Origin of asteroid rotation rates in catastrophic impacts

Abstract

The rotation rates of asteroids, which are deduced from periodic fluctuations in their brightnesses¹, are controlled by mutual collisions^2–8. The link between asteroid spin and collision history is usually made with reference to impact experiments on centimetre-scale targets, where material strength governs the impact response^2,3,9–11. Recent work, however, indicates that for objects of the size of most observed asteroids (≥1 km in diameter), gravity rather than intrinsic strength controls the dynamic response to collisions^12–14. Here we explore this idea by modelling the effect of impacts on large gravitating bodies. We find that the fraction of a projectile's angular momentum that is retained by a target asteroid is both lower and more variable than expected from laboratory experiments, with spin evolution being dominated by 'catastrophic' collisions that eject ∼50 per cent of the target's mass. The remnant of an initially non-rotating silicate asteroid that suffers such a collision rotates at a rate of ∼2.9 per day, which is close to the observed mean asteroid rotation rate of ∼2.5 d^–1. Moreover, our calculations suggest that the observed trend in the mean spin frequency for different classes of asteroids⁴ (2.2 d^–1for C-type asteroids, 2.5 d^–1 for S-type, and 4.0 d^–1 for M-type) is due to increasing mean density, rather than increasing material strength.