Observational Constraints on Water Sublimation from 24 Themis and 1 Ceres

TL;DR

This study sets upper limits on water sublimation rates from 24 Themis and 1 Ceres using [OI] emission, indicating very low surface activity compared to comets, with implications for understanding water ice on these bodies.

Contribution

It provides the first upper limits on water sublimation rates from Themis and Ceres based on [OI] emission observations, highlighting their low activity levels.

Findings

Water production rates are less than 4.5 x 10^27 mol/s for Themis.

Water production rates are less than 4.6 x 10^28 mol/s for Ceres.

Surface activity is less than 2 x 10^-4 of the surface area.

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 H$_2$O production rate on these bodies derived using a search for [OI]6300 Angstrom emission. For Themis, the water production is less than 4.5 $\times$ 10$^{27}$ mol s$^{-1}$, while for Ceres our derived upper limit is 4.6 $\times$ 10$^{28}$ mol s$^{-1}$. The derived limits imply a very low fraction of the surface area of each asteroid is active ($<$ 2 $\times$ 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.