Now out from JGR:Plan (Journal of Geophysical Research: Planets) vol. 52, #21:
Elder, C.M. Boulders on Bennu: Low Apparent Thermal Inertia Caused by Thermal Fatigue
Fractures art. e2025GL116256 2025GL116256
We have samples returned from Bennu, but… grain sizes are mostly pebbles (millimeters), at best gravel (about an inch). Bennu, an astronomical body, is not a grain, it’s a system. How does that system work- there’s the geophysics. The OSIRIS-REx probe determined, from orbit, the thermal inertia (heat retention/loss capacity) of Bennu as a body. But the returned grains show a different thermal inertia value. At first glance, the answer might be that Bennu is covered in a regolith- a ‘soil’ of loosely held grains. The porosity and poor contact of this grain system would then act as insulation, making Bennu (and not single grains) hold onto its heat.
Does this hypothesis hold up? No, because we also saw boulders there. Boulders are not regolith, and are, so it seems, neither packed nor unpacked. And these boulders also fail to match the thermal inertias of the grain sample. Another hypothesis is that the boulders are covered in a thin dust/pebble layer. Yet another is that the boulders are, yes, porous, because they’re fractured, not truly solid. Elder takes us through this last possibility.
Also note that Bennu was seen by OSIRIS-REx to shed rocks into space- an active asteroid. This argues for the cracked-boulder explanation, because the cracking process would, in extreme cases, launch the rock fragments. Hey, two birds, one stone.