EVOLUTION OF THE NORTH POLAR GYPSUM DUNES: VARIATIONS IN THE MORPHOLOGY AND COMPOSITION OF THE OLYMPIA UNDAE SAND SEA

The North Polar dunes on Mars are intriguing due to their unique gypsum (CaSO ₄•2H₂O) composition, proximity to the polar ice deposits, and seasonal variations. We have documented the locations and seasons of HiRISE and CRISM images across the Olympia Undae Sand Sea to investigate seasonal changes in morphology and composition of the dunes as well as spatial changes of the dunes from east to west with decreasing gypsum concentration. Comparison of overlapping HiRISE images collected at different seasons reveals the presence of darker dunes with light-toned interdune materials in summer, while the dunes appear frosted with a lighter color than the interdune materials in winter. During winter, the frost can accumulate to such an extent that it appears to mask the dune’s ripples in some images. The distribution of the light-toned interdune patches is not uniform. Instead, they appear in random clusters throughout the sand sea. Previous studies have noted that the light-toned patches in interdunes have a weaker gypsum signal than the crest and slope of the dunes. Interestingly, the morphology of the light-toned patches changes across the sand sea. In the eastern region, where gypsum is more prevalent, the bright patches contain ripples and some polygonal cracks. Towards the west, the light-toned patches contain mostly polygonal cracking and rarely any ripples. The difference in the morphology and color of the interdune materials from east to west of the Olympia Undae sand sea suggest that there is a dynamic system with changing processes producing these light-toned patches. Furthermore, the difference in morphology between the interdunes and the dunes also supports the idea that there is a change in composition from the dune to the interdune materials. Continuing studies of the interdune materials are key to understanding the composition of these enigmatic features.