POSSIBLE ICE-WEDGE POLYGONS IN UTOPIA PLANITIA, MARS, AND THEIR POLEWARD LATITUDINAL GRADIENT
Résumé
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. Rising mean-temperatures induce ice-wedge degradation and the collapse of uplifted margins, giving these polygons a distinctly high-centred appearance [1-7]. Sand or soil-sand admixtures also are associated with polygon-margin fills and may generate individual fields of lcps and hcps [e.g 7]. The closely-set spatial association of lcps and hcps in wet periglacial-landscapes on Earth, however, often is indicative of ice-wedge polygons, albeit in disparate phases of evolution. Fig. 2. Possible thermokarst/polygon complex in UP. LCPs adjacent to scarp on left side of image (black arrow); HCPs above and to the right (white arrow) (HiRISE ESP_026094_2250; 44.657 o N, 111.415 o E). North is up. Image credit, NASA/JPL/Univ. of Arizona. LCPs/HCPs in UP: The presence of lcps/hcps at the mid-latitudes of both Martian hemispheres, as well as the spatial and possibly periglacial-association of these polygons with alas-like landforms (Fig. 2), are noted in the literature [e.g. 8-12]. On the other hand, questions concerning the range, density or sparcity of lcp and/or hcp distribution in UP (Figs. 3-4) have not been explored fully. Fig. 3. Dense distribution of lcps near crater central-peak (HiRISE ESP_011523_2235; 42.953 o N, 115.670 o E). North is up. Image credit, NASA/JPL/ University of Arizona. One of the keynotes of our study is evaluating the ratio of lcp/hcp distribution by latitude. If a statistical analysis of this distribution shows that the presence of 50 m 100 m 50 m 2121.pdf 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)