Thermophysical Investigation of Asteroid Surfaces. II. Factors Influencing Grain Size

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 data set using thermal conductivity models. Following our previous work, we perform multivariate linear model fits to the grain size data set and quantify its dependency on diameter and rotation period. We find that the preferred model indicates that asteroid grain sizes are inversely dependent on object size for <10 km asteroids and exhibit 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. These results indicate that both impact weathering and thermal fatigue are relevant regolith evolution mechanisms. We run post-hoc t-tests between spectral groups to infer the influence of composition on regolith grain sizes. We find that M-type (including suspected metal-rich objects) and E-type asteroids have larger grain sizes relative to our population sample and that P-type asteroids have distinctly smaller grains than other groups.

Experimentally Induced Thermal Fatigue on Lunar and Eucrite Meteorites—Influence of the Mineralogy on Rock Breakdown

“…Therefore, we cannot exclude that for rocks of specific petrology, abrasion could be dominated by micro‐flaking rather than micro‐meteoroid bombardment. As remarked by MacLennan & Emery, 2022, a regolith formation solely by accumulating fine‐grained soil due to the breakdown by thermal fatigue will essentially come to halt after a certain soil thickness is accumulated that exceeds the thermal skin depth. In any case, on the Moon, catastrophic shattering of any rock size and type will occur by meteoroid impact at a rate higher than by abrasion (Hörz et al., 1974) allowing constant formation of micro‐flakes.…”

Multifactor characterization of space weathering among siliceous asteroids. I: Grain size effects

“…In addition to using already published asteroid spectra, we surveyed asteroids that have modeled grain size estimates based on thermal inertia determinations (MacLennan & Emery, 2022). Our telescopic spectral survey acquired NIR spectra for 51 S-complex objects.…”

“…To assess which multilinear model best characterizes the variation in the data, we compare their adjusted r 2 (r adj 2 ) and Bayesian information criterion (BIC ) parameters. As in MacLennan and Emery (2022), we prefer a model with the highest r adj 2 and the lowest BIC. Model M-6, which incorporates all three independent variables and a breakpoint in q, is therefore preferred over all others.…”

Surface Heterogeneity, Physical, and Shape Model of Near-Earth Asteroid (52768) 1998 OR2

“…Indeed, during the first epoch, the NEOWISE observations covered multiple rotations of OR2 and yielded a well-characterized thermal light-curve (see Figure 15). We used the detailed nonconvex shape model derived in this work and a TPM to estimate the emitted thermal flux of OR2 according to the approach of MacLennan & Emery (2022). The TPM used one-dimensional heat transfer and accounted for self-shadowing/heating effects to calculate surface temperatures for a bolometric Bond albedo of A = 0.06.…”