Abstract
Polygonal networks of patterned ground are a common feature in cold-climate environments. They can form through the thermal contraction of ice-cemented sediment (i.e. formed from fractures), or the freezing and thawing of ground ice (i.e. formed by patterns of clasts, or ground deformation). The characteristics of these landforms provide information about environmental conditions. Analogous polygonal forms have been observed on Mars leading to inferences about environmental conditions. We have identified clastic polygonal features located around Lyot crater, Mars (50°N, 30°E). These polygons are unusually large (>100 m diameter) compared to terrestrial clastic polygons, and contain very large clasts, some of which are up to 15 metres in diameter. The polygons are distributed in a wide arc around the eastern side of Lyot crater, at a consistent distance from the crater rim. Using high-resolution imaging data, we digitised these features to extract morphological information. These data are compared to existing terrestrial and Martian polygon data to look for similarities and differences and to inform hypotheses concerning possible formation mechanisms. Our results show the clastic polygons do not have any morphometric features that indicate they are similar to terrestrial sorted, clastic polygons formed by freeze-thaw processes. They are too large, do not show the expected variation in form with slope, and have clasts that do not scale in size with polygon diameter. However, the clastic networks are similar in network morphology to thermal contraction cracks, and there is a potential direct Martian analogue in a sub-type of thermal contraction polygons located in Utopia Planitia. Based upon our observations, we reject the hypothesis that polygons located around Lyot formed as freeze-thaw polygons and instead an alternative mechanism is put forward: they result from the infilling of earlier thermal contraction cracks by wind-blown material, which then became compressed and/or cemented resulting in a resistant fill. Erosion then leads to preservation of these polygons in positive relief, while later weathering results in the fracturing of the fill material to form angular clasts. These results suggest that there was an extensive area of ice-rich terrain, the extent of which is linked to ejecta from Lyot crater.
Highlights
Terrestrial polygonal networks of centimetre- to decametrescale patterned ground are common in cold-climate regions
Unique to each polygon type to aid with identification, this is key for their use as morphological analogues for features observed on Mars
These polygons are enigmatic in that the clasts that demarcate the polygon sides are up to 15 metres across, with an average polygon diameter of 130 metres. This is significantly larger than morphologically similar polygons observed on Earth or on Mars (e.g., Fig. 1) which are found with maximum diameters of tens of metres (Washburn, 1956; Balme et al, 2009; Treml et al, 2010; Feuillet et al, 2012; Soare et al, 2016)
Summary
Terrestrial polygonal networks of centimetre- to decametrescale patterned ground are common in cold-climate regions. We have identified polygonal clast-bounded networks around Lyot crater, Mars These polygons are enigmatic in that the clasts that demarcate the polygon sides are up to 15 metres across, with an average polygon diameter of 130 metres. This is significantly larger than morphologically similar polygons observed on Earth or on Mars (e.g., Fig. 1) which are found with maximum diameters of tens of metres (Washburn, 1956; Balme et al, 2009; Treml et al, 2010; Feuillet et al, 2012; Soare et al, 2016). A better understanding of these features could provide useful information about the environment around Lyot, as well as the material that they are composed of
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