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 temperature variations on a rotating cometary nucleus and the corresponding sublimation and condensation processes make it probable that the surface is rough, with many caverns, cracks, and pores on size scales from meters down to the sub-millimeter range. Present work describes first results of theoretical models able to describe some of the aspects of the “shape-forming” processes active on a cometary nucleus. Monte Carlo simulations and analytical methods are used to study the gas flow within a porous surface layer. Dust pores with sublimation from ice filled bottoms as well as ice covered walls are considered. It is found that the erosion of an ice-filled channel embedded in a matrix composed of non-volatile material is effectively limited by the gas recondensing at the bottom due to the back flux of molecules reflected from the side walls. Inhomogeneities of the sublimating bottom tend to be leveled out, resulting in a sublimation surface of slightly parabolic shape. Hence it is correct to use Clausing formula for gas flux out of porous media modelled by a bundle of tubes, even if the shape of the sublimating surface at the bottom of each tube is a priori not known.