Are the Dorsa Argentea on Mars eskers?

The Dorsa Argentea are an extensive assemblage of ridges in the southern high latitudes of Mars. They have previously been interpreted as eskers formed by deposition of sediment in subglacial meltwater conduits, implying a formerly more extensive south polar ice sheet. In this study, we undertake the first large-scale statistical analysis of aspects of the geometry and morphology of the Dorsa Argentea in comparison with terrestrial eskers in order to evaluate this hypothesis. The ridges are re-mapped using integrated topographic (MOLA) and image (CTX/HRSC) data, and their planar geometries compared to recent characterisations of terrestrial eskers. Quantitative tests for esker-like relationships between ridge height, crest morphology and topography are then completed for four major Dorsa Argentea ridges. The following key conclusions are reached: (1) Statistical distributions of lengths and sinuosities of the Dorsa Argentea are similar to those of terrestrial eskers in Canada. (2) Planar geometries across the Dorsa Argentea support formation of ridges in conduits extending towards the interior of an ice sheet that thinned towards its northern margin, perhaps terminating in a proglacial lake. (3) Variations in ridge crest morphology are consistent with observations of terrestrial eskers. (4) Statistical tests of previously observed relationships between ridge height and longitudinal bed slope, similar to those explained by the physics of meltwater flow through subglacial meltwater conduits for terrestrial eskers, confirm the strength of these relationships for three of four major Dorsa Argentea ridges. (5) The new quantitative characterisations of the Dorsa Argentea may provide useful constraints for parameters in modelling studies of a putative former ice sheet in the south polar regions of Mars, its hydrology, and mechanisms that drove its eventual retreat.FEGB is funded by STFC grant ST/N50421X/1 and is grateful for bursaries provided by The Open University, The Ogden Trust and the British Society for Geomorphology. SJC was funded by a Leverhulme Trust Grant RPG-397. We would like to thank Peter Fawdon for his technical advice. We are grateful to RD Storrar for providing raw data for the Canadian eskers, and to both RD Storrar and H Bernhardt for their insightful reviews of the manuscript.This is the final version of the article. It first appeared from Elsevier via https://doi.org/ 10.1016/j.icarus.2016.03.02

Recent Basal Melting of a Mid-Latitude Glacier on Mars

Evidence for past basal melting of young (late Amazonian), debris-covered glaciers in Mars’ mid-latitudes is extremely rare. Thus, it is widely thought that these viscous flow features (VFFs) have been perennially frozen to their beds. We identify an instance of recent, localized wet-based mid-latitude glaciation, evidenced by a candidate esker emerging from a VFF in a tectonic rift in Tempe Terra. Eskers are sedimentary ridges deposited in ice-walled meltwater conduits and are indicative of glacial melting. We compare the candidate esker to terrestrial analogues, present a geomorphic map of landforms in the rift, and develop a landsystem model to explain their formation. We propose that the candidate esker formed during a transient phase of wet-based glaciation. We then consider the similarity between the geologic setting of the new candidate esker and that of the only other candidate esker to be identified in association with an existing mid-latitude VFF; both are within tectonic graben/rifts proximal to volcanic provinces. Finally, we calculate potential basal temperatures for a range of VFF thicknesses, driving stresses, mean annual surface temperatures, and geothermal heat fluxes, which unlike previous studies, include the possible role of internal strain heating. Strain heating can form an important additional heat source, especially in flow convergence zones, or where ice is warmer due to elevated surface temperatures or geothermal heat flux. Elevated geothermal heat flux within rifts, perhaps combined with locally-elevated strain heating, may have permitted wet-based glaciation during the late Amazonian, when cold climates precluded more extensive wet-based glaciation on Mars

Landforms indicative of regional warm based glaciation, Phlegra Montes, Mars

Viscous flow features (VFF) occur in the mid-latitudes of Mars and have characteristics consistent with being glaciers. Climate models suggest that martian glaciers are cold-based systems in which meltwater has never been widely produced. VFF are common in Phlegra Montes, a mountain range in the mid-latitudes of the northern hemisphere of Mars. However, in Phlegra Montes, the presence of an esker associated with an extant Amazonian Period VFF provides evidence that warm-based glacial processes did formerly operate. The problem at the centre of this paper is that the glacial meltwater responsible for this esker could have been produced as a consequence of its setting in a graben, with locally enhanced geothermal heating having been the driver of melt, not systemic heating associated with a regional warm-based regime. Given this uncertainty, this paper aims to determine if there are indicators of more widespread warm-based glacial processes in Phlegra Montes. The paper briefly describes the distribution and characteristics of VFF across the region, before focussing on the search for key landforms considered diagnostic of erosion by warm-based glaciers. From our observations, including discriminant morphometrics, we conclude that the landscape of Phlegra Montes is indicative of widespread warm-based glacial processes, including subglacial scour, linear abrasion and the incision of subglacial meltwater channels. Our findings have significance in constraining the contexts and process environments within which liquid water has been produced during the Amazonian Period on Mars and point to several lines of future research into martian glaciation, climate and landscape evolution.FEGB is part of the PALGLAC research team supported by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (Grant agreement No. 787263). FEGB also acknowledges funding from the Science and Technology Facilities Council (STFC) grant ST/N50421X/1

Morphometry of a glacier-linked esker in NW Tempe Terra, Mars, and implications for sediment-discharge dynamics of subglacial drainage

We present a systematic, metre-scale characterisation of the 3D morphometry of an esker on Mars, and the first attempt to reconstruct the multi-stage dynamics of esker formation on Mars. Eskers are sinuous ridges comprising sediment deposited by meltwater draining through ice-confined tunnels within or beneath glaciers. Detailed morphometric insights into eskers on Mars are important for (i) informing morphometric tests of whether sinuous ridges elsewhere on Mars are eskers, and (ii) informing modelling experiments which aim to reconstruct the glaciological and environmental controls on esker formation on Mars. We use a digital elevation model generated from High Resolution Imaging Science Experiment (HiRISE) images to characterise the height and width of an extremely rare esker associated with a late-Amazonian-aged viscous flow feature (debris-covered glacier) in NW Tempe Terra, Mars. Our measurements suggest that the NW Tempe Terra esker is a ‘stacked’ formation comprising an underlying ‘lower member’ ridge that is superposed by a narrower ‘upper member’ ridge. We used a novel morphometric approach to test whether the apparent stacking records two distinct esker deposition regimes (either within the same drainage episode, or within temporally-separated drainage episodes). This approach posits that esker crest morphology is controlled by primary esker formation processes and, by extension, that portions of eskers with similar crest morphologies should have similar morphometric relationships. We predicted the morphometric relationships described by the constituent upper and lower member ridges based on ‘reference relationships’ observed for morphologically-similar portions of the esker where no evidence of stacking was observed. Our observations corresponded well with the predicted relationships, supporting our stacked esker hypothesis. We propose conceptual models, which invoke spatial and temporal variations in sediment supply and meltwater discharge, to explain the stacked morphology. These models are informed by morpho-sedimentary relationships observed along eskers on Earth.This work was un-dertaken at The Open University as part of a PhD studentship held by FEGB and funded by the Science Technology and Facil-ities Council (STFC) grant ST/N50421X/1. Manuscript preparation was supported by The Open University (internal funding) and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (ERC Advanced Grant PALGLAC 787263). We also gratefully acknowledge UK Space Agency grants ST/L00643X/1, ST/R001413/1, and ST/R001383/1 (MRB); ST/R001405/1, ST/P001262/1, and ST/S00145X/1 (SRL); and ST/R/001375 (AH). SJC is supported by the French space agency, CNES