Surface Properties of Asteroids from Mid-Infrared Observations and Thermophysical Modeling

Abstract

The subject of this work is the physical characterization of asteroids, focusing on the thermal inertia of near-Earth asteroids (NEAs). Thermal inertia governs the Yarkovsky effect, a non-gravitational force which significantly alters the orbits of asteroids up to \sim 20 km in diameter. Yet, very little has previously been known about the thermal inertia of small asteroids including NEAs. Observational and theoretical work is reported. The thermal emission of asteroids has been observed in the mid-infrared (5-35 {\mu}m) wavelength range using the Spitzer Space Telescope and the 3.0m IRTF. A detailed thermophysical model (TPM) has been developed and extensively tested; this is the first detailed TPM shown to be applicable to NEA data. Our main result is the determination of the thermal inertia of 5 NEAs, increasing the total number of NEAs with measured thermal inertia to 6. For two of our targets, previously available estimates are refined. Our results allow the first determination of the typical thermal inertia of NEAs, which is around 300 J s^{-1/2} K^{-1} m^{-2}, larger than the typical thermal inertia of large main-belt asteroids (MBAs) by more than an order of magnitude. In particular, thermal inertia appears to increase with decreasing asteroid diameter. Our results have been used by colleagues to estimate the size dependence of the Yarkovsky effect, thus explaining the apparent difference in the size-frequency distribution of NEAs and similarly sized MBAs. Thermal inertia is a very sensitive indicator for the presence or absence of particulate material on the surface. Our results indicate that even sub-km asteroids are covered with coarse regolith.