Rocks with extremely low thermal inertia at the OSIRIS-REx sample site on asteroid Bennu

The OSIRIS-REx mission recently returned a sample of rocks and dust collected from asteroid Bennu. We analyzed the highest-resolution thermal data obtained by the OSIRIS-REx Thermal Emission Spectrometer (OTES) to gain insight into the thermal and physical properties of the sampling site, including rocks that may have been sampled, and the immediately surrounding Hokioi Crater. After correcting the pointing of the OTES datasets, we find that OTES fortuitously observed two dark rocks moments before they were contacted by the spacecraft. We find thermal inertias of 50–150 (± 50) J m–2 K–1 s–1/2 for these two rocks — exceptionally low even compared with other previously analyzed dark rocks on Bennu (180–250 J m–2 K–1 s–1/2). Our simulations indicate that monolayer coatings of sand- to pebble-sized particles, as observed on one of these rocks, could significantly reduce the apparent thermal inertia and largely mask the properties of the substrate. However, the other low-thermal-inertia rock that was contacted is not obviously covered in particles. Moreover, this rock appears to have been partially crushed, and thus potentially sampled, by the spacecraft. We conclude that this rock may be highly fractured and that it should be sought in the returned sample to better understand its origin in Bennu’s parent body and the relationship between its thermal and physical properties.