Possible (closed system) pingo and ice-wedge/thermokarst complexes at the mid latitudes of Utopia Planitia, Mars
On Earth, hydrostatic or closed-system pingos (CSPs) are perennial ice-cored mounds formed by the freeze-thaw cycling of water when or as thermokarst lakes lose their water by drainage or evaporation. The mounds vary in shape from circular or sub-circular to elongate, are sub-kilometre in their long axes, and may reach decametres in height. Often, the mounds show summit depressions, radial fractures and single tiers or benches. As the CSPs degrade, small-scale debris flows or slumps occur; end-stage degradation typically exhibits debris-laden ramparts elevated metres above the surrounding terrain.Typically, the CSPs originate and develop where deep and continuous permafrost is ice-rich, i.e. infused with excess ice, as it is in the Tuktoyaktuk Coastlands of northern Canada, mid to northern Alaska and northeast Russia. In these areas or regions the terrain often is populated by: a) thermokarst lakes and alases (thermokarst lake basins absent of water); b) fields of small-scale ice-wedge polygons (either low-centred or high-centred relative to the polygon margins); and, c) melt-water filled or emptied polygon-margin troughs or -corner pits. Individual troughs and pits connected by means of the local topographical gradient comprises beaded stream systems or networks. Here, we report, describe and discuss a suite of candidate CSPs at the mid-latitudes of Utopia Planitia (UP) (~35-50° N; ~80-115° E). The morphologies, scale and height of the Martian mounds, as well as the surface textures and characteristics, landscape features and spatially-associated landforms, are consistent with the CSPs observed in ice-rich regions on Earth. The UP mounds are nested in or reside adjacent to: flat-floored, possibly thermokarstic depressions; small-scale polygons (some of which exhibit low or high centres relative to their margins); and, connected polygon-margin depressions or pits that mirror beaded-stream systems in ice-rich terrain on Earth. The size-frequency distribution of craters in our study region precludes a surface younger than ~100 Ma. Thus, the CSP-like mounds in UP and the pre-mound periglacial-revisions of the landscape presented by us may be markers of freeze-thaw cycling that occurred much earlier in the Amazonian Epoch than most of the current discussions about periglaciation in UP suggest.