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In this paper a possible explanation for an unexpected ortho.para-water ratio in the gas clouds of comets is given. The description is based on the quantum-mechanical density matrix formalism and the spin temperature concept. Only the nuclear spin system is treated quantum-mechanically. Employing the model of a four spin system, created by two nearest neighbour water molecules, spin eigenstates and their dynamics under the influence of their mutual dipolar interactions are studied. It is shown that a fast conversion between ortho- and para-states occurs on a msec time scale, caused by the intermolecular homonuclear magnetic dipolar interaction. Moreover the spin eigenstates of water in an ice crystal are determined by magnetic dipolar interactions and are not given by normal ortho- and para-H₂O states of gaseous water. As a result of this the spin temperature of gaseous water evaporated from ice depends strongly on its evaporation history and the ortho.para-ratio of water molecules are only an indirect measure of the temperature of ice crystals from where they descend. This result could explain the unexpected experimentally observed ortho.para-ratios in the clouds of comets.

Keywords: Ortho/Para-Water, Spin Temperature, Dipolar Interaction, Comets, Ice