Variations in Martian surface composition and cloud occurrence determined from thermal infrared spectroscopy: Analysis of Viking and Mariner 9 data

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Abstract
Viking infrared thermal mapper (IRTM) and Mariner 9 infrared interferometer spectrometer (IRIS) data were examined for evidence of spatial variations in the composition of surface materials and the abundance of atmospheric condensates. The IRTM instrument collected broadband spectral radiance in bands centered at 7, 9, 11, and 20 μm. These measurements were converted to emissivities at 9, 11, and 20 μm and used to construct multispectral images at five seasons (Ls 90–180°, 180–360°, 0–90°, 90–180°, and 180–360°) and at local times of 6–8, 8–10, 10–12, 12–14, and 14–16 H (24 H equals one Martian day). During dusty periods (Ls 180–360°), these images clearly revealed the presence of suspended dust. During clear periods (Ls 0–180°) the seasonal and diurnal variations in water‐ice cloud occurrence could be mapped using the 11 μm IRTM band. Clouds tended to occur over the Tharsis region and the region between 0 and 60°W near the equator, and to evolve from dispersed hazes to more localized hazes during the day. As northern summer progressed during the Viking period, the northern clouds transitioned from a diffuse haze to a more concentrated zone of clouds in the equatorial region with a well‐defined northern boundary. During clear periods the IRTM multispectral data reveal significant absorptions at 9, 11, and 20 μm that are strongly correlated with surface albedo. These absorptions are produced by surface materials, and individual four‐point spectra show compositional variations within both dark and bright material types. The IRIS instrument collected high‐spectral resolution (2.4 cm−1) data from 6 to 50 μm. Unfortunately, only ∼85 daytime spectra out of the ∼21,000 total collected were acquired after the intense global dust storm of 1971 subsided (after Ls 330°). The close agreement of these spectra, convolved to IRTM resolution, with the IRTM surface spectra confirms that they also represent surface, rather than atmospheric, materials. The IRTM four‐point spectra show similarities with plagioclase feldspar minerals, clays, and pyroxenes, although the low spectral resolution of IRTM prevents definitive mineral identification. Carbonates do not compose a significant fraction of the dark‐region materials. The IRIS surface spectra show similarities with feldspars, clays, and pyroxenes in the 8–12 μm region but do not match any of these minerals well in the 18–25 μm region. Overall, the IRTM and IRIS data are consistent with previous suggestions of mafic to ultramafic compositions, but data provide stronger evidence that unweathered plagioclase feldspars may be significant components of the present surface. Together, the IRTM and IRIS reveal the presence of absorption features associated with surface materials which should permit detailed surface compositional mapping using future thermal IR spectral measurements.