Hematite Coatings Match Tes Spectra of Sinus Meridiani, Mars

Introduction. Lane et al. [1] and Christensen et al. [2] conclude that the 1996 Global Surveyor Thermal Emission Spectrometer (TES, ~1700–200 cm) spectra of regions in Sinus Meridiani match only coarsely particulate hematite. Here we show spectra of hematite coatings that match the TES signatures. The coating option is important because (1) hematite coatings can require little water to form, which may significantly change current interpretations and explain the apparent absence of other alteration minerals; and (2) one 2003 rover is planned for this site, so researchers need to prepare for possible measurement of hematite coatings by the rover instruments. This includes adding coating signatures to interpretation libraries and addressing measurement issues related to a hematite coating. Spectral contrast. Hematite has three absorption bands centered near 18, 23, and 33 μm. These strong bands are called “reststrahlen bands.” When reststrahlen bands are observed as troughs with good spectral contrast, that indicates the presence of smooth-surfaced material in one of two broad forms: (1) closely packed or cemented fine particles (e.g., a coating or duricrust); (2) large particles. There are two broad options because material can scatter light through two processes: surface and volume scattering [3]. A strong band is produced by high reflectance from the surface (surface scattering) when high opacity within the band gives it a mirrorlike property. This produces reflectance peaks called "reststrahlen bands." In emission, the surface reflects radiance inward at reststrahlen bands, causing an emissivity trough (e.g. Fig. 1 “solid surface”) [4]. When unconsolidated particles are small enough for light to survive passage through the grain, volume absorption (volume scattering) occurs [5]. When volume scattering dominates, the reststrahlen bands appear as emission peaks (Fig. 1, lower trace). Coarse vs. coating. Volume scattering dominates for optically thin materials, and most materials become optically thin at small particle sizes. This causes very small hematite particles to have very low spectral contrast (e.g., Fig. 1 upper trace), which is contrary to the TES observations. This reasoning has been used to rule out the presence of finely particulate hematite as the source of the observed TES signatures. However, when optically thin particles are close (~wavelength) together, they scatter coherently, and behave as if they were larger particles [5]. This occurs for cemented, fine particles (e.g. as may occur in duricrust and desert varnish). Differences in the relative contribution from surface and volume scattering will affect the band depths, shapes, and widths.