The most common method for estimating surface grain size of asteroids is by determining thermal inertia using thermophysical models. Calculating accurate values of thermal inertia is a difficult process requiring a shape model, thermal-infrared observations obtained over broad viewing geometry, and detailed thermophysical modeling. Nevertheless, thermal inertia is a sensitive probe of surface regolith properties (Christensen et al. 2003), and therefore is of great importance in the design of instrumentation and observing strategies for asteroid missions where knowledge of surface characteristics is critical. Yet, thermal inertia alone cannot uniquely describe the full complexity of asteroid surface properties. This was true for OSIRIS-REx target (101955) Bennu whose thermally derived grain size estimates did not accurately represent the rough, bouldered surface observed by the spacecraft (Dellagiustina et al. 2019).
Radiative transfer models are some of the most widely used tools for compositional analyses of planetary bodies and have the opportunity to provide a comprehensive understanding of asteroid surface properties when used in conjunction with thermal modeling. In application to silicate-rich asteroids, radiative transfer models have almost exclusively been used to derive olivine to pyroxene abundance ratios. However, new formulas have been developed for deriving mineralogy from visible and near-infrared spectra that display prominent olivine and pyroxene (1 and 2 micron) absorption bands (e.g. Burbine et al. 2007, Reddy et al. 2011). Furthermore, the effects of non-compositional parameters (temperature, phase angle, grain size) have been well characterized, allowing for a more detailed analysis of these parameters with radiative transfer models.
We present a new implementation of the Hapke radiative transfer model to constrain grain size for unresolved asteroid surfaces. This technique can be applied to a large number of targets including near-Earth and Main Belt asteroids. This model is optimized for investigating S/Q type asteroids whose spectra are dominated by olivine and pyroxene absorption bands. Results from this study compliment thermal grain size estimates, when they exist, and provide standalone constraints on surface properties for a much larger number of near-Earth and Main Belt asteroids.
We will present a validation of the model against ordinary chondrite meteorites and well-studied near-Earth asteroids (e.g. Eros, Itokawa) with thermal inertia and spacecraft observations.
This work is supported by the NASA NEOO program, grant number NNX17AH06G.