Thermophysical Investigation of Asteroid Surfaces II: Factors Influencing Grain Size

TL;DR

This study estimates asteroid surface grain sizes from thermal inertia data, revealing size and rotation dependencies, and highlights the roles of impact and thermal fatigue in regolith evolution, with compositional variations affecting grain size.

Contribution

It introduces a method to estimate asteroid regolith grain sizes from thermal inertia data and analyzes their dependencies on size and rotation period, considering compositional differences.

Findings

Grain size inversely related to asteroid size below 10 km

Rotation period influences grain size differently above and below ~5 hours

Metal-rich asteroids tend to have larger grains, P-types smaller

Abstract

Asteroid surfaces are subjected to mechanical weathering processes that result in the development and evolution of regolith. Two proposed mechanisms--impact bombardment and thermal fatigue--have been proposed as viable and dominant weathering processes. Previously, we compiled and estimated thermal inertias of several hundred asteroids (mostly in the main-belt) for which we determined dependencies on temperature, diameter, and rotation period. In this work, we estimate grain sizes of asteroid regoliths from this large thermal inertia dataset using thermal conductivity models. Following our previous work we perform multi-variate linear model fits to the grain size dataset and quantify its dependency on diameter and rotation period. We find that the best-fit model fit indicates that asteroid grain sizes are inversely dependent on object size for <10 km asteroids and exhibits no relationship above this size cutoff. Rotation period and grain size show a positive relationship when the rotation period is greater than ~5 hr, and an inverse relationship below this rotation period. We conclude that both impact weathering and thermal fatigue are significant regolith evolution mechanisms. Furthermore, we run post-hoc t-tests between spectral groups to identify compositional differences among our asteroid set. Notably, suspected metal-rich, M-type and E-type asteroids have larger than expected grain sizes, and P-types have distinctly smaller grains than other groups