Abstract
Abstract 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) are important factors that separately influence thermal inertia. In this work we present new thermal inertia values for 239 asteroids and model all three factors in a multivariate model of thermal inertia. Using multiepoch infrared data of this large set of objects observed by WISE, we derive the size, albedo, thermal inertia, surface roughness, and sense of spin using a thermophysical modeling approach that does not 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 of 220 objects from the literature to construct a multivariate model and quantify the dependency on asteroid diameter, rotation period, and surface temperature. This multivariate model, which accounts for codependencies between the three independent variables, identifies asteroid diameter and surface temperature as strong controls on thermal inertia.
Highlights
The thermophysical characterization of regolith—the unconsolidated, heterogeneous, rocky material covering the surface of other planetary bodies—is an important part of understanding the processes and evolution of airless bodies of the solar system
We applied the method of MacLennan & Emery (2019) to Wide-field Infrared Survey Explorer (WISE) multiepoch observations in order to estimate the effective diameter, geometric albedo, thermal inertia, and surface roughness for 239 asteroids (Table 3)
Our thermal inertia estimates are consistent with previous values from the literature for individual objects (Figure 5) and for objects with similar size and rotation period
Summary
The thermophysical characterization of regolith—the unconsolidated, heterogeneous, rocky material covering the surface of other planetary bodies—is an important part of understanding the processes and evolution of airless bodies of the solar system. By comparing thermal observations to thermophysical models, the regoliths of asteroids can be characterized by their thermal inertia (Γ). Thermal inertia is defined as G = krcs , where k is the effective thermal conductivity of the regolith, ρ is the bulk density, and cs is the bulk specific heat capacity. Thermophysical models (TPMs) are often used to derive the thermal inertia of a body by comparing the observed fluxes to those estimated from the model. Smaller asteroids such as (433) Eros, (162173) Ryugu, (101955) Bennu, and (25143) Itokawa have approximate, estimated thermal inertias of, respectively
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