SURFACE ROUGHNESS OF BENNU DERIVED FROM OSIRIS-REX THERMAL EMISSION SPECTROMETER OBSERVATIONS

Planetary bodies with rough surfaces exhibit a broad range of kinetic temperatures attributable to the diversity of slopes and their orientation to the Sun. This anisothermality increases with increasingly rough surfaces and, if left unaccounted for, can impact the calibration and interpretation of spectral datasets. Here we present results examining the surface roughness of asteroid (101955) Bennu using radiance observations acquired by the OSIRIS-REx Thermal Emission Spectrometer (OTES). Following the methods of Bandfield et al. (2015), OTES radiance observations are compared to simulated radiance spectra predicted from a coupled thermal and Gaussian roughness model. The Gaussian roughness model is similar to that employed by Bandfield et al. (2018) and makes use of a vectorial ray-tracing routine to statistically account for shadowing and off-nadir viewing geometry. For application to Bennu, the equilibrium thermal model was replaced with a thermal diffusion model (KRC; Kiefer, 2013) where diurnal temperatures were predicted for a range of slopes, azimuth orientations, and latitudes. For a representative set of RMS slope distributions, the predicted thermal radiance of every slope/azimuth combination is weighted by the statistical probability of its occurrence, its potential to be shadowed by neighboring surfaces, and its likelihood to be visible within the OTES field of view. Once weighted, the predicted radiances are summed to yield a simulated “rough” radiance spectrum for a given latitude, local time, and observation geometry in order to be compared to individual OTES spots. Data for this investigation were acquired during two distinct mission phases: (1) Detailed Survey where the spacecraft slewed to scan the payload deck across the asteroid surface and (2) Orbital B, C, and R where the spacecraft entered into a low altitude terminator (+/- ~30 minutes local solar time) orbit and the payload deck was pointed nadir. When plotted collectively, these data show significant variability in brightness temperature slope to long wavelengths with greater slope near local dawn and dusk, a tell-tale sign of strongly anisothermal behavior, and thus a “thermally rough” surface. We will compare these slopes to those derived from our roughness thermal model and discuss the implications for the surface of Bennu.