Doublet craters and the tidal disruption of binary asteroids

Abstract

Out of approximately 28 known terrestrial impact craters with diameters greater than 20 km, at least 3 are doublets created by the nearly simultaneous impact of objects of comparable size. The large size and separation of these doublets rule out atmospheric breakup as the agent of dispersion, while previous investigations of fragmentation within the Roche Limit are similarly unable to account for the dispersion of the doublets. Both the frequency of occurrence and the morphology of the craters and their ejecta blankets make very low angles of impact highly improbable. In this paper we investigate the possibility that tidal disruption of a population of compact binary asteroids might account for doublet craters. After detailed orbital integrations we find that although such asteroids are often disrupted by tidal forces well outside the Roche limit, the magnitude of the separations, averaged over a random population of impactors, is too small to account for the observed doublet craters. The only rational explanation we can find for the occurrence of doublet craters is an initial population of kilometer-size Earth-crossing objects containing between 10 and 20% of binaries separated by four times or more of their mean diameter. We offer this result as a prediction that may be tested by future radar observations of Earth-approaching asteroids.