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Developments in Deriving Best-Fit Thermal Inertia of the Surface of Mars Using THEMIS Images

Introduction: The Thermal Emission Imaging System (THEMIS) aboard Mars Odyssey has, since 2001, provided extensive coverage of the surface of Mars in 10 IR (6.78–14.88μm, at 100m/pix) bands. One quantity of interest is thermal inertia (measured in J m -2 s -1/2 K -1 or tiu [1]), which represents a material’s thermal response to changes in temperature of its surroundings. It is defined by taking the square root of the product of the density, thermal conductivity and heat capacity of a material. In a geologic context these quantities may be influenced by particle size, shape and degree of induration. Thermal inertia provides a level of thermophysical information allowing interepretation of ambiguous units. Typical values range from < 150 tiu for dust to > 1000 tiu for bedrock. Deriving thermal inertia from THEMIS (rather than TES) is especially desirable due to the high spatial resolution, which is commensurate with the scale of many light-toned outcrops that are of geologic interest. We enhance the method of [2] to derive best-fit thermal inertia and albedo of the surface of Mars by addressing issues regarding accuracy and spatial resolution. Method to derive thermal inertia: By modeling martian surface temperatures using the thermal model of [3] diurnal temperature curves are calculated for a large range of surface albedo, thermal inertia, slope and azimuth. For the geographic region under study, we obtain pairs of overlapping day and night THEMIS IR images and spatially align them. Using band 3 (7.93μm) for daytime and band 4 (8.56μm) for night images, brightness temperatures are calculated, which loosely represent the diurnal thermal extremes. Topography is obtained for the region (previously from 128 pixel/degree MOLA data) and reprojected to THEMIS spatial resolution (100m/pixel). Slope and azimuth data are then calculated from the resulting DEM. A lookup table produced from the thermal model comprises a list of surface temperatures at the time of day of each THEMIS image calculated for a wide range of values for albedo, thermal inertia, slope and azimuth. The observed brightness temperatures for each overlapping pixel are then best-fit with this table to solve for thermal inertia and albedo. Time offset of THEMIS images to daily thermal inflexions: To minimize error in the algorithm, we found that local time of acquisition of each THEMIS IR image must be very close to or at the diurnal thermal extremes. THEMIS images are often not. We produce a method for calculating the excursion from the diurnal thermal inflexion of a given THEMIS image using the thermal model (figure 1).