Laser Raman spectroscopy of varnished basalt and implications for in situ measurements of Martian rocks

Earth-based and orbital spectroscopy and Viking Lander data strongly suggest that rocks on Mars are coated with indurated dust deposits. Rock coatings will influence mineralogical measurements to be acquired in situ from Mars revers and other surface missions, in that most techniques will detect the coatings and/or coatings plus underlying minerals. In this paper, we present results of experiments on varnished basalt cobbles from the Lunar Crater Volcanic Field, Nevada, using laser Raman spectroscopic microprobe measurements obtained with a 514.5 nm Ar+ laser and irradiation spot sizes from 1 to 20 mu m. Measurements from thin sections and fresh, unprepared rock surfaces clearly allow identification of igneous minerals. Varnish spectra acquired from thin sections as well as from unprepared exterior rock surfaces typically show broad peaks consistent with the fine-grained nature of varnish and the dominance of manganese oxide and clay minerals. Several areas also show sharp Raman peaks due to phosphates and anatase, indicative of the heterogeneity of varnish deposits. Raman spectra taken through varnished rock surfaces reveal the mineralogy of coatings (where varnish is >15 mu m in thickness) and a combination of coatings and igneous minerals in areas with thinner varnish. Given the abundance of rock coatings observed at the Viking and Pathfinder landing sites, results indicate that techniques such as Raman spectroscopy (and any others with comparable penetration depths) will sample some combination of coatings and underlying rock materials. The ability to make many in situ measurements across samples, coupled with the ability to expose fresh rock surfaces, will be needed to unambiguously characterize both the coatings and the rocks on Mars from future landers and/or revers.