Fine-regolith production on asteroids controlled by rock porosity

Spacecraft missions have observed regolith blankets of unconsolidated subcentimetre particles on stony asteroids^1-3. Telescopic data have suggested the presence of regolith blankets also on carbonaceous asteroids, including (101955) Bennu⁴ and (162173) Ryugu⁵. However, despite observations of processes that are capable of comminuting boulders into unconsolidated materials, such as meteoroid bombardment^6,7 and thermal cracking⁸, Bennu and Ryugu lack extensive areas covered in subcentimetre particles^7,9. Here we report an inverse correlation between the local abundance of subcentimetre particles and the porosity of rocks on Bennu. We interpret this finding to mean that accumulation of unconsolidated subcentimetre particles is frustrated where the rocks are highly porous, which appears to be most of the surface¹⁰. The highly porous rocks are compressed rather than fragmented by meteoroid impacts, consistent with laboratory experiments^11,12, and thermal cracking proceeds more slowly than in denser rocks. We infer that regolith blankets are uncommon on carbonaceous asteroids, which are the most numerous type of asteroid¹³. By contrast, these terrains should be common on stony asteroids, which have less porous rocks and are the second-most populous group by composition¹³. The higher porosity of carbonaceous asteroid materials may have aided in their compaction and cementation to form breccias, which dominate the carbonaceous chondrite meteorites¹⁴.

Asteroid (101955) Bennu's weak boulders and thermally anomalous equator

Thermal inertia and surface roughness are proxies for the physical characteristics of planetary surfaces. Global maps of these two properties distinguish the boulder population on near-Earth asteroid (NEA) (101955) Bennu into two types that differ in strength, and both have lower thermal inertia than expected for boulders and meteorites. Neither has strongly temperature-dependent thermal properties. The weaker boulder type probably would not survive atmospheric entry and thus may not be represented in the meteorite collection. The maps also show a high-thermal inertia band at Bennu's equator, which might be explained by processes such as compaction or strength sorting during mass movement, but these explanations are not wholly consistent with other data. Our findings imply that other C-complex NEAs likely have boulders similar to those on Bennu rather than finer-particulate regoliths. A tentative correlation between albedo and thermal inertia of C-complex NEAs may be due to relative abundances of boulder types.