  Water (H2O) ice is an important solid constituent of many astrophysical
environments. To comprehend the role of such ices in the chemistry and
evolution of dense molecular clouds and comets, it is necessary to understand
the freeze-out, potential surface reactivity, and desorption mechanisms of such
molecular systems. Consequently, there is a real need from within the
astronomical modelling community for accurate empirical molecular data
pertaining to these processes. Here we give the first results of a laboratory
programme to provide such data. Measurements of the thermal desorption of H2O
ice, under interstellar conditions, are presented. For ice deposited under
conditions that realistically mimic those in a dense molecular cloud, the
thermal desorption of thin films (~50 molecular layers) is found to occur with
zero order kinetics characterised by a surface binding energy, E_{des}, of 5773
+/- 60 K, and a pre-exponential factor, A, of 10^(30 +/- 2) molecules cm^-2
s^-1. These results imply that, in the dense interstellar medium, thermal
desorption of H2O ice will occur at significantly higher temperatures than has
previously been assumed.
