Fine-regolith production on asteroids controlled by rock porosity

• Published in: Nature (London) Vol. 598; no. 7879; pp. 49 - 52
• Main Authors: Cambioni, S., Delbo, M., Poggiali, G., Avdellidou, C., Ryan, A. J., Deshapriya, J. D. P., Asphaug, E., Ballouz, R.-L., Barucci, M. A., Bennett, C. A., Bottke, W. F., Brucato, J. R., Burke, K. N., Cloutis, E., DellaGiustina, D. N., Emery, J. P., Rozitis, B., Walsh, K. J., Lauretta, D. S.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 07.10.2021; Nature Publishing Group
• Subjects: 639/33/445/215; 639/33/445/848; Apollo asteroids; Asteroid missions; Asteroids; Astrophysics; Carbonaceous chondrites; Comminution; Earth and Planetary Astrophysics; Humanities and Social Sciences; Meteoroids; Meteors & meteorites; multidisciplinary; Physics; Porosity; Regolith; Rocks; Science; Science (multidisciplinary); Spacecraft; Temperature
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-021-03816-5

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) Bennu 4 and (162173) Ryugu 5 . However, despite observations of processes that are capable of comminuting boulders into unconsolidated materials, such as meteoroid bombardment 6 , 7 and thermal cracking 8 , 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 surface 10 . 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 asteroid 13 . By contrast, these terrains should be common on stony asteroids, which have less porous rocks and are the second-most populous group by composition 13 . The higher porosity of carbonaceous asteroid materials may have aided in their compaction and cementation to form breccias, which dominate the carbonaceous chondrite meteorites 14 . The absence of fine regolith on the asteroid Bennu is due to the high porosity of its rocks, which compress rather than fragment after impacts and exhibit slow thermal cracking.