THE MINERALOGY OF NATURAL ROCK SURFACES FROM THERMAL INFRARED SPECTROSCOPY: TERRESTRIAL STUDIES

Data returned from the Miniature Thermal Emission Spectrometer (Mini-TES) aboard the Mars Exploration Rovers are providing new insight into the mineralogy of Martian surface materials at local scales. Two of the primary goals of the Mini-TES experiments are to determine the primary bulk-compositions of rocks and the alteration state of rocks at the landing sites. With the exception of surfaces that have been freshly exposed by grinding with the Rock Abrasion Tool, the Mini-TES instrument exclusively measures spectra of natural rock surfaces, which contain information about physical and chemical weathering processes on Mars. We are studying the mineralogy of various natural rock surfaces using various mineralogical techniques and thermal infrared emission spectroscopy to understand how to separate interpretations of bulk-rock mineralogy from weathered surface mineralogy using spectra of natural surfaces. The key processes affecting natural rock surfaces are: 1) removal/destruction of primary phases by physical-chemical processes, 2) in-situ replacement of primary phases with secondary phases, 3) precipitation of secondary phases in void spaces, and 4) coating of rock surfaces. In terrestrial samples, the differential removal of primary phases changes the proportions of different primary minerals, complicating rock classifications from spectral analysis. Secondary alteration minerals may occur within the bulk rock, may be coupled to the rock exterior (e.g. coatings and rinds), or may not occur at all in altered rocks. Spectral features that are attributable to secondary phases within rocks are not limited to well-defined spectral features of crystalline materials – poorly crystalline silicate alteration products, which are spectrally similar to silicate glasses, may occur within or on rock surfaces. Thermal emission spectroscopy may be one of the best techniques for studying the composition and structure of volumetrically small alteration products on rock surfaces.