EVOLUTION OF ICE SURFACES WITHIN POROUS NEAR-SURFACE LAYERS ON COMETARY NUCLEI

The small-scale structure of a cometary surface layer determines to a large extent the way in which cometary activity develops. The strong t emperature variations on a rotating cometary nucleus and the correspon ding sublimation and condensation processes make it probable that the surface is rough, with many caverns,cracks, and pores on size scales f rom meters down to the sub-millimeter range. Present work describes fi rst results of theoretical models able to describe some of the aspects of the ''shape-forming'' processes active on a cometary nucleus. Mont e Carlo simulations and analytical methods are used to study the gas f low within a porous surface layer. Dust pores with sublimation from ic e filled bottoms as well as ice covered walls are considered. It is fo und that the erosion of an ice-filled channel embedded in a matrix com posed non-volatile material is effectively limited by the gas reconden sing at the bottom due to the back flux of molecules reflected from th e side walls. Inhomogenities of the sublimating bottom tend to be leve led out, resulting in a sublimation surface of slightly parabolic shap e. Hence it is correct to use Clausing formula for far flux out of por ous media modelled by a bundle of tubes, even if the shape of the subl imating surface bottom of each tube is a priori not known. (C) 1998 El sevier Science Ltd. All rights reserved.