“Icy” scarp exposures, “ice-rich” overburdens and ephemeral climate-warming at Mars' mid-latitudes in the very late Amazonian epoch

Abstract

Most chronologies of Mars' climate history suggest that Mars' climate has undergone a relatively linear evolution, i.e. from cool and ephemerally-wet Late Noachian/Early Hesperian Epoch boundary conditions to the very Late Amazonian Epoch where extreme aridity, sparse water-vapour pressure and largely sub-zero (celsius) mean temperatures rule at non-polar latitudes. Against this backdrop, icy-surface accumulation and glacial features, forms and landscapes are expected; the freeze-thaw cycling of water at or near the surface and the associated development of ice-rich periglacial landscapes rooted in wet processes are not. Here, we use an icy (glacial) scarp exposure (HiRISE image ESP 0500477_1220) and its possible ice-rich (periglacial) overburden as a case study of other icy-scarp exposures on Mars. We suggest that the icy flow-forms that embrace our case-study scarp and others elsewhere (see Table-1) either could be: relicts of a high-amplitude glacial system and regional climate-variations; or of a post-glacial transition characterised by low-amplitude climate oscillations that drove rhythmic slope-activity. We also propose that the icy scarp exposures, orientation and retrogression is the work of preferential insolation/sublimation and of regional/local winds. Often, wind-streaks and blow-outs are observed adjacent to icy-scarp headwalls. These features are the result of headwall erosion by the work of wind and of icy/lithic mass loss by sand-blasting or enhanced radiative exposure. We also note that some of the wind-streaks and blow-outs vary over time (through to the present day) in: presence/absence; surface coverage, and/or darkness/lightness. This suggests that aeolian activity is diachronic and current. Based on a suite of morphological similarities with ice-rich permafrost regions on Earth such as the Tuktoyaktuk Coastlands in northern Canada, we propose that the icy-scarp and ice-rich overburden at our case-study location are pre- and post-dated by features and forms that are/were ice-rich too, i.e. formed by the freeze-thaw cycling of water. These features and forms include: 1) contiguous low and high centred polygons, possibly underlain by (aggraded or degraded) ice wedges at the margins; 2) interconnected channels of polygon corner/trough pits, perhaps formed by the sublimated loss of ice-wedges; and, 3) alas-like depressions, suggestive of thermokarst that has undergone ice-loss, in this case, by sublimation. Ice depletion by sublimation is not-inconsistent with ice enrichment by the freeze-thaw cycling of water and there is no necessary reason to believe that enrichment and depletion periods on Mars were/are concomitant. Moreover, ice-rich landscapes on Earth have been observed to develop in extremely short periods of time, as little as ∼10⁻¹ years. As such, small and favourable windows of climatic opportunity could be/have been sufficient to generate ice-rich landscapes on Mars in the very Late Amazonian Epoch. This could be/have been the case especially where: a) water currently is meta-stable; b) near-surface perchlorate salts (brines) have been detected or are hypothesised; and, c) regional volcanism has occurred in the recent past.