Constraining the Surface Properties in the Tharsis Region of Mars with TES and THEMIS Data

Surfaces on Mars that display time-dependent variability are readily recognized when they occur in regions where there is significant contrast between dark and bright surfaces. A layer of dust only a few microns thick can increase the albedo of a surface without altering the thermal inertia. Time-variable albedo is most likely due to the presence of a mobile layer of dust that accumulates after large-scale dust storms, and is then removed by surface winds. In most cases, low-albedo features brighten after dust storms, and then get progressively darker with time. While this is clearly observed on dark surfaces, a similar mobile dust layer may be present in uniformly bright regions. TES data has provided information about the thermophysical properties of the surface of Mars. Rock abundance, thermal inertia, and albedo have been used in combination to generate three-dimensional interpretations of the surface materials, with sensitivity from a few microns to centimeters into the subsurface. THEMIS images have been used to identify surfaces which have variable surface characteristics at an even higher spatial resolution. Nighttime data provide thermophysical measurements to constrain the bulk thermal inertia to a depth of a few centimeters. Daytime thermal observations are sensitive to the albedo of the top few microns as well as the inertia. Visible imagery provides surface reflectivity on a scale that individual features and surface roughnesses can be observed and correlated with temperature measurements. We have investigated isolated features in the Tharsis-Amazonis-Daedalia region which display low (< 0.2) albedo and moderately high thermal inertia (150-400 Jm-2K-1s-1/2), on an otherwise low inertia, high albedo surface. Distinct changes in the materials exposed at the surface have been observed in both THEMIS visible and daytime infrared observations. Using TES and THEMIS data to quantify surface characteristics and provide geomorphic context, we have constrained the thickness of the mobile surface layer, and the thermal inertia of the underlying materials in a few localities. Using this information, other regions of Tharsis have been identified that potentially have a mobile dust layer on top of equally bright surfaces, though without measurable changes in albedo. This suggests that the widespread Tharsis low-inertia region is not experiencing uniform yearly net dust accumulation.