EVIDENCE OF LANDSCAPE MODIFICATION IN AND AROUND THE ARGYRE IMPACT BASIN, MARS, BY "WET" PERIGLACIAL PROCESSES

We have used all relevant HiRISE, MOC, THEMIS and CTX images in and around the Argyre impact basin (AIB; ~30-70⁰S; ~300-160⁰E), Mars, to identify and then map a close spatial-assemblage of three landforms whose origin could be indicative of an ice-rich and thaw-modified (regional) regolith: (a) polygonised terrain comprised of unsorted and small-sized polygons (~150m in diameter); (b) putative (relict) debris-flows (kms in length); and, (c) mantled terrain (extant in some instances, dissected in others). Heretofore, the possible connexion between each of these landforms and the occurrence of an ice-rich landscape at the northern or southern mid-latitudes of Mars has been debated by numerous workers; whether thaw has contributed to the formation or modification of these landscapes has been a question of ongoing interest and of far-reaching consequence. Here we present and discuss the occurrence of low-centred polygons (a polygon sub-type) in the general population of small-sized polygons in and around the AIB. On Earth, low-centred polygons are a commonplace characteristic of periglacial landscapes in regions such as the Tuktoyaktuk Coastlands (TC) of northern Canada where small-sized polygons form by thermal-contraction cracking and ice wedges underlie the marginal cracks or troughs that comprise the polygon boundaries. In the TC, the accumulation (or aggradation) of ice wedges is the product of the (seasonal) migration of snow-melt into polygon cracks and the subsequent freezing of this melt-water in situ. If and when the ice wedges degrade (thaw) then the troughs overlying them lose elevation relative to the polygon centres; this gives the polygons a low-centred appearance. Although mineral-filled troughs may show a similar range of elevation-related morphologies, i.e. sand-wedge polygons in the Antarctic Dry Valleys, they are not associated spatially with landforms, i.e. debris flows (current or relict), that are geological markers of a landscape modified by “wet” processes. We suggest that the low-centred polygons observed by us are part of a landform triumvirate that when bundled together is more consistent with stable liquid-water during the Late Amazonian epoch in and around the AIB than has been thought possible hitherto.