LOW-CENTRED POLYGONS, “WET” GULLIES AND THE LATITUDE-DEPENDENT MANTLE: MAPPED EVIDENCE OF LANDSCAPE MODIFICATION IN AND AROUND THE ARGYRE IMPACT BASIN OF MARS BY FREEZE-THAW CYCLING

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 ~4 Gya. This notwithstanding, the regional landscape continues to be altered by a multiplicity of geological and geomorphological processes. Three landforms, each of them perhaps rooted in the freeze-thaw cycling of water, feature prominently in the list of recent (Late Amazonian Epoch, perhaps as recently as a few million years ago) alterations: (a) small-sized (≤~20m in diameter) and unsorted polygons that exhibit elevated margins or shoulders, giving them a low-centred appearance; in “wet” permafrost environments on Earth these low-centred polygons (LCPs) often are underlain by ice-wedges; (b) gullies, seemingly formed by “wet” flow(s) and incised by the LCPs; and, (c) a putative (and possibly ice-rich) latitude-dependent mantle (LDM) that underlies the LCPs and spatially-convergent “wet” gullies in all of our observations.

Here, we present a new geological map of the Argyre region (~290-360⁰ E; ~30-72⁰ S) that integrates these observations. The map plots the spatial convergence of the LCPs, “wet” gullies and the putative LDM at the middle to the high (near-polar) latitudes of the Argyre region and shows that the landform triumvirate overlies geological units of all ages. Hitherto, the presence of LCPs has been reported only fleetingly in the literature and only in as much as they have been observed in a few Late Amazonian Epoch thermokarst-like depressions in mid-Utopia Planitia. However, the LCPs observed by us are widespread in the Argyre region and their spatial convergence with the “wet” gullies and putatively ice-rich LDM suggests that freeze-thaw cycling could have enjoyed a far greater extent and range in this area of Mars than has been thought possible.