High latitude patterned grounds on Mars: Classification, distribution and climatic control

Patterned grounds such as polygonal features and slope stripes are the signature of the presence of ground ice and of temperature variations in cold regions on Earth. Identifying similar features on Mars is important to understand its past climate as well as the ground ice distribution. In this study, young patterned grounds are classed and mapped from the systematical analysis of Mars Observer Camera high resolution images. These features are located poleward of 55° latitude which fits the distribution of ground ice found by the Neutron Spectrometer onboard Mars Odyssey. Thermal contraction due to seasonal temperature variations is the predominant process of formation of polygons formed by cracks which sizes vary from 15 to 300 m. The small (<40 m) widespread polygons are very recent and degraded by the desiccation of ground ice from the cracks which enhances the effect of ice sublimation. The large polygons (50 to 300 m) located only around the south CO₂ polar cap indicate the presence of ground ice and thus outline the limit of the CO₂ ice cap. They could be due to the blanketing of water ice deposits by the advances and retreats of the residual CO₂ ice cap during the last thousand years. Large (50-250 m) and homogeneous polygons similar to ice wedge polygons, hillslope stripes and solifluction lobes may indicate that specific environments such as crater floors and hillslopes could have been submitted to freeze-thaw cycles, possibly related to higher summer temperatures in periods of obliquity higher than 35°. These interpretations must be strengthened by higher resolution images such as those of the HiRise mission of the Mars Reconnaissance Orbiter because locations with past seasonal thaw could be of major interest as potential landing sites for the Phoenix mission.