Thermal inertia (TI) represents the resistance of a material to changes in temperature and is calculated using thermal conductivity, thermal capacity, and density. It has been applied to investigate numerous surface properties on Mars by modeling the response of materials to heating and cooling. The Daedalia Planum region on Mars was formed in part from a series of large basaltic lava flows originating from the southwest flank of the Arsia Mons volcano. This flow field has had very few thermal infrared (TIR) studies in the past because of the higher albedo, which implies mantling of dust and therefore little to no capability to determine composition using TIR spectroscopy. The ongoing research as part of this NASA-funded project has found, however, that Daedalia represents an ideal location to investigate some of the most recent volcanic history on Mars because of the relatively young age of this flow field and preservation state of the flows. To fully understand the thermophysical response of these lava flows, fieldwork was completed at a Mars analog site, the rhyolite Mono Craters and Domes (MCD) in central California. However, a model-based TI is more complex in terrestrial applications due mainly to the atmosphere. An approximation, apparent thermal inertia (ATI), is used to determine grain size, soil moisture, and mantling properties. Materials with small grain size and/or low soil moisture will have low ATI, whereas larger rocks will display a higher ATI.
The North Coulee flow in the MCD chain is covered in areas with thick tephra deposits from younger eruptions and the surface size distribution ranges from ashy to blocky (Figure 1). Two issues that occur during the ATI calculation that can produce inaccurate values were identified in this study. The first issue is mixing of different particle sizes on the surface. The detection of coarse particles may be obscured by finer ones, so it is critical to improve the identification of the particle size distribution. Furthermore, the method for detecting whether a region on the ground is dominated by mantling, the continuous cover of the surface by a fine particle size, or checkboard mixing of larger blocks plus fines in low-lying regions must be addressed. The second potential complication is caused by sub-pixel shadowing created by large blocks and slopes. This shadowing will lower the visible albedo and daytime temperature, which combine to artificially raise ATI values (Figure 2). To accurately assess the subpixel distribution of blocks, multispectral data from orbital sensors with spatial resolutions ranging from 1.85 to 90 m/pixel are being analyzed, along with samples, GPS, and photogrammetry data acquired during fieldwork. Sub-pixel distributions and shadowing are being evaluated and quantified to determine the accuracy at each spatial resolution. Those results are then compared to the ATI data to evaluate the relationship between block size, shadowing and temperature. Additionally, analysis of samples and field observations confirmed the composition and grain size concentrations. With an improved understanding of the TI complications, the thermophysical response of the flows in Daedalia Planum can be better evaluated. The Mars based research seeks to determine the accuracy of extracting useful thermophysical properties to constrain the possible composition, eruption rates, and flow emplacement properties using a wide array of datasets: (1) orbital TIR image data (Figure 3) and (2) flow morphology and geological mapping derived from high-resolution orbital visible data (Figure 4). High resolution data from CTX and HiRISE are used to investigate the flow structures and degree of mantling. TIR spectral and thermophysical data from THEMIS and TES are compared both between and along the flows to understand specific spectral and thermophysical heterogeneities. Identified variations will be used to extrapolate information on the emplacement processes and lava characteristics using the spectral data as well as the subsequent conditions leading to the mantling. Analyses of surface thermophysical properties and flow morphology reveal that individual flows in Daedalia Planum respond differently to diurnal heating, suggesting that the area is not completely (or uniformly) mantled.
Figure 1. Photograph taken at the Mono Domes field site showing different surface mixtures.
Figure 2. Calculated ATI image of the North Coulee, Mono Domes
Figure 3 THEMIS IR daytime temperature mosaic of the study area showing variation in the thermophysical response of the lava flows. Lava flow boundaries by Crown et al. (2015).
Figure 4. CTX image of neighboring smooth and rugged flows showing both the mapped boundary and the locations of regions of interest.
Future work includes application of a thermal model for Mars compared to the thermal inertia (TI) derived from orbit to identify percentage abundance of large outcrops and sand on the surface of these flows. This information will then be used to determine the mantling or mixing relationship on the surface and for possible compositional analysis of areas identified as containing a significant exposure of lava flow outcrops. Ultimately, the errors identified and constrained in the analog study will be applied to determine the relationship between thermal inertia and block size on the MCD and Daedalia flows. This approach has a wide range of applications including understanding surface change with time during future active lava flow emplacement on Earth and the distribution of flow outcrop versus eolian mantling on other lava flows on Mars.