Investigating the Origin of Gypsum in Olympia Undae.

Abstract:

The Olympia Undae Sand Sea, near the North Polar Ice Cap, contains the largest known deposit of gypsum discovered on Mars. The formation of gypsum requires liquid water, hinting that it formed in the North Polar region under circumstances vastly different from today’s Martian environment. The late Amazonian age gypsum dunes, likely sourced from materials of the late Hesperian-early/mid-Amazonian ages, could suggest wetter periods during these ages. Since its discovery, various hypotheses have been suggested for the source of gypsum in the north polar dunes. One predominant hypothesis suggests gypsum formed in the early-middle Amazonian-aged Basal Unit that underlies the North Polar Layered Deposits (NPLD) and has been eroded from this unit. Another theory suggests formation from in-situ aqueous alteration of sulfide and high-Ca bearing pyroxene materials present in the dunes and/or formation of evaporitic gypsum crystals within pore spaces. Water for its formation is suggested to be sourced from nearby channels carved by meltwater during melting of the NPLD. Studies using CRISM and OMEGA hyperspectral data detected that spectral signatures of sulfates in the Basal Unit have been largely inconclusive. We hypothesize the spectra of sulfate-bearing rocks and soils are dominated by water ice and dust, which occlude spectral signatures diagnostic of poly-hydrated sulfates. To explore this connection between the Basal Unit and the gypsum dunes in greater detail, we use Dynamic Aperture Factor Analysis/Target Transformation (DAFA/TT). This algorithm has capabilities to detect target mineral signatures in complex mixed environments that obscure mineral signatures. DAFA/TT has, for example, been used to detect the spectral signatures of small and/or spectrally weak exposures of hydrous mineral deposits on Mars with CRISM data. The factor analysis algorithm derives a set of orthogonal eigenvectors from hyperspectral data. By fitting library spectra with the eigenvectors to within a least-squares Root Mean Square threshold, the presence of target minerals is strongly indicated. Here we show that sulfate minerals are present in the basal unit via application of DAFA/TT to a set of CRISM observations (FRT0000287f, FRT0000c58f, FRT00002f91, FRT000024506).