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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, Comment: accepted to The Planetary Science Journal

The thermal inertia of an asteroid is an indicator of the thermophysical properties of the regolith and is determined by the size of grains on the surface. Previous thermophysical modeling studies of asteroids have identified or suggested that object size, rotation period, and heliocentric distance (a proxy for temperature) as important factors that separately influence thermal inertia. In this work we present new thermal inertias for 239 asteroids and model all three factors in a multi-variate model of thermal inertia. Using multi-epoch infrared data of a large (239) set of objects observed by WISE, we derive the size, albedo, thermal inertia, surface roughness, and sense of spin using a thermophysical modelling approach that doesn't require a priori knowledge of an object's shape or spin axis direction. Our thermal inertia results are consistent with previous values from the literature for similarly sized asteroids, and we identify an excess of retrograde rotators among main-belt asteroids < 8 km. We then combine our results with thermal inertias from the literature to construct a multi-variate model and quantify the dependency on asteroid diameter, rotation period, and surface temperature. This multi-variate model, which accounts for co-dependencies between the three independent variables, identified asteroid diameter and surface temperature as strong controls on thermal inertia., Comment: accepted to Planetary Science Journal 12.06.2021

The near-Earth objects (NEOs) (3200) Phaethon and (155140) 2005 UD are thought to share a common origin, with the former exhibiting dust activity at perihelion that is thought to directly supply the Geminid meteor stream. Both of these objects currently have very small perihelion distances, which results in them having perihelion temperatures of or exceeding 1000 K. The current activity from Phaethon is relevant to the destruction of NEOs close to the Sun, which most likely has produced meteor streams linked to asteroids in the past. We model the past thermal characteristics of Phaethon and 2005 UD using a combination of a thermophysical model (TPM) and orbital integrations of each object. Temperature characteristics such as maximum daily temperature, maximum thermal gradient, and temperature at varying depths are extracted from the model, which is run for a predefined set of a and e. Next, dynamical integrations of orbital clones of Phaethon and 2005 UD are used to estimate the past orbital elements of each object. These dynamical results are then combined with the temperature characteristics to model the past evolution of thermal characteristics such as maximum (and minimum) surface temperature and thermal gradient. We find that dwarf planet (2) Pallas is unlikely to be the parent body for Phaethon and 2005 UD, and it is more likely that the source is in the inner part of the asteroid belt. The orbital histories of Phaethon and 2005 UD are characterized by cyclic changes in, resulting in perihelia values periodically shifting between present-day values and 0.3 au. We find that the subsurface temperatures are too large over this timescale for water ice to be stable unless actively supplied somehow. The near-surface thermal gradients strongly suggest that thermal fracturing may be very effective at breaking down surface regolith., Comment: accepted 9.May.2021

Thermophysical Models (TPMs), which have proven to be a powerful tool in the interpretation of the infrared emission of asteroid surfaces, typically make use of a priori obtained shape models and spin axes for use as input boundary conditions. We test then employ a TPM approach - under an assumption of an ellipsoidal shape - that exploits the combination of thermal multi-wavelength observations obtained at pre- and post-opposition. Thermal infrared data, when available, at these observing circumstances are inherently advantageous in constraining thermal inertia and sense of spin, among other physical traits. We show that, despite the lack of a priori knowledge mentioned above, the size, albedo, and thermal inertia of an object are well-constrained with precision comparable to that of previous techniques. Useful estimates of the surface roughness, shape, and spin direction can also be made, to varying degrees of success. Applying the method to WISE observations, we present best-fit size, albedo, thermal inertia, surface roughness, shape elongation and sense of spin direction for 21 asteroids. We explore the thermal inertia's correlation with diameter, after accounting for its dependance on heliocentric distance.

We present the results of a search for the reactivation of active asteroid 176P/LINEAR during its 2011 perihelion passage using deep optical observations obtained before, during, and after that perihelion passage. Deep composite images of 176P constructed from data obtained between June 2011 and December 2011 show no visible signs of activity, while photometric measurements of the object during this period also show no significant brightness enhancements similar to that observed for 176P between November 2005 and December 2005 when it was previously observed to be active. An azimuthal search for dust emission likewise reveals no evidence for directed emission (i.e., a tail, as was previously observed for 176P), while a one-dimensional surface brightness profile analysis shows no indication of a spherically symmetric coma at any time in 2011. We conclude that 176P did not in fact exhibit activity in 2011, at least not on the level on which it exhibited activity in 2005, and suggest that this could be due to the devolatization or mantling of the active site responsible for its activity in 2005., Comment: 17 pages, 9 figures; published in AJ