EVOLUTION OF THE NORTH POLAR GYPSUM DUNES: MINERALOGICAL VARIATIONS ACROSS THE DUNES AND INTERDUNE REGIONS

The gypsum-bearing dunes of the Olympia Undae Sand Sea provide insights into modern polar processes. Recent advances in CRISM image processing and hyperspectral mapping are enabling improved characterization of the gypsum (CaSO₄•2H₂O) and associated materials comprising the North Polar dunes. We have processed and analyzed four CRISM images in the eastern part of Olympia Undae where gypsum-like spectral features are the strongest. We collected spectra of multiple 3x3 pixel regions across the dunes, dune crests, and bright patches in interdune regions of these CRISM images, averaged these spectra for each class of feature and compared these spectral averages to lab spectra of gypsum and other Ca sulfates. Strong sulfate spectral features are observed across the dunes and dune crests and their spectral signatures tend to be very similar for both of these. However, our analyses reveal changes from the dunes to the interdune region. The gypsum-like dune spectra include a triplet at 1.45, 1.49, 1.54 μm, and features at 1.75, 1.94-1.95, 2.22, and 2.27 μm in CRISM image FRT0000825F in eastern Olympia Undae where the dune spectra resemble gypsum most closely. The spectra of these lighter-toned interdune regions have higher reflectance values and weaker sulfate features, indicating that less sulfate may be present and that additional bright components may be accumulating here in these shallow deposits. Additionally, spectra of the interdune regions exhibit broader bands near 1.75-1.77 and 1.92-1.95 μm and the other bands are less well resolved. These spectral features indicate a change from gypsum to dehydrated gypsum (CaSO₄•nH₂O, 2>n>0.5) based on comparison with lab spectra. In CRISM images of dunes 350-400 km to the west we observed further changes in the spectral features where the spectral signatures of the dune material are similar to dehydrated gypsum and the spectra of the interdune regions are more consistent with bassanite (CaSO₄•½H₂O). We are investigating potential causes for these changes in the Ca sulfates present in the Olympia Undae Sand Sea.