Identification of a primordial asteroid family constrains the original planetesimal population

1Marco Delbo’, 2Kevin Walsh, 1Bryce Bolin, 3Chrysa Avdellidou, 1Alessandro Morbidelli
Science 357, 1026-1029 Link to Article [DOI: 10.1126/science.aam6036]
1Université Côte d’Azur, CNRS–Lagrange, Observatoire de la Côte d’Azur, CS 34229–F 06304 Nice Cedex 4, France.
2Southwest Research Institute, 1050 Walnut Street, Suite 300, Boulder, CO 80302, USA.
3Scientific Support Office, Directorate of Science, European Space Agency, Keplerlaan 1, NL-2201 AZ Noordwijk ZH, Netherlands.
Reprinted with permission from AAAS

A quarter of known asteroids is associated with more than 100 distinct asteroid families, meaning that these asteroids originate as impact fragments from the family parent bodies. The determination of which asteroids of the remaining population are members of undiscovered families, or accreted as planetesimals from the protoplanetary disk, would constrain a critical phase of planetary formation by unveiling the unknown planetesimal size distribution. We discovered a 4-billion-year-old asteroid family extending across the entire inner part of the main belt whose members include most of the dark asteroids previously unlinked to families. This allows us to identify some original planetesimals, which are all larger than 35 kilometers, supporting the view of asteroids being born big. Their number matches the known distinct meteorite parent bodies.

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