THE INFLUENCE OF CO ICE ON THE ACTIVITY AND NEAR-SURFACE DIFFERENTIATION OF COMET NUCLEI

The role of unclathrated CO ice in the activity and near-surface differentiation of comet nuclei is investigated. A computer program used in the authors' previous work (Fanale and Salvail 1987, Icarus 72, 535-554) has been adapted to model a comet nucleus composed of a homogeneous mixture of H2O ice, CO ice, and dust in a Halley-type orbit. The difference in the volatility of CO and H2O ices results in greatly different gas production rates, and recession of the CO ice surface occurs, forming a porous surface layer of H2O ice and dust. Temperatures of H2O and CO ice surfaces, fluxes of CO and H2O gases and, depths of the CO ice are presented as functions of orbital position. Also considered are the pressures of subsurface CO as they relate to the stability of the overlying H2O ice and dust mantle and the possibility of dust or H2O ice grain emission. Possible pressures are in the range 3 × 102 dyn cm-2 to 1 × 106 dyn cm-2. These are insufficient to fragment annealed H2O ice, yet capable of overcoming the lithostatic pressures of many meters of unannealed H2O ice and emitting free surface grains in the 1-μm range. However, they are comparable to the inferred range of ice-poor mantle strength and may account for episodic regional changes in activity. It has been found that CO ice should always exist within 3 m from the surface and that CO gas production exceeds H2O gas production beyond 2.5 AU. This has implications for planned in situ elemental analysis of nuclei. CO gas production from "new" comets can be as much as 3 orders of magnitude greater before perihelion. After perihelion there is little difference in CO production for "new" and periodic comets. The results have been found not to depend significantly on the permeability of the water ice mantle or the estimated variability of the mass fraction of CO. © 1990.