Laboratory Simulation of Cometary Structures

Abstract

The properties of a porous mineral crust on the surface of an icy cometary nucleus and the crust's influence on the thermal regime and gas production in the nucleus have been studied by laboratory simulation experiments. A nucleus model of H2O ice with the impurity of graphite particles has been shown to display the same temperature and surface albedo as those determined for Comet Halley’s nucleus by the VEGA 1, VEGA 2, and Giotto spacecraft. The effective thermal conductivity of a crust with a density of 0.5 × 102 kg m−3 to 0.7 × 102 kg m−3 is less than 10−1 W m−1 K−1, while the crust’s strength (103 to 104 Pa) is not sufficient to withstand its erosion by the sublimating gases. A crust that is 1 cm thick lowers the gas production of the nucleus model by one order of magnitude. The destruction of the crust, and the gas and dust production of Comet Halley’s nucleus can be explained either by a spotty surface on the nucleus or, more likely, by the presence of volatile impurities such as CO2 with concentrations of 1 × 10−2 to 3 × 10−2 in the H2O ice under the crust.