Sorted (clastic) polygons in the Argyre region, Mars, and possible evidence of pre- and post-glacial periglaciation in the Late Amazonian Epoch

Abstract

The Argyre basin and associated rim-materials in the southern highlands of Mars are ancient, having been formed by the impact of a large body ∼3.9Gya. Despite its age, the regional landscape exhibits a wide range of geological/geomorphological modifications and/or features, e.g. fluvial, lacustrine, aeolian, glacial and periglacial. Collectively, this bears witness to the dynamic evolution of the Argyre region from the deep past through to, perhaps, the present day.Here, we present three principal findings that point to at least two distinct episodes of periglaciation, separated by a possible glacial-interval, during the very Late Amazonian Epoch in eastern Aonia Terra (AT), i.e. on the western flank of the Argyre basin. These findings are the product of our circum-Argyre study of all HiRISE images (∼35–65°S and ∼290–350°E).(1)(a) The first periglacial episode involves the development of small-sized (∼15–25m in diam.) and clastically-“sorted polygons” (SPs). The SPs are observed at eighteen locations within eastern AT. Hitherto, the presence of SPs in this region has been reported at one location alone. No other observations of SPs in the southern hemisphere of Mars have been documented. Morphologically similar landforms develop in cold-climate (permafrost) landscapes on Earth by means of periglacial processes, i.e. freeze–thaw cycling, segregated-ice formation, cryoturbation and frost heave.(b) We ascribe a periglacial origin to the SPs in eastern AT on the basis of this similarity of form and, no less importantly, on the close spatial-association of the SPs with blockfields (whose weathered “clastic” products are the building blocks of periglacial sorting on Earth), gelifluction-like lobes and possible “wet” gullies. Where similar assemblages occur in terrestrial permafrost-landscapes, the presence of liquid water and of boundary conditions tolerant of freeze–thaw cycling, are observed or inferred.(c) Fifteen of the eighteen SP locations are clustered longitudinally (44.4–57.5°S; 289.9–302.4°E). This is inconsistent with the latitudinal- and (obliquity-driven) dependency of freeze–thaw cycling in the Late Amazonian Epoch hypothesised by many workers in the discipline.(2) The second periglacial episode is highlighted by the development of small-sized and clastically non-sorted polygons (NSPs). These polygons could have formed by means of a “dry” cryotic process, i.e. thermal-contraction cracking.(3) The NSPs incise (and thus postdate) a light-toned mantle, thought by numerous workers to comprise an “ice-dust” admixture. At some of the locations where the putatively icy-mantle has undergone apparent ablation, underlying SPs are observed. This suggests that the SPs predate the mantle and, derivatively, the NSPs as well.The proposed geochronology of “wet-based SP – icy mantle – dry-based NSP” (periods and interval) is entirely new to the community. Moreover, it underlines the possibility that periglacial and glacial boundary-conditions, at least in our study area, may have oscillated much more substantially in the very Late Amazonian Epoch than many workers have thought possible.