New Thermal Model of the Sub-Surface Layer of Comets

Thermal modeling is an essential tool for investigations of cometary activity and other typical cometary phenomena. Various thermal models have been published to date but their usefulness is compromised by lack of knowledge of the thermal properties of cometary matter. However, recent ground-based measurements of Hale-Bopp (Kührt, 2002) and space-based observations of 9P/Tempel 1 with the Deep Impact spacecraft (Groussin et al., 2007) provide crucial constraints on the physical properties of the nuclei. It is now clear that the heat conductivity of cometary matter is extremely low. Analytical results based on a simplified model lead to the hypothesis that under these circumstances, near perihelion, a thermal wave penetrates the surface material with an average speed lower than that of surface erosion. This effect, together with the low thermal skin depth, implies that subsurface layers are considerably less heated than indicated by previous models. Consequently, water ice sublimation should be confined to the surface and immediate sub-surface layers. In order to quantify this hypothesis we have developed and tested a novel thermal conduction code which allows the strongly nonlinear heat conduction equation and the erosion to be treated consistently by means of a moving surface boundary condition, the so called Stefan problem. A crucial aspect of our method is the numerical treatment of energy conservation. First results of our numerical computations are presented. O. Groussin et al., Surface temperature of the nucleus of Comet 9P/Tempel 1, Icarus 187, 16-25, 2007 E. Kührt, From Hale-Bopp's Activity to Properties of its Nucleus, Earth, Moon and Planets 90, 1-4 / June, 200