Thermal infrared spectral characteristics of martian dust deposits and evidence for atmosphere-regolith interactions

It has been hypothesized that the dust component of the Martian surface is globally homogeneous, based on chemical similarity between landing sites and spectral similarity from select areas within bright regions. We tested that hypothesis by producing the first near-global data set of surface spectral emissivity (excluding polar regions) across the ∼233–508 and 825–1650 cm⁻¹ (∼20–50 and 6–12 μm) spectral ranges from Mars Global Surveyor Thermal Emission Spectrometer data and using various data reduction techniques to search for any spectral heterogeneity in bright regions that might be present. We found no unequivocal evidence for spectral heterogeneity, supporting the hypothesis that dust is globally homogenized. The global emissivity product permits new spectral parameter maps and preliminary assessments of atmosphere-regolith interactions. We produced the first map of the Christiansen feature (CF) and show that, unlike on the Moon, where CF is a proxy for bulk silica content, CF position on Mars is primarily associated with dust cover. We produced an updated map of the 1630 cm⁻¹ emissivity peak that arises from bound H₂O in fine-particulate material and show that the peak is nearly ubiquitous across the Martian surface, including in dark regions with relatively low dust cover. This is attributed to minor amounts of dust disproportionally contributing to the spectral signal in the ∼1630 cm⁻¹ region. We show that regions within the equatorial dust deposits with higher annual modeled frequency of nighttime CO₂ frosts are more likely to have lower emissivity in the ∼1350-1400 cm⁻¹ region, consistent with a higher fraction of unconsolidated dust. This provides the first spectral evidence for a previously hypothesized regolith gardening process via a diurnal CO₂ cycle, representing an important surface-atmosphere interaction that may contribute to near-surface porosity and affect diffusive exchange of H₂O between atmosphere and hydrated solids (ice, minerals) in the regolith.