The loss and depth of CO2 ice in comet nuclei

Abstract

An analytical model has been developed to simulate the chemical differentiation of a homogeneous, initially unmantled cometary nucleus composed of water ice, putative unclathrated CO2 ice, and silicate dust in specified proportions. Selective sublimation of any free CO2 ice present in a new comet should produce a surface layer of water ice and dust overlying the undifferentiated core. This surface layer modifies the temperature of buried CO2 ice and restricts the outflow of gaseous CO2. On each orbit, water sublimation closer to perihelion temporarily reduces the thickness of the water ice and dust layer and liberates dust. Most of the dust is blown off the nucleus, but a small amount of residual dust remains on the surface (cf. H. L. F. Houpis, W. H. Ip, and D. A. Mendis, 1986, Astrophys. J., in press). Our model includes the effects of nucleus rotation, arbitrary orientation of the rotation axis, latitude, heat conduction into the interior of the nucleus, restriction of CO2 gas outflow by the water ice and dust layer, and the use of thermal conductivities for both amorphous and crystalline water ice as appropriate, featuresthat were not included in the Houpis et al. model. The model also accounts for the erosion of the water ice surface, which Houpis et al. appear to have accounted for and which is an important effect. Specifically, we investigate the effects of varying the permeability of the surface water ice layer, the mass fraction of CO2, the orbit and the latitude, using the orbital parameters of Comets Halley and Tempel 2. It is found that CO2 gas production should exceed H2O gas production beyond ∼3 AU, and at 1 AU CO2 gas production should be between 20 to 25% of H2O gas production. The depth of CO2 ice and the variation in the depth of CO2 ice throughout an orbit are affected significantly by the perihelion of the orbit. The effects due to water ice permeability are significant but much less than expected on the basis of flow area. Latitude and CO2 concentration produce relatively small effects. Under all conditions considered here, CO2 ice should always be found within ∼1 m from the surface of comet nuclei if it is present as a free species to begin with. This result is probably generally valid for unmantled portions of most comets and qualitatively simulates the behavior of an abundant, highly volatile component in an H2O/silicate matrix. Comparison of these and similar results with observations could yield information regarding the permeability and chemical composition of cometary material and suggest sampling strategies to minimize fractionation effects. The method is applicable to other nonwater ices.