From Coma Abundances to Nucleus Composition. K. Altweg, P. Ehrenfreund, J. Geiss, and W. F. Huebner (eds.) Proceedings of the ISSI-Workshop: The Origin and Composition of Cometary Material,

Abstract

A major goal of comet research is to determine conditions in the outer solar nebula based on the chemical composition and structure of comet nuclei. The old view was to use coma abundances directly for the chemical composition of the nucleus, However, since the composition of the coma changes with heliocentric distance, r, the new ivew is that the nucleus composition must be determind from analysis of coma mixing ratios as a function of r. Taking advantage of new observing technology and the early detection of the very active comet Hale-Bopp (C/1995 O1) allows us to determine the coma mixing ratios over a large range of heliocentric distances. In our analysis we assume three sources for the coma gas: (1) the surface of the nucleus (releasing water vapour), (2) the interior of the porous nucleus (releasing gases from ices and hydrocarbon polycondensates trapped and contained in coma dust). Molecules diffusing inside the nucleus are sublimated by heat transported into the interior. The mixing ratios in the coma are modeled assuming various chemical compositions and structural parameters of the spinning nucleus as it moves in its orbit from large heliocentric distance through perihelion. We have combined several sets of observational data of Comet Hale-Bopp for H2O (from OH) and CO, covering the spectrum range from radio to UV. Many inconsistencies in the data were uncovered and reported to the observers for a reanalysis. Since post-perihelion data are still parse, we combinde pre- and post-perihelion data. The resulting mixing ratio of CO relative to H2O as a function of r is presented with a preliminary analysis that still needs to be expanded further. Our fit to the that H2O is 30% near perihelion.