Thermophysical properties of Bennu and the OSIRIS-REx sample sites

Asteroid (101955) Bennu is the target of NASA’s OSIRIS-REx mission, which will return a sample of ≥60 grams of regolith from its surface. To map spectral and thermophysical property variations across the surface of Bennu, the OSIRIS-REx spacecraft carries two instruments capable of measuring its thermal-infrared emission: the OSIRIS-REx Thermal Emission Spectrometer (OTES), and the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS). Analyses of global data collected by OTES and OVIRS with the Advanced Thermophysical Model (ATPM) find spatial variations in Bennu’s thermal inertia and surface roughness that characterize the boulder populations on Bennu. The global averages of these properties are comparable to those determined for asteroid (162173) Ryugu by JAXA’s Hayabusa2 mission. Additionally, high-spatial-resolution data acquired at the Nightingale and Osprey sample sites resolve the thermophysical properties of centimeter-scale regolith with relatively few meter-scale or larger boulders. Both the Nightingale and Osprey craters have lower thermal inertia than their surroundings, indicating that the centimeter-scale particles may be smaller than the diurnal thermal skin depth. Although advances have been made in models used to predict regolith particle size from thermal inertia, a precise size comparison is still complicated by assumptions of the material properties of the regolith particles in these craters. Laboratory thermophysical analysis of the returned sample(s) will therefore provide ground truth for the interpretation of remote sensing infrared data of asteroidal surfaces.