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

TL;DR

This study validates that orbital thermal data can reliably determine particle sizes of Martian dunes by comparing ground-based and orbital measurements, and assesses the influence of small-scale features on thermal inertia estimates.

Contribution

It demonstrates the consistency of particle size measurements from orbital and ground data and evaluates the impact of small-scale surface features on thermal property interpretations.

Findings

Orbital and ground measurements of dune particle sizes are consistent (~110-350 microns).

Small-scale ripples and thin layers do not significantly affect thermal inertia estimates.

Thermal inertia and grain size determinations are reliable for homogeneous Martian surface materials.

Abstract

In this work, we compare the thermophysical properties and particle sizes derived from the Mars Science Laboratory (MSL) rover's Ground Temperature Sensor (GTS) of the Bagnold dunes, specifically Namib dune, to those derived orbitally from Thermal Emission Imaging System (THEMIS), ultimately linking these measurements to ground-truth particle sizes determined from Mars Hand Lens Imager (MAHLI) images. In general, we find that all three datasets report consistent particle sizes for the Bagnold dunes (~110-350 microns, and are within measurement and model uncertainties), indicating that particle sizes of homogeneous materials determined from orbit 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 (cm-m scale) ripples, and 2) thin layering of indurated/armored materials. To first order, we find small scale ripples and thin (approximately centimeter scale) layers do not significantly affect the determination of bulk thermal inertia from orbital thermal data determined from a single nighttime temperature. Modeling of a layer of coarse or indurated material reveals that a thin layer (< ~5 mm; 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 grain sizes of relatively homogeneous materials derived from nighttime orbital data should be considered as reliable, as long as there are not significant sub-pixel anisothermality effects (e.g. lateral mixing of multiple thermophysically distinct materials).