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B Schmidt (UCLA-IGPP), R H Brown, D S Lauretta (LPL)
Since the primary source of information about comets continues to be studies of the coma, it is necessary to understand the relationship of the coma with the bulk ice. In particular, the unsettled issue of deuterium enrichment versus terrestrial water observed in the deuterium to hydrogen (D/H) ratio of comets Hale-Bopp, Hyakutake, and Halley is provocative.
We initiated an experimental study of comet sublimation in which water ice samples were produced in a vacuum by mixing pure D2O into pure de-ionized H2O producing a prescribed D/H ratio from 0-51%. These samples were placed in a radiationshielded apparatus where the base of the 6-inch column of ice was kept at a constant temperature ~165 K. The surface was exposed to incident light from a Xe lamp for periods ranging between 140 and 1127 hours. The resulting sublimated material was delivered to a QMS gas analyzer and a mass spectrum was produced. The temperature and pressure inside the vacuum-cryostat system was also monitored. We observed an upward trend in the D/H ratio in the evaporated material independent of the bulk composition, over the duration of each experiment. Additionally, sudden outbursts were observed in each sample. Sharp pressure spikes of up to 38% occurred indicating an increase in flux from the surface were followed by simultaneous temperature drops. During these outbursts, D/H ratios of up to 56% enrichment over the bulk sample ratio were measured. The periodicity of these pressure-temperature changes has also been analyzed. The complexity of the behavior indicates that diffusion kinetics play a key role in the establishment of a deuterium enriched surface layer that is subject to quasi-periodic rapid alteration. These results suggest that the production of the coma is a complex process and that measurements of the coma are not necessarily representative of the bulk comet composition.
The author(s) of this abstract have provided an email address for comments about the abstract: britneys@ucla.edu
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.