Morphological comparison of polygonal patterned ground across the Arctic and Antarctic: Implications for polygon formation on Earth and Mars

Polygonal patterned ground (polygons) is ubiquitous in polar periglacial regions. It is thought to form due to repeated fracturing of the ground during freeze-thaw as a result of fluctuations in air temperature and soil conditions. Polygons can channelise overland flow in their troughs and guide groundwater flow during permafrost thaw, providing pathways for channel network development. Given the importance of polygons on local hydrology and geomorphology in cold regions, a key knowledge gap exists: We do not yet understand the evolution of existing polygons or the formation of new polygons under a changing climate. This is especially important as climate change is causing cold region water budgets to change, driving landscape change. We investigate the morphologic characteristics that are associated with polygons to indirectly examine their formation mechanism. We extensively map polygon morphologies across the McMurdo Dry Valleys in Antarctica, Prudhoe Bay in Alaska, as well as on Devon Island and Axel-Heiberg Island in the Canadian High Arctic using high-resolution DEMs derived from LiDAR data. We use a semi-automatic mapping tool based on adaptive thresholding to accelerate and scale our efforts while also improving reproducibility. We calculate surface slope and roughness for baseline lengths of 3m to 300m to investigate how and whether polygon morphology varies with local and regional topography. Overall, we quantify how polygon shape and form varies by proximity to important hydrological features. For example, in the McMurdo Dry Valleys, polygons are more often orthogonal and low-centred when they are closer to streams and glacier termini, but are characteristically hexagonal and high-centred  elsewhere. Orthogonal polygons are characterised by a smoother surface compared to hexagonal polygons across all baseline lengths, bounded by a rough `ridge’ on one side and a stream on the other. Further, on Axel-Heiberg, polygons that formed within the last 60 years are more orthogonal the closer they are to a lake. These observations suggest that polygon shape is controlled by soil moisture.  It is commonly accepted that polygons form as a result of thermal contraction cracking followed by ice- or sand-wedge formation, and field studies suggest that the formation of ice-wedges over sand-wedges can be explained by elevated soil or air moisture. Sand-wedges potentially are more deformable than ice-wedges, allowing for the fracture network to evolve and relax into a hexagonal pattern, whereas ice-wedges would preserve the initial, orthogonal, pattern. Consequently, we hypothesise that the number and size of ice-wedges decreases along soil moisture gradients, and hence polygons farther away from water sources evolve into hexagonal shapes over repeated fracture cycles. This would mean that existing streams and other water sources set up gradients in the amount of ice stored throughout a polygon field, which in turn will influence both surface and groundwater flow during permafrost thaw, pointing towards complex interactions between polygons and landscape evolution in a changing climate.