Quantifying Sub‐Meter Surface Heterogeneity on Mars Using Off‐Axis Thermal Emission Imaging System (THEMIS) Data

Surface heterogeneities below the spatial resolution of thermal infrared (TIR) instruments result in anisothermality and can produce emissivity spectra with negative slopes toward longer wavelengths. Sloped spectra arise from an incorrect assumption of either a uniform surface temperature or a maximum emissivity during the temperature‐emissivity separation of radiance data. Surface roughness and lateral mixing of different sub‐pixel surface units result in distinct spectral slopes with magnitudes proportional to the degree of temperature mixing. Routine Off‐nadir Targeted Observations (ROTO) of the Thermal Emission Imaging Spectrometer (THEMIS) are used here for the first time to investigate anisothermality below the spatial resolution of THEMIS. The southern flank of Apollinaris Mons and regions within the Medusae Fossae Formation are studied using THEMIS ROTO data acquired just after local sunset. We observe a range of sloped TIR emission spectra dependent on the magnitude of temperature differences within a THEMIS pixel. Spectral slopes and wavelength‐dependent brightness temperature differences are forward‐modeled for a series of two‐component surfaces of varying thermal inertia values. Our results imply that differing relative proportions of rocky and unconsolidated surface units are observed at each ROTO viewing geometry and suggest a local rock abundance six times greater than published results that rely on nadir data. High‐resolution visible images of these regions indicate a mixture of surface units from boulders to dunes, providing credence to the model.