Characterizing the thermal infrared spectral effects of optically thin surface dust: Implications for remote-sensing and in situ measurements of the martian surface

A spectral contribution different from that observed for thick dust mantles has been identified in many of the in situ measurements of rocks and regolith acquired by the Miniature Thermal Emission Spectrometer (Mini-TES) instruments on the Mars Exploration Rovers (MER). This spectral contribution is thought to be caused by optically thin surface dust and if not corrected can greatly hinder the mineralogical interpretation of rock surfaces. The focus of this study is the characterization of key radiative processes that are necessary to understand the spectral contributions produced by optically thin surface dust. An understanding of these radiative processes is important to be able to reproduce, predict, and correct their contribution in thermal infrared (TIR; ~200-2000cm^-1; 5-50μm) datasets. By combining TIR spectroscopic laboratory measurements and radiative transfer (RT) modeling, we have reproduced and quantified the spectral contributions produced by optically thin surface dust in the TIR spectral range. TIR laboratory measurements were acquired of basaltic rocks and gold diffuse reflectors (GDR) mantled with varying amounts of optically thin dust. The spectral contributions of optically thin dust as observed by Mini-TES were not observed in the laboratory measurements of the dusty basaltic rocks, but were observed in the measurements of the dusty GDR's. For the dust to contribute spectral features the dust must maintain a thermal contrast with the underlying surface. This thermal contrast was not achieved for the dusty basaltic rocks. Using our RT model, laboratory spectra of the dusty basaltic rocks and GDR's were reproduced. Our RT model appears to reproduce the spectral features attributed to the dust in the laboratory measurements to first order and can quantify the relationship between dust coatings and measured radiance. After validating the RT model against the TIR laboratory measurements, it was then used in an initial application to reproduce measurements acquired by the Mini-TES. By characterizing the spectral behavior of the dust, including the potential for thermal contrast between the dust and the substrate, it is possible to better understand and interpret TIR spectra of dust mantled surfaces.