Spectra of Cemented, Hematite-rich Material and Tes Spectra of Sinus

Introduction. Lane et al. [1] and Christensen et al. [2] conclude that 1996 Global Surveyor Thermal Emission Spectrometer (TES, ~1700–200 cm) spectra of regions in Sinus Meridiani are consistent with hematite. The observed band strengths are inconsistent with unconsolidated, nanophase hematite dust, but are consistent with laboratory spectra of pure, coarselyparticulate hematite. We seek to extend hematite samples studied past pure end-members to include materials such as naturally occurring hematite-rich cemented materials (e.g. duricrust and desert varnish) in order to determine whether they may match the spectra recorded of Mars. Here we discuss why cemented finely particulate materials can also exhibit good spectral contrast, and we present an interesting spectral signature of a hematiterich patch on a ferricrete hand sample. Spectral contrast. A material can scatter light through two processes: surface and volume scattering [3]. A strong band is produced by a Fresnel reflection from the surface (surface scattering) when high opacity within the band gives it a mirror-like property. This mirror-like reflectance produces reflectance peaks called "reststrahlen bands." In the case of emission, the surface of a body reflects radiance inward at reststrahlen bands [4]. Thus reststrahlen reflectance from the surface of the grain reduces emerging radiation at the reststrahlen feature, causing an emissivity trough. When unconsolidated particles are small enough for light to survive passage through the grain, volume scattering occurs [5]. When volume scattering dominates, the reststrahlen bands are offset slightly toward longer wavelength, and appear as emission peaks. Surface scattering dominates for opaque materials, and smooth surfaces enhance spectral contrast [3,6]. Volume scattering dominates for optically thin materials, and most materials become optically thin at small particle sizes. However, when optically thin particles are pressed close (~wavelength) together, they scatter coherently, as if they were larger, opaque particles dominated by surface scattering [5]. When they are both close together and smooth-surfaced, they have high spectral contrast. This occurs for smooth-surfaced cemented, fine particles (e.g. as can occur in duricrust and desert varnish). Thus when reststrahlen bands are observed as troughs with good spectral contrast, that indicates the presence of large, smooth particles, and/or smoothsurfaced, closely packed or cemented fine particles. Differences in the relative contribution from surface and volume scattering will affect the band depth and shape. Pecharroman and Iglesias [7] show variations in hematite spectral character caused by different particle shapes and thus scattering effects. In particular, they observe variations in the width of the 575 cm band and in the relative band contrasts. Fig. 1 shows a spectrum measured of coarsely particulate hematite [8]. The dashed trace shows a spectrum measured of a hematite-rich spot on a ferricrete hand sample. The ferricrete is heterogeneous [9], and spectra of the hand sample varied with the composition. This spectrum is broadly consistent with hematite.