Small body shapes and spins reveal a prevailing state of maximum topographic stability

Abstract

Over the past thirty years, spacecraft missions, Earth-based radar experiments, and telescopic observations have revolutionized our knowledge of Main Belt asteroids, near-Earth asteroids, and comet nuclei. As a result of this effort, we now possess high resolution shape models and well-constrained spin rates, pole orientations, and basic surface properties for a few dozen such small bodies. Here we present the results of a geomorphological examination of 32 such small body shape models along with their associated spin properties, and show that small body shape, gravity, and spin combine to gradually drive the surface towards a condition of maximum topographic stability; that is, a state of low topographic variation, low slopes, and low surface erosion (mass-wasting) rates. Of the 32 bodies investigated, 15 (47%) reside within this ‘zone of maximum topographic stability’, and when asteroid lightcurve-derived rotation rate and body elongation estimates are included, 1941 (70%) of 2791 well-observed asteroids reside within this zone. This finding indicates that given a mobile surface layer and sufficient time, small body surfaces naturally tend to erode, and their spin states gradually evolve, towards a state of maximum topographic stability, which also corresponds to a state of lowest internal stress. This erosional effect is most prominent on bodies several kilometers and larger in diameter, where YORP induced spin-state changes are small.