Physics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p11a0102s&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #P11A-0102
Physics
6225 Mars, 6299 General Or Miscellaneous
Scientific paper
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 (VSMOW) 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 radiation shielded 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 sample material over the duration of each experiment, independent of initial composition. Additionally, sudden outbursts were observed in. Sharp pressure spikes of up to 38% 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 isotopic and time-varying behavior of these outbursts is presented. 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. IN addition, we present new analysis of NMS data from the Giotto Spacecraft which exhibits a spatially dependent D-H ratio, demonstrating that many assumptions made in the analysis of spacecraft and telescope data are not robust. HDO and H2O are rarely measured at the same time, which our study suggests will affect the accuracy of the resulrs. We contend that the production of the coma is a complex process and that simple measurements of the coma are not necessarily representative of the bulk comet composition. This study sheds new light on the dynamics of comets, and the debate surrounding the role of these object in the early solar system.
Brown Harvey R.
Lauretta Dante S.
Schmidt Britney Elyce
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