Observational constraints on water sublimation from 24 Themis and 1 Ceres

Abstract

Recent observations have suggested that there is water ice present on the surfaces of 24 Themis and 1 Ceres. We present upper limits on the H2O production rate on these bodies derived using a search for [O I]6300 Å emission. For Themis, the water production is less than 4.5 × 1027 mol s−1, while for Ceres our derived upper limit is 4.6 × 1028 mol s−1. The derived limits imply a very low fraction of the surface area of each asteroid is active (<2×10−4), though this estimate varies by as much as an order of magnitude depending on thermal properties of the surface. This is much lower than seen for comets, which have active areas of 10−2–10−1. We discuss possible implications for our findings on the nature of water ice on Themis and Ceres.