Thermal infrared (2.5–13.5 μm) spectroscopic remote sensing of igneous rock types on particulate planetary surfaces

TLDR

Scattering in fine particulate regoliths suppresses fundamental molecular vibration bands, making them unreliable for mineral identification, and environmental factors further alter these spectral features. The study seeks alternative spectral features that can more effectively reveal composition in remote sensing of planetary surfaces. Prominent absorption bands arise from trace water, hydroxyl, or carbonate in the 2.7–4.0 µm region, whose contrast increases as particle size decreases. Overtones and combination tones in the 3.0–7.0 µm range preserve mineralogical information, but only a few minerals are uniquely identifiable; however, general rock type can be determined using the Christiansen frequency and a transparency window, and even minute amounts of volatiles are detectable.

Abstract

Fundamental molecular vibration bands are significantly diminished by scattering. Thus such bands in spectra of fine particulate regoliths (i.e., dominated by <5‐μm particles), or regoliths displaying a similar scale of porosity, are difficult to use for mineralogical or rock type identification. Consequently, other spectral features have been sought that may be more useful in spectroscopic remote sensing of composition. We find that mineralogical information is retained in overtones and combination tones of the fundamental molecular vibrations in the 3.0‐ to 7.0‐μm region, but that relatively few minerals have a sufficiently distinctive band structure to be unambiguously identified with currently available techniques. More significantly, identification of general rock type, as defined by the SCFM chemical index (SCFM = SiO 2 /SiO 2 + CaO + FeO + MgO), is possible using spectral features associated with the principal Christiansen frequency and with a region of relative transparency between the Si‐O stretching and bending bands. However, environmental factors may affect the appearance and wavelengths of these features. Finally, prominent absorption bands may result from the presence of relatively small amounts of water, hydroxyl or carbonate, because absorption bands exhibited by these materials in the 2.7‐ to 4.0‐μm region, where silicate spectra are otherwise featureless, increase strongly in spectral contrast with decreasing particle size. Such materials are thus detectable in very small amounts in a particulate regolith composed predominantly of silicate minerals.