Characterization of the Effects of Anisothermality on OSIRIS-REx Thermal Emission Spectrometer Data

The OSIRIS-REx Thermal Emission Spectrometer (OTES) has observed spectral variability over the surface of Bennu. Variations from the mean Bennu spectrum most commonly occur in the 800-1000 cm^-1 range and at < ~400 cm^-1. Previous laboratory work has demonstrated that thermal IR spectral variability can be caused by a number of factors unrelated to composition, including but not limited to particle size, albedo, and degree of space weathering. Analyses of Diviner Lunar Radiometer long-wavelength thermal IR data demonstrated that surface anisothermality caused by surface roughness on the Moon leads to strong spectral variability that must be accounted for in any compositional analyses.

Imagery and thermal inertia values indicate an apparent dearth of fine particles, suggesting that particle size variations likely play a small role, if any, in the observed spectral variability. Bennu exhibits albedo variations, but its overall low albedo likely results in minimal variation in the surface thermal gradient, suggesting that the albedo also does not contribute to spectral variability. On the other hand, we observe that OTES spectra calibrated to emissivity using a single temperature display negative slopes towards lower frequencies that are consistent with surface anisothermality, likely caused by shadowed surfaces in the OTES detector field of view. This is supported by the positive correlation of the band depth of the 350 cm^-1 feature with mean emission angle of the OTES spectra. To address this, the OTES team has implemented a multi-temperature emissivity calibration method and is working to assess whether any residual effects are present in the spectra as a result of this method.

We performed a series of forward modeling tests to constrain the spectral variability that could be caused by surface anisothermality and determine whether all of the observed variability could be caused by surface roughness. These tests indicate that much of the global spectral variability observed between 800 and 1000 cm^-1 and at 350 cm^-1 cannot be explained by anisothermality and is likely due to differences in composition.

Quantitative modeling of the OTES data will benefit from a correction for the effects of anisothermality. We have begun preliminary efforts to address anisothermality in the data set and will report on our progress.