Abstract DGP2026-51

Polygonal grounds are landscape features commonly associated with permafrost regions, where they form through freeze-thawing cycles and frost-related processes. However, polygonal patterns also occur on saline alluvial sediments in the hyper-arid Atacama Desert, but they received rather little attention. For these hyper-arid polygons, a permafrost origin is not plausible due to the lack of enduring sub-zero temperatures and limited water availability. To investigate their surface geometry, subsurface structure and potential cracking mechanisms, we combined drone-based morphometric terrain analysis with field experiments and sedimentological and geochemical analyses. Our results reveal that polygons exhibit diverse surface morphologies and are composed of siliciclastic sediment that is cemented by salts vertically stratified according to their solubility. Less soluble calcium sulfates, primarily gypsum and anhydrite, dominate the upper profile (~0–0.5 m), whereas more soluble halite and nitrates occur at greater depths (>0.5 m). Polygon centers are separated by V-shaped sand wedges up to 0.5 m wide and 1 m deep, displaying vertical lamination indicative of repeated cracking and infilling. Polygon geometry is strongly influenced by the local topography: polygons become more elongated on steeper slopes and are aligned either parallel or perpendicular to slope orientation. The presence of hydrated and dehydrated calcium sulfates suggests a cracking mechanism driven by episodic rainfall, where dehydration–hydration cycles induce volumetric changes and fracturing in cemented alluvium. In contrast, for polygons mainly cemented by halite, a thermal cracking genesis is more plausible, driven by large diurnal surface temperature variations and subsequent infilling by aeolian transported sand. While gypsum dehydration–hydration cycles require periodic water input, saline thermal contraction cracking can occur in the absence of liquid water, making it a particularly relevant analogue for polygonal ground formation in saline deposits on Mars.