Obliquities of "Top-Shaped" Asteroids May Not Imply Reshaping by YORP Spin-up

TL;DR

The paper explains that the low obliquities of top-shaped asteroids like Bennu are not necessarily caused by YORP-induced reshaping, as obliquity evolution occurs on shorter timescales than shape changes.

Contribution

It demonstrates that low obliquities in top-shaped asteroids do not imply YORP-driven reshaping, challenging previous assumptions about their formation.

Findings

Obliquity timescales are much shorter than shape spin-up times.

Low obliquities are expected regardless of reshaping mechanisms.

YORP effect's influence on obliquity does not confirm reshaping history.

Abstract

The timescales over which the YORP effect alters the rotation period and the obliquity of a small asteroid can be very different, because the corresponding torques couple to different aspects of the object's shape. For nearly axisymmetric, "top-shaped" near-Earth asteroids such as 101955 Bennu, spin timescales are an order of magnitude or more longer than obliquity timescales, which are of order 10^5 to 10^6 yr. The observed low obliquities (near 0 or 180 degrees) of top-shaped asteroids do not constitute evidence that they acquired their present shapes and spins through YORP spin-up, because low obliquities are expected regardless of the spin-up or reshaping mechanism.