MEASURED EFFECTS OF DESERT VARNISH ON THE MIDINFRARED SPECTRA OF WEATHERED ROCKS AS AN AID TO TIMS IMAGERY INTERPRETATION

The thermal infrared spectral properties (7-12 mum) of natural rock su rfaces from Silver Lake, CA, are investigated. Although the reflectanc e of weathered rocks is largely a function of the quartz content in ro cks, the presence of desert varnish (clay coating) on rocks reduces th e reflectance and spectral contrast with features unique to the rock s pectra persisting if varnish is thin. Thick varnish has a spectrum wit h a reflectance peak near 9.6 mum, due to clays, and resembles the spe ctra of clay-rich playa surfaces. Comparison of laboratory reflectance spectra for varnish and weathered rock samples, with TIMS emissivity spectra for Silver Lake, suggest that TIMS signatures for felsic rocks are dominated by weathered rock and rock debris. In contrast, it is l ikely that varnish plays an important role in the TIMS signatures of m afic rocks.