4.8 Article

Crater population on asteroid (101955) Bennu indicates impact armouring and a young surface

Journal

NATURE GEOSCIENCE
Volume 15, Issue 6, Pages 440-+

Publisher

NATURE PORTFOLIO
DOI: 10.1038/s41561-022-00914-5

Keywords

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Funding

  1. NASA [80NSSC18K0226, NNG12FD66C, NNM10AA11C]
  2. Canadian Space Agency
  3. French space agency CNES
  4. European Union [870377]
  5. Academies of Excellence of the IDEX JEDI of Universite Cote d'Azur
  6. Italian Space Agency (ASI) under ASI-INAF [2017-37-H.0]

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This study investigates the impact of impactor characteristics and target surface physics on crater formation on the asteroid Bennu. The results show that the population of boulders controls the transition from crater formation to armouring. The estimated crater retention age of Bennu's surface is significantly younger than previous estimates.
The impactor-to-crater size scaling relationships that enable estimates of planetary surface ages rely on an accurate formulation of impactor-target physics. An armouring regime, specific to rubble-pile surfaces, has been proposed to occur when an impactor is comparable in diameter to a target surface particle (for example, a boulder). Armouring is proposed to reduce crater diameter, or prevent crater formation in the asteroid surface, at small crater diameters. Here, using measurements of 1,560 craters on the rubble-pile asteroid (101955) Bennu, we show that the boulder population controls a transition from crater formation to armouring at crater diameters -2-3 m, below which crater formation in the bulk surface is increasingly rare. By combining estimates of impactor flux with the armouring scaling relationship, we find that Bennu's crater retention age (surface age derived from crater abundance) spans from 1.6-2.2 Myr for craters less than a few meters to -10-65 Myr for craters >100 m in diameter, reducing the maximum surface age by a factor of >15 relative to previous estimates. The range of crater retention ages, together with latitudinal variations in large-crater spatial density, indicate that ongoing resurfacing processes render the surface many times younger than the bulk asteroid.

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