The desiccation process of cometary surfaces

Comets and their surface are mixtures of refractory and icy material. Space missions like Rosseta gave estimates on how much ice is present at a comet on global scale. On local (millimetre to micrometre) scale the distribution of ices inside the porous regolithic structure is less well understood. Nevertheless, in active periods the distribution of ices might change the thermodynamic behaviour and thereby affect global outgassing rates and dust activity. To address this, we developed a one-dimensional thermophysical outgassing model, which allows to simulate gas activity depending on the chosen structural model. For now, we consider two possible porous structures, one where ice is located around agglomerates filling the larger macropores and another where the ice is present within the agglomerate, which affects the gas diffusivity less.  The thermophysical model couples the heat and gas diffusion equations and allows ices to sublimate and redeposit depending on the local temperature and pressure. Figure 1 illustrates the simulated desiccation process, hence how the ice content φ changes with time and depth when the irradiation is assumed to be constant. The front (shaded regions) moves deeper with time as no dust activity is assumed. We find that a part of the sublimated material does redeposit in deeper layers forming a layer of high ice fraction (sinter layer), which is visible as a region where the ice content exceeds the initial 10 %. Simulations of single agglomerates show that most of the additional ice will freeze out in the larger pores around the agglomerate, thereby clogging the pore space beneath. We further discuss the effects of the different models on the thermal conductivity and analyse properties of a forming sublimation front.