The Origin of Gypsum in the Mars North Polar Region

Introduction: The OMEGA instrument on Mars Express has recently detected the largest gypsum deposit on Mars within the north polar sand sea [1]. Here we discuss the unique geologic context and origin of this evaporite deposit. Gypsum on Earth and Mars: Gypsum is a hydrated Ca-sulfate (2H 2 O • CaSO 4). Though gypsum can be formed, for example, as a hydrothermal deposit [e.g., 2] or as an alteration product of iron sulfide [e.g., 3], the largest deposits form as evaporites in acidic water. Gypsum is soft and thus easily susceptible to physical weathering and is light in color. While on Earth there are dunes composed of gypsum (e.g. White Sands, NM), such dunes are rare due to the mechanical weakness of the mineral; gypsum can survive saltation for only relatively short distances. On Mars, in addition to the north polar deposit, the OMEGA team has tentatively identified gypsum as one constituent in the layered deposits of Juventae Chasma [4]. The wide range in elevations of sulfate-containing outcrops in Valles Marineris has lead the authors to suggest that the gypsum in Juventae Chasma did not likely originate as an evaporite deposit. The MER Opportunity rover has also detected small amounts of Ca-sulfate salts at Meridiani Planum [5] which may have been created by evaporation of fluids involved with weathering of basalts [6]. Observations in the North Polar Region: Langevin et al. [1] have identified gypsum in the north polar region mainly by the strong 1.94 m absorption feature in OMEGA spectra of the sand sea. In Fig. 1, we map the 1.94 m feature using the 1.927 m OMEGA channel in order to reduce contamination by atmospheric CO 2 ; it is essentially a map of gypsum concentration (purple: 6% band strength; red: >25%). We are currently investigating the translation of band strength into weight or volume percent. We find that gypsum exists only in the presence of dunes. In other words, OMEGA does not detect gypsum anywhere where dunes are not present. We have further investigated this relationship to the dunes by examining MOC images both of the high gypsum areas (red in Fig. 1) and of the dunes further to the west (within the main sand sea) which contain no detectable gypsum. We find that there is no correlation of gypsum concentration with dune