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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. Plain Language Summary: Orbital thermal infrared (TIR) spectral and temperature data are used to determine numerous planetary surface properties, providing insight into how the planet's surface has evolved. This paper applies a new methodology to examine temperature mixing of surfaces with different units and particle sizes (i.e., rock and dust). Using the Thermal Emission Imaging Spectrometer (THEMIS) TIR data at 100 m/pixel, we model emissivity spectra and surface temperature to derive the abundance of these different surface units. TIR spectra are sensitive to sub‐pixel temperature differences at small scales. Surfaces with various components heat and cool at differing rates creating temperature differences within each pixel. The extracted TIR spectra will have a negative slope proportional to the degree of the temperature difference. Using a series of post‐sunset TIR images from different viewing angles, we model the surface temperature and spectral slope to derive the percent of surface units present. Matching these results to our observations allows for determining each component's particle size and abundance. Additionally, this method creates a thermophysical model that predicts a rock abundance six times greater than models derived from nadir viewing observations. This is critical for understanding modern surface evolution and future exploration, such as landing site selection. Key Points: Using Routine Off‐nadir Targeted Observations (ROTOs) of Mars Odyssey, we acquire directional thermal infrared (TIR) spectra of the surfaceTIR spectral slopes from the ROTO data enable extraction of sub‐pixel anisothermal heterogeneities at fine spatial scalesA thermal inertia mixing model is used to quantify sub‐pixel temperature mixing produced by a checkerboard mixing of surface units [ABSTRACT FROM AUTHOR]