The periglacial landscape at the Phoenix landing site

The Mars Scout mission Phoenix landed in the northern high latitudes of Mars in an area thought to be dominated by subsurface ground ice, and has provided a unique detailed examination of periglacial morphology and topography that is not visible from orbit. The landing site and surrounding terrain is dominated by polygonal patterned ground typical of the Martian northern plains. The polygons exhibit a gentle U-shaped trough and rounded mound topography, with an average trough depth of 17 cm and an average surface slopes of only 6.7 degrees. Polygons an average 4.3 m across form a pattern superimposed on all other geologic landforms, including larger 20-25 m polygons and kilometer long linear ridges. The polygon morphology and the predominance of subsurface ice-cemented soil are consistent with the formation of subsurface "sand wedges'' in a permanently frozen arid climate. Small-scale soil furrows and cracks into the ice-cemented soil indicate that the thermal contraction process responsible for the formation of the polygons is active today. Rocks appear to be sorted within the polygon troughs, indicating a process of cryoturbation within the permanently frozen polygons. The complexity of the patterns, crosscutting relations, and rock distributions, indicate advanced maturity of the polygon networks and shifting crack patterns, possibly facilitated by crack and wedge annealing during times of low obliquity.