Morphological Development of Polygonal Patterned Ground and Associated Sand Wedges in Dry Valleys, Antarctica

Polygonal patterned ground forms characteristic patterns on Earth and Mars. Here I present and interpret morphological properties as revealed from field measurements, differential GPS, and ground based LIDAR. The continued cycling of polygonal patterned ground can lead to surface renewal. Initially the ice-rich ground contracts during cooling and cracks form as the tensile stress exceeds the strength of the ice-cemented ground. In the hyperarid Dry Valleys, the cracks open and sand is injected to depth. As the ground warms the cracks close but, since the cracks now contain sand and cannot fully close, the edges of the cracks deform and the deformation propagates into the polygon. This repeated cycle of crack opening, sand infilling, and cracks closing leads to the characteristic ridge and trough pattern. While it appears symmetrical, it is apparent that sand wedge growth is highly directional and cracks effectively are migrating as sand accumulates preferentially on one side of the crack. The relative migration of the cracks leads to surface renewal, in some cases on a time-scale as short as tens of thousands of years. The cause of the asymmetric growth of the sand wedge is likely due to climatic gradients. Snow accumulates in troughs preferentially on the downwind side and this is reflected in measured moisture gradients in the sand wedges. Asymmetric warming due to solar aspect may also influence the direction of sand wedge growth. It was also noted that polygons on slopes often have a lower relief than the slope gradient. This is seen in polygon areas whose surfaces have totally been renewed by sand-wedge growth. It is apparent in polygonal patterned ground on gentle hill slopes that have undergone many cycles and the growth of sand wedges has spanned across the entire polygon; the polygon centers are nearly level or are sloping much more gently than the hill slope. This leveling process provides an index of the activity of the polygons relative to the hillslope processes and that may have general applicability to other regions on Earth and on Mars.