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Grain surfaces play a central role in the formation and desorption ofmolecules in space. To form molecules on a grain surface, adsorbed speciestrapped in binding sites must be mobile and migrate to adjacent sites. Thermalhopping is a popular mechanism for the migration of adsorbed species when thegrain surface is warmed up by stellar radiation. However, previous studiesdisregarded the fact that grains can be spun-up to suprathermal rotation byradiative torques (RATs) during grain heating process. To achieve an accuratemodel of surface astrochemistry, in this paper, we study the effect of grainsuprathermal rotation by RATs on thermal hopping of adsorbed species on icygrain mantles. We find that centrifugal force due to grain suprathermalrotation can increase the mobility of radicals on/in the ice mantle compared tothe prediction by thermal hopping, and we term this mechanism ro-thermalhopping. The rate of ro-thermal hopping depends both on the local radiationenergy density (i.e., grain temperature) and gas density, whereas thermalhopping only depends on grain temperature. We calculate the decrease in graintemperature required by ro-thermal hopping to produce the same hopping rate asthermal hopping and find that it increases with increasing the diffusion energyand decreasing the gas density. We finally study the effect of grainsuprathermal rotation on the segregation of ice mixtures and find thatro-thermal segregation of CO$_2$ from H$_2$O-CO$_2$ ices can occur at muchlower temperatures than thermal segregation reported by experiments. Ourresults indicate that grain suprathermal rotation can enhance mobility,formation, desorption, and segregation of molecules in icy grain mantles.

Chemistry on rotating grain surfaces: ro-thermal hopping and segregation of molecules in ice mantles