Advances in the Mineralogy and Potential Formation Processes of Sulfates and Cl-Salts in the North Polar Dunes at Olympia Undae on Mars

The Olympia Undae Sand Sea of the North Polar region contains unique gypsum-rich dunes that provide insights into modern polar processes. Detailed characterization of gypsum and associated minerals is now feasible at the tens of meters scale due to advances in CRISM image processing and is revealing compositional variations across the dunes and interdune regions. Dunes with the strongest gypsum signatures are present in the eastern part of Olympia Undae and contain spectral features at 1.75, 1.94, 2.22, 2.27 and 2.48 μm as well as a triplet at 1.45, 1.49, and 1.54 μm. The bright interdune patches in the east are filled with ripples (as seen in HiRISE images) and have spectra consistent with gypsum but are brighter, indicating the presence of an additional spectrally bright material as well (likely a sulfate or chloride salt).

Traveling west, the dunes have slightly weaker gypsum-like spectral bands that are shifted slightly and could be resulting from dehydration of the gypsum or the presence of additional alteration minerals. Specifically, the band at 1.75 µm in the eastern dunes is shifted to 1.78 µm, which is observed in spectra of bassanite and some hydrated Ca chlorides. Continuing from central to western Olympia Undae the ~1.94 µm band becomes substantially weaker, indicating a much lower abundance of gypsum. Polygonal cracks can be seen in HiRISE images of the interdune regions that are reminiscent of evaporitic-type formations accompanied by secondary salt precipitation. The interdune regions also have weak spectral signatures consistent with a mixture of hydrated Ca and Mg sulfates and possibly hydrated Ca chlorides. These interdune regions in central to western Olympia Undae may be providing glimpses of the Planum Boreum basal unit below the dunes.

We are also investigating CRISM and HiRISE images bordering the Cavi region in order to gain insights into formation of the evaporitic-type salts under the Olympia Undae dunes. Surface materials at the Cavi region are hydrated but exhibit spectral properties different from those of gypsum and bassanite. Instead, spectra of dunes and regolith at the Cavi region have features similar to the spectra of hydrated ferric sulfates and perchlorates. Some hydrated chlorides may also be present. Ice and ice-regolith mixtures are also observed there.

MOLA maps reveal a lower elevation in eastern Olympia Undae where the gypsum is strongest. Thus, if more water pooled here at the time of alteration of the basal unit, then more gypsum may have formed in this depression. This area also contains the highest abundance of bright gypsum-bearing ripples, suggesting wind reworking of bright polygonal surfaces as a mechanism for extracting gypsum from the basal unit. Additionally, wind patterns from east to west could be spreading gypsum westward and at the same time dehydrating the gypsum. Additionally, frost is frequently observed on the dunes and interdune regions in winter and spring and could be altering the mineralogy and morphology.