Martian dust mantling and surface composition: Interpretation of thermophysical properties

The surface block abundance, fine component thermal inertia, thermal emissivity, and relative atmospheric dust content have been determined for selected regions on Mars, using broadband infrared spectral properties measured at 7, 9, 11, and 20 m by the Viking infrared thermal mapper (IRTM). Each of the surface and atmospheric properties produce characteristic spectral signatures which have been successfully modeled. The surface was modeled by a mixture of a high‐inertia ‘block’ component and a fine component, with wavelength dependent emissivity. The abundance of blocks or patches of exposed high‐inertia material varies from 5 to 20% between 0 and 30°N. Low‐inertia regions have the fewest blocks: these may be exposed patches of bonded material. The ubiquity of nonaeolian materials exposed on the surface suggests that there has not been substantial net deposition within this region. However, some dust deposition may be occurring in low‐inertia regions. Observed global variations in the fine component inertia may be due to changes in the degree of bonding of the surficial material. The surface thermal emissivity is strongly correlated with albedo. The nearly linear decrease from near unity at all wavelengths for materials with an albedo greater than 0.28, to 0.98, 0.94, 0.92, and 0.92 at 7, 9, 11, and 20 m, respectively, for materials with an albedo of 0.10 suggests that material on the surface of Mars is composed of mixtures of the light, high‐emissivity and dark, low‐emissivity components. During the relatively clear period studied, the atmospheric opacity was found to vary spatially, being uniformly ‘low’ between 30 and 60°N and uniformly ‘high’ between 0 and −30° S, Between 0 and 30°N the opacity was higher over dark regions than over bright, low‐inertia regions. This variability is probably due to local, surface property‐dependent differences in the rates of dust generation and removal. Dark regions are sources of dust through removal of material deposited by global storms. Dust accumulates in bright, low‐inertia regions.