Terrestrial Rock Varnish: a Key to Understanding the Surface Composition of Mars

The thermal infrared spectral signatures of two weathered terrestrial basalts were studied to better understand spectral signatures measured of weathered surfaces on Mars. Here we examine whether thermal infrared spectra (5-25 µm) may be used to differentiate between a desert varnish and a glassy, sili-cate cooling rind. Our preliminary results indicate the spectral signatures of both poorly ordered silicates are so similar that it may require a different method or wavelength range to differentiate between the two. Background: An important goal of remote sensing studies of Mars is to determine the surface composition , enabling scientists to better understand the past environmental conditions and plate tectonic history of Mars and to locate deposits of interesting minerals as a factor for landing site selection. For example, thermal infrared signatures recorded by the 1996 Thermal Emission Spectrometer (TES, 6-50 µm) may be used to examine the surface composition. TES spectral signatures are interpreted by comparison to laboratory spectra of known materials. Accurate interpretations require a full understanding of the spectral signatures of all the materials that may be present. Since it is reasonable to consider whether the surface of Mars may include sili-cates with a glassy cooling rind or a desert varnish, we desire to examine the signatures of these materials. Terrestrial desert varnish, generally less than 50 µm thick, consists of small particles [1] comprising mostly clays surrounded by oxides of iron and manganese [2, 3]. The small particles are cemented together so that they appear to an infrared spectrometer as if they are large particles, which produces a strong spectral band contrast. This also gives varnish its shiny appearance. On Earth rock varnish may have a microbial origin [3]. Clays are transported from an external source and deposited on the rocks [4, 5]. Manganese-concentrating bacteria, which bloom during wet periods, oxidize the manganese in the rock. These oxides cause the clay to adhere to the rock surface, producing varnish [5]. Raymond et al. [1] found manganese-rich stromatolites in rock varnish, which reinforces the idea that manganese oxidizing bacteria are involved in varnish formation. Krinsley et al. [6] suggested a varnish formation model that involves biomineralization and diagenesis. If the surface layer composition matches that of the bulk composition, and in particular if the surface layer shows no manganese enrichment, then we use this as evidence that the surface layer is a cooling rind rather than a desert varnish. Thus a …