre elevated margins or shoulders, giving them a low-centred appearance; in wet permafrost environments on Earth low-centred polygons (LCPs) often are underlain by ice-wedges. (b) Gullies, seemingly formed by wet flow(s) and incised by the LCPs. (c) A putative (possibly ice-rich) latitude-dependent mantle (LDM) that underlies the LCPs and spatially-convergent wet gullies in all of our observations. These landforms occur from the middle to the high (near-polar) latitudes of the region and overlie geological units of all ages. Hitherto, the presence of the LCPs has been reported only fleetingly in the literature and only in as much as they have been observed on the walls of a few scalloped depressions in mid-Utopia Planitia. By contrast, we report the ubiquitous occurrence of the LCPs in and around the Argyre impact-basin on gully-margins and adjacent slopes as well as on relatively-flat inter-crater terrain. On the basis of three separate but related lines of reasoning we hypothesise that ice wedges could be present beneath LCP margins in our study region. If we are correct then these LCP sites are geomorphological expressions of boundary conditions that were relatively warm and wet in the recent past. First, substantial ice-wedging in permafrost environments on Earth requires the availability of meltwater. If, as some researchers propose, the formation of some wet gullies at the martian mid-latitudes is induced by the localised (slope-side and crater-wall) thaw of the underlying LDM at the gully sites themselves, then meltwater also should be available for ice-wedging and the formation of LCPs at these sites. Interestingly, LCPs are observed on gully-channel walls as expected if meltwater is associated with gully formation; however, in some instances the LCPs are also observed on the slope-side terrain that extends for hundreds of metres beyond the channel walls and even above the gully alcoves. This suggests that the distribution of icy terrain affected by thaw could be much more substantial than has been suggested hitherto. Second, LCPs that are identical in shape and scale to the slope-side LCPs are observed on relatively flat inter-crater terrain (also underlain by the LDM) that is distal from the wet gullies. By contrast, here, their distribution extends for kilometres. This too could be indicative of meltwater being more extensive in the regional landscape than most workers have thought possible. Third, on Earth the dryness or the wetness of a permafrost environment determines whether LCP margins are underlain by sand or by ice. When the observed LCPs and major deposits of sand in the study region are plotted on our new map of the Argyre impact-basin, we see that these deposits are rarely proximal. On the other hand, the LDM underlies the LCPs in all instances and, if modified by thaw, seems to be a likelier source of margin fill for the LCPs than sand. (C) 2014 Elsevier Inc. All rights reserved.