Separation of thermal inertia and roughness effects from Dawn/VIR measurements of Vesta surface temperatures in the vicinity of Marcia Crater

Abstract

A physical model of small scale roughness has been applied to the analyses of VIR (Visible InfraRed mapping spectrometer) – measured surface temperatures in the Marcia Crater region of asteroid Vesta. Model-generated surface temperatures which include the effects of minicrater-induced flux enhancements and viewing geometry are compared with the measured surface temperature dependence on solar incidence and emission angles. Results indicate that an extremely low thermal inertia (I=<5JK−1m−2s−1/2) and high roughness (rms slopes ∼35°) characterizes almost all of the areas in and around Marcia Crater. The one clear exception is the relatively cool pitted terrain region within Marcia Crater which requires either a higher thermal inertia value (I∼10) or significantly less roughness to match the VIR data. The I∼5 finding is the lowest value found for the regolith of any airless body in the inner Solar System and is difficult to reproduce in either laboratory measurements or theoretical models of the thermal conductivity of dry granular materials in vacuum.