From coma abundances to nucleus composition

A major goal of comet research is to determine conditions in the outer sola r nebula based on the chemical composition and structure of comet nuclei. T he old view was to use coma abundances directly for the chemical compositio n of the nucleus. However, since the composition of the coma changes with h eliocentric distance, r, the new view is that the nucleus composition must be determined from analysis of coma mixing ratios as a function r. Taking a dvantage of new observing technology and the early detection of the: very a ctive Comet Hale-Bopp (C/1995 O1) allows us to determine the coma mixing ra tios over a large range of heliocentric distances, In our analysis we assume three sources for the coma gas: (1) the surface o f the nucleus (releasing water vapor), (2) the interior of the porous nucle us (releasing many species more volatile than water), and (3) the distribut ed source (releasing gases from ices and hydrocarbon polycondensates trappe d 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 para meters of the spinning nucleus as it moves in its orbit from large heliocen tric 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 in consistencies in the data were uncovered and reported to the observers for a reanalysis. Since post-perihelion data are still sparse, we have combined pre- and post-perihelion data. The resulting mixing ratio of CO relative t o H2O as a function of r is presented with a preliminary analysis that stil l needs to be expanded further. Our fit to the data indicates that the tota l CO release rate (from the nucleus and distributed sources) relative to th at of H2O is 30% near perihelion.