VISIBLE-NEAR INFRARED SPECTROSCOPY OF THE NORTHERN PLAINS OF MARS FROM MARS EXPRESS OMEGA DATA

Visible/near-infrared spectral data provide compositional constraints on the northern plains of Mars. Telescopic observations suggest dark regions, including the northern plains, are composed of basaltic materials with thin coatings of Fe-oxides and/or palagonitic grains. Global scale mapping with spectral data from the Mars Express OMEGA instrument shows that the northern plains of Mars contain only weak absorptions attributable to mafic minerals and an overall spectral slope that could be indicative of grain coatings (Poulet et al., in press). The presence of a hydration feature at 3 microns (wavelength) that increases in strength with latitude suggests the northern plains are hydrated and their surface composition is influenced by climatic and seasonal volatile-driven processes (Milliken et al. in press). It has been proposed, based on thermal infrared spectral observations, surface chemistry at the Mars Pathfinder Landing site, and regional geomorphic context, that the northern plains may well contain geochemically altered materials. However, the lack of specific spectral bands related to certain alteration phases (e.g. clay minerals, zeolites, and opaline silica) seems inconsistent with an interpretation of altered surfaces in the northern plains. Still it is possible that a combination of effects related to composition, surface texture, and particle size could be the reason for these confusing results. In this presentation, we discuss the spectroscopic properties of the northern plains of Mars observed in OMEGA data. We also present laboratory measurements that provide some insights relevant to connecting OMEGA visible/near-infrared observations to thermal infrared spectroscopic observations. Specifically, we claim that an interpretation of secondary silica from thermal infrared data is consistent with OMEGA observations if the northern plains surfaces are weakly chemically weathered basaltic materials with thin (micron-scale), alumina- or iron-bearing, silica-rich coatings. Such a scenario would seemingly be consistent with the volatile-driven surface geology interpreted from image data, but real understanding of how aqueous chemical weathering may have occurred in this region may require some reevaluation of role of liquid water in recent Mars history.