Asteroid Regolith Weathering: A Large-Scale Observational Investigation

As bodies lacking an atmosphere or significant protection from solar wind particles, asteroids are subject to processes that modify the physical state and spectral properties of their regoliths. By investigating the relevant factors that contribute towards asteroid regolith modification, this work will provide crucial insight into the nature of these processes. I propose and test two major hypotheses: 1) that physical (mechanical) breakdown is caused by both meteoroid bombardment and thermal fatigue cycling, and thus regolith grain size depends on asteroid size and rotation period, and 2) changes in spectral properties (space weathering) are due to solar wind bombardment and depend on an object's mineralogy, sun-distance, and surface age. I develop and validate a thermophysical modeling (TPM) approach that analyzes multiepoch (pre- and post-opposition) thermal infrared observations for asteroids without prior shape or spin information, in order to determine various thermophysical properties -- chiefly the thermal inertia. This TPM approach is applied to over 250 asteroids to determine their thermal inertia. Combining other thermal inertia datasets with mine, for a total of over 300 objects, a characteristic grain size is estimated for each object. Next, a multiple linear model is used to quantify the grain size dependence on asteroid diameter and rotation period, which are both shown to be statistically-significant model predictors. I also identify grain size differences between spectral groups -- namely the M-types, which exhibit 4 times larger regolith grains, on average. Spectral data from meteorite and irradiated samples, spanning the visible and near-infrared regions, are used to develop an index to quantify the degree of space weathering. This space weathering index is applied to asteroid spectral observations and used in a multi-linear model to determine the predictor variables that increase the perceived amount of asteroid space weathering. Perihelion distance, diameter, and the average sun distance are found as statistically-significant factors in the multiple linear model. I also present evidence that regolith grains smaller than 0.5 mm enhance the extent of space weathering.

Chapter 1 of this document was previously published in The Astronomical Journal under the title "Thermophysical Modeling of Asteroid Surfaces Using Ellipsoid Shape Models"