The Thermophysical Properties of the Bagnold Dunes, Mars: Ground-Truthing Orbital Data

We compare the thermophysical properties and particle sizes derived from the Mars Science Laboratory rover's Ground Temperature Sensor of the Bagnold dunes, specifically Namib dune, to those derived orbitally from Thermal Emission Imaging System, ultimately linking these measurements to ground truth particle sizes determined from Mars Hand Lens Imager images. In general, we find that all three datasets report consistent particle sizes for the Bagnold dunes (similar to 110-350m and are within measurement and model uncertainties), indicating that particle sizes of homogeneous materials inferred from temperature measurements and thermophysical models are reliable. Furthermore, we examine the effects of two physical characteristics that could influence the modeled thermal inertia and particle sizes, including (1) fine-scale (centimeter to meter scale) ripples and (2) thin layering of indurated/armored materials. To first order, we find that small-scale ripples and thin (approximately centimeter scale) layers do not significantly affect the determination of bulk thermal inertia from orbital thermal data using a single nighttime temperature. Modeling of a layer of coarse or indurated material reveals that a thin layer (< similar to 5mm; similar to what was observed by the Curiosity rover) would not significantly change the observed thermal properties of the surface and would be dominated by the properties of the underlying material. Thermal inertia and particle sizes of relatively homogeneous materials derived from nighttime orbital data should be considered as reliable, as long as there are no significant subpixel anisothermality effects (e.g., lateral mixing of multiple thermophysically distinct materials). Plain Language Summary The Mars Science Laboratory Curiosity rover spent approximately 20 Martian days interrogating an active sand dune field informally named the Bagnold dunes. The suite of measurements made by Curiosity provide a unique opportunity to link orbital data to ground truth. In this specific instance, we investigate if particle sizes derived from orbital data are reliable. Using a numerical model that describes how surface temperature varies as a function of a variety of input parameters, including observation geometry (e.g., season and time of day) and surface properties (e.g., reflectance, slope, azimuth, and elevation), we can model the response of the surface to various particle sizes. By using Curiosity's Ground Temperature Sensor as the link between fine-scale Mars Hand Lens Imager ground truth data and the Thermal Emission Imaging System's orbital perspective, we conclude that indeed particle sizes determined from orbital observations are reliable. Furthermore, we find that relatively thin armoring lags of coarser particles and dune ripples do not dramatically affect these orbitally determined particle sizes.