On the Prediction of CO Outgassing from Comets Hale-Bopp and Wirtanen

The gas flux from a volatile icy-dust mixture is computed using a comet nucleus thermal model in order to study the evolution of CO outgassing during several apparitions from long-period Comet Hale-Bopp and short-period Comet Wirtanen. The comet model assumes a spherical, porous body containing a dust component, one major ice component (H₂O), and one minor ice component of higher volatility (CO). The initial chemical composition is assumed to be homogeneous. The following processes are taken into account: heat and gas diffusion inside the rotating nucleus; release of outward diffusing gas from the comet nucleus; chemical differentiation by sublimation of volatile ices in the surface layers and recondensation of gas in deeper, cooler layers. A 2-D time dependent solution is obtained through the dependence of the boundary conditions on the local solar illumination as the nucleus rotates. The model for Comet Hale-Bopp was compared with observational measurements (Biver et al., 1999). The best agreement was obtained for a model with amorphous water ice and CO, assuming that a part of the latter is trapped by the water ice, another part is condensed as an independent ice phase. The model confirms that sublimation of CO ice at large heliocentric distance produces a gradual increase in the comet's activity as it approaches the Sun. Crystallization of amorphous water ice begins at 7 AU from the Sun, but no outbursts were found. Seasonal effects and thermal inertia of the nucleus material lead to larger CO outgassing rates as the comet recedes from the Sun. In the second part of this work the model was run with the orbital parameters of Comet Wirtanen. Unlike Comet Hale-Bopp, the predicted CO outgassing from Comet Wirtanen is almost constant throughout its orbit. Such behavior can be explained by a thermally evolved and chemically differentiated comet nucleus. This revised version was published online in June 2006 with corrections to the Cover Date.