Influence of porosity on the thermal behaviour of comet surfaces

Comet nuclei are assumed to be of porous structure. A consequence of the porosity of sublimating matter is that sublimation will take place also at "internal" surfaces and not only at the outer surface. This considerably modifies the thermal and sublimation behaviour. The surface material will be cooler if compared to a non-porous surface while the temperature gradients increase. The resulting comet diurnal and depth variations of temperature and outgassing properties are described in some detail in their dependence on heliocentric distance for different porosities, pore radii and heat conductivities for a model nucleus on the orbit of. comet 46P/Wirtanen. The results are compared to astronomically observed properties of this comet. Based on that, "most appropriated" numerical values or ranges of parameters which can be used to characterize the properties of comet 46P/Wirtanen are identified. (C) 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.