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Recent observations have suggested that there is water ice present on thesurfaces of 24 Themis and 1 Ceres. We present upper limits on the H$_2$Oproduction rate on these bodies derived using a search for [OI]6300 Angstromemission. 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 areaof each asteroid is active ($<$ 2 $\times$ 10$^{-4}$), though this estimatevaries by as much as an order of magnitude depending on thermal properties ofthe surface. This is much lower than seen for comets, which have active areasof 10$^{-2}$ - 10$^{-1}$. We discuss possible implications for our findings onthe nature of water ice on Themis and Ceres.

Observational Constraints on Water Sublimation from 24 Themis and 1 Ceres