Confined tumbling state as the origin of the excess of slowly rotating asteroids

TL;DR

This paper proposes a rotational evolution model for asteroids that explains the excess of slow rotators observed, suggesting they are mainly tumblers influenced by collisions and internal friction, with implications for their internal structure.

Contribution

The study introduces a model linking asteroid tumbling and slow rotation distribution, constraining their internal properties and suggesting a porous or regolith-rich structure.

Findings

The model reproduces the observed distribution of slow and fast rotators.

The product of rigidity and quality factor is estimated at ~4×10^9 Pa.

Rubble pile asteroids likely have porous structures or thick regolith layers.

Abstract

The rotational distribution of asteroids as a function of their size is used {as a diagnostic of} their physical properties and evolution. Recent photometric surveys from the Gaia mission, allowing observation of asteroids with long spin periods (for example $\geq 24$h), found an excessive group of slow rotators and a gap separating them from faster rotators, which is unexplained by current theories. Here we developed an asteroid rotational evolution model capable of reproducing the observed distribution. {We suggest that this distribution is regulated by the competition between collisions and internal friction dampening of "tumblers" -asteroids with unstable rotation vectors, and that the slow rotator group is mainly populated by tumblers.} {We constrain the product of the rigidity and quality factor, which relates to the body's viscosity, to $\mu Q \sim 4 \times 10^9~$Pa. This number, two orders of magnitude smaller than the one assumed for monolithic boulders,} implies that {rubble pile} asteroids could have a porous structure or a thick regolith layer, and undergo stronger tidal effects.