New View of the Martian Surface: Themis Global Thermal Inertia Mosaic

Introduction: Thermal inertia derived from Thermal Emission Spectrometer (TES) data at 20-ppd [1-2] is the highest spatial resolution global thermal inertia dataset currently available. This valuable dataset is important for understanding regional variations in thermal inertia, and for inferring the physical nature of the surface at 10s to 100s of km scales. However, the resolution of TES (~3 km per pixel) is a major limitation, as this spatial resolution is often not sufficient to infer the physical properties of, or relative differences between, morphologic features observed in highresolution images, including Mars Orbiter Camera (MOC) [3], Thermal Emission Imaging System (THEMIS) [4], and High Resolution Imaging Science Experiment (HiRISE) [5] or to understand geologic processes acting on local scales. Many features that have important implications for the geologic and climactic history of Mars, including exposed bedrock or layered deposits, are too small to be uniquely identified in TES data. In this study, we are generating a new global thermal inertia mosaic using high resolution THEMIS data. The THEMIS instrument onboard Mars Odyssey is obtaining infrared observations of the martian surface at the highest spatial resolution to date (100 m per pixel) [4]. These data are currently used to calculate thermal inertia values of local regions (100-m scales) and to improve the interpretation of particle size distributions and material properties of local surfaces observed in high-resolution visible images [e.g. 612]. This new mosaic has facilitated an improved understanding of the nature of moderate thermal inertia surfaces. Specifically indurated material may be the primary cause of moderate thermal inertia surfaces. Features such as aeolian derived sediment, rocky material, crater ejecta, exposed ridges, cliffs, and crater rims all act to further increase the thermal inertia in the local regions where they are present, but these features alone do not explain the moderate thermal inertia values observed. Method: We are creating a global thermal inertia mosaic in 30° latitude by 30° longitude bins, resulting in 48 individual thermal inertia mosaics (Figure 1). These mosaics will then be pieced together to complete a global thermal inertia mosaic between ±60° latitude at a spatial resolution of 256 ppd (equivalent to ~230 m per pixel at the equator). The 48 individual mosaics and the global mosaic, in both image (e.g. png, tiff, and jpeg) and data (e.g. vicar) formats, will be made available to the community through the Planetary Data System (PDS). To generate this mosaic, thermal inertia values are first derived from individual THEMIS nighttime infrared images using the technique of Fergason et al. [10]. The brightness temperature of the surface is determined by fitting a Planck curve to band 9 (centered at 12.57 μm) calibrated radiance that has been corrected for instrumental effects [4]. The THEMIS band 9 temperatures are converted to a thermal inertia by interpolation within a 7-dimensional look-up table using latitude, season, local solar time, atmospheric dust opacity, thermal inertia, elevation (atmospheric pressure), and albedo as input parameters. Season, latitude, and local solar time are determined from spacecraft ephemeris. The remaining model input parameters (albedo, elevation, and atmospheric dust opacity) are obtained from external datasets. The absolute accuracy of the THEMIS thermal inertia is ~20%. Uncertainties in the THEMIS derived thermal inertia values are primarily due to: 1) instrument calibration; 2) uncertainties in model input parameters at the resolution of the THEMIS instrument; and 3) thermal model uncertainties [10].