PREDICTING EMITTED RADIANCE OF ROUGH LUNAR SURFACES TO INFORM VOLATILE DISTRIBUTION

Daytime infrared observations of planetary bodies are sensitive to surface roughness which complicate illumination conditions at micron to meter scales. Pioneering work by Bandfield et al. (2016) revealed that illuminated surfaces on the Moon may be 100—200 K warmer and remain thermally isolated from shadowed surfaces only centimeters away. This anisothermality significantly influences observed emitted radiance so much that repeat thermal observations of the same surface at the same local time may differ by upwards of 20 K depending on the observing geometry. Since emitted thermal radiance is similar in magnitude to reflected solar radiance in the 3.0 micron region at lunar daytime temperatures, roughness poses a significant challenge for thermal corrections and spectral interpretations of the OH/H₂O 3 μm feature (Bandfield et al., 2018).

Here we present an update to the bi-directional roughness thermal model developed in Bandfield et al. (2016, 2018). Updates to the model include an optimized vectorial raytracing function that computes illumination conditions of synthetic surfaces, producing computationally efficient lookup tables of the likelihood of illumination of surface facets with specific slope, azimuth orientations. The illumination statistics computed by the model are body-agnostic and scale-independent, allowing them to be applied to a variety of planetary bodies (current efforts concern the Moon, Mars, Phobos, Bennu, and other asteroids). This model will be released as an open source Python package called Roughness, which will facilitate rough surface modeling for bodies throughout the solar system.

Here, we apply a radiative equilibrium thermal model to validate the Roughness model under lunar daytime conditions. We show good agreement between our model predictions and Diviner measured brightness temperatures. We also apply our first principles thermal model to the task of removing the thermal tail that masks the 3 μm feature in Moon Mineralogy Mapper spectra. Our results suggest that diurnal variability observed in previous studies of the 3 μm feature could be an artifact of untamed roughness effects. Our work provides key constraints on the nature of equatorial lunar volatiles and underscores the importance of accounting for roughness in infrared observations of planetary surfaces.