| Summary: | International audience Introduction: Here, we describe and evaluate: a) the presence and distribution in Utopia Planitia (UP), Mars (40-50 o N, 110-124 o E), of small-sized polygons , (10-25 m in diameter), with low centres (lcps) or high centres (hcps) relative to their margins; b) the spatial , perhaps periglacial, association of these polygons and thermokarst-like depressions or basins; and, c) statistical data that support the hypothesis that ice-wedges underlie lcp/hcp margins. LCPs/HCPs on Earth: Geographically-expansive complexes of ice-wedge polygons (be they lcps or hcps), thermokarst, thermokarst lakes and alases, i.e. thermokarst depressions of basins absent of water, are commonplace in the Tuktoyaktuk Coastlands (TC) of northern Canada and the Yamal peninsula (YP) of eastern Russia [1-4]. In these and similar arctic-regions sur-face/near-surface water is abundant, freeze-thaw cycling is ubiquitous and the permafrost is ice-rich to depth [1-4, Fig. 1]. Ice-rich permafrost comprises excess ice: "the volume of ice in the ground which exceeds the total pore-volume that the ground would have under natural unfro-zen-conditions" [5]. Ice lenses, veins, wedges or larger masses of consolidated ice are typical examples of excess ice [5]. Thermokarst comprises excess ice. This makes it particularly sensitive to volumetric inflation as ice ag-grades, when mean temperatures remain stable or fall, or volumetric deflation as ice degrades, when mean temperatures rise and meltwater is evacuated by drainage or evaporation from the thaw zone. Fig. 1. Near-surface ice and ice wedges at Peninsula Point, SW of Tuktoyaktuk: (a-c) recessional terraces resulting from thermal destabilisation of coastline. (d) Surface depressions above degrading ice-wedges; massive-ice exposures cen-tre/centre left. Image credit, R. Soare. Spatially-associated assemblages of lcps and hcps also are geological markers of climate change. Stable or falling mean-temperatures engender ice-wedge aggrad-ation and the uplift of polygon margins. ...
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