WATER CLUSTERS IN THE COMA OF COMET HALLEY AND THEIR EFFECT ON THE GAS-DENSITY, TEMPERATURE, AND VELOCITY

The formation of water clusters of all sizes by water recondensation in an expanding cometary atmosphere is investigated. Several competing algorithms are used, and the present uncertainties concerning the physical properties of the clusters are carefully taken into account. Numerical estimates appropriate to Comet P/Halley at the time of the flyby observations are given. The results strongly suggest that about 15% of the water recondenses into clusters of 100 to 1000 water molecules which disappear beyond a cometocentric distance of about 500 nucleus radii. Surprisingly, these predictions are found to be weakly sensitive to the existing uncertainties concerning the mathematical model of the nucleation process and the physical properties of the water clusters. The most probable observable effect of these large clusters is a strong perturbation of the water radial temperature profile at all altitudes including beyond that of cluster disparition. Recondensation also increases the gas outflow velocity in the inner coma: the present results suggest that the gas velocities derived in situ by the Giotto mass spectrometer could be about 30% below the coma-averaged outflow velocities. This study does not support previously published speculations concerning the detectability of chemical or optical effects associated with the formation of small water clusters, particularly of the dimer (H2O)2. It also excludes the detectability of the continuum emissions from the clusters, at any wavelength because of the masking effect of the coma dust. Finally, it indicates that recondensation cannot form a dense halo of macroscopic ice grains: such a halo, if discovered in the future, can only result from surface fragmentation. © 1990.