Abstract

Thermal inertia derived from temperature observations is critical for understanding surface geology and assessing potential landing sites on Mars. Derivation methods generally assume uniform surface properties for any given observation. Consequently, horizontal heterogeneity and near-surface layering may yield apparent thermal inertia that varies with time of day and season. To evaluate the effects of horizontal heterogeneity, I modeled the thermal behavior of surfaces containing idealized material mixtures (dust, sand, duricrust, and rocks) and differing slope facets. These surfaces exhibit diurnal and seasonal variability in apparent thermal inertia of several 100 tiu, ¹ even for components with moderately contrasting thermal properties. To isolate surface effects on the derived thermal inertia of Mars, I mapped inter-annual and seasonal changes in albedo and atmospheric dust opacity, accounting for their effects in a modified derivation algorithm. Global analysis of three Mars years of MGS-TES² data reveals diurnal and seasonal variations of ∼200 tiu in the mid-latitudes and 600 tiu or greater in the polar regions. Correlation of TES results and modeled apparent thermal inertia of heterogeneous surfaces indicates pervasive surface heterogeneity on Mars. At TES resolution, the near-surface thermal response is broadly dominated by layering and is consistent with the presence of duricrusts over fines in the mid-latitudes and dry soils over ground ice in the polar regions. Horizontal surface mixtures also play a role and may dominate at higher resolution. In general, thermal inertia obtained from single observations or annually averaged maps may misrepresent surface properties. In lieu of a robust heterogeneous-surface derivation technique, repeat coverage can be used together with forward-modeling results to constrain the near-surface heterogeneity of Mars.

¹tiu ≡ J m^-2 K^-1 s^{-½ 2}Mars Global Surveyor Thermal Emission Spectrometer