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[16.09] The Photolysis of Cometary H₂O

A. L. Cochran (University of Texas), D. G. Schleicher (Lowell Observatory)

Since H₂O ice represents 80% of the icy component of cometary nuclei, understanding the photolysis of H₂O is important for understanding cometary chemistry. The photodissociation of H₂O is strongly controlled by the solar UV flux. Therefore, the lifetime of H₂O against photodissociation should be a strong function of solar activity. We (Cochran and Schleicher, Icarus 105, 234, 1993) studied H₂O photolysis in order to understand the nature of the change of the lifetimes with solar activity. However, the data set was limited and did not include many observations when the Sun was quiescent. We found that the activity of the Sun had an important influence on the photodissociation but could not determine accurately the functional form of the effect of changing UV flux. Since the time of that original paper, we have gathered many observations of cometary comae during periods of low solar activity along with some new observations during maximum solar activity. In this paper, we will show that the situation is quite complicated. We observe the distribution of the OH gas in the coma and, if we were to assume that the distribution changes shape only because of changing photodissociative lifetime, we would find that dissociation lifetimes were different for different comets on the same night. This is not plausible. Instead, we have explored the alternative that the outflow velocity is different for different comets (since in reality we measure scale lengths and not lifetimes). In this paper, we will show the outcome of our study and will explore what aspects of the comets cause the changing parameters.

This work was supported by NASA.

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.