Periglacial landforms at the Phoenix landing site and the northern plains of Mars

We examine potentially periglacial landforms in Mars Orbiter Camera (MOC) and High Resolution Imaging Science Experiment (HiRISE) images at the Phoenix landing site and compare them with numerical models of permafrost processes to better understand the origin, nature, and history of the permafrost and the surface of the northern plains of Mars. Small-scale (3-6 m) polygonal-patterned ground is ubiquitous throughout the Phoenix landing site and northern plains. Larger-scale (20-25 m) polygonal patterns and regularly spaced (20-35 m) rubble piles (localized collections of rocks and boulders) are also common. Rubble piles were previously identified as "basketball terrain" in MOC images. The small polygon networks exhibit well-developed and relatively undegraded morphology, and they overlay all other landforms. Comparison of the small polygons with a numerical model shows that their size is consistent with a thermal contraction origin on current-day Mars and are likely active. In addition, the observed polygon size is consistent with a subsurface rheology of ice-cemented soil on depth scales of about 10 m. The size and morphology of the larger polygonal patterns and rubble piles indicate a past episode of polygon formation and rock sorting in thermal contraction polygons, while the ice table was about twice as deep as it is presently. The pervasive nature of small and large polygons, and the extensive sorting of surface rocks, indicates that widespread overturning of the surface layer to depths of many meters has occurred in the recent geologic past. This periglacial reworking has had a significant influence on the landscape at the Phoenix landing site and over the Martian northern plains.