Effect of porosity and non-planarity on icy grain surface chemistry in molecular clouds

The recent observation by the Rosetta probe of large amounts of O2 in the coma of comet 67P (3.8 +/- 0.85% relative to water) was unexpected. Such an amount of O2 and its strong correlation with water indicate that molecular oxygen was incorporated in the comet during its formation when icy grains agglomerated to form the cometary nucleus. This suggests that the production of O2 in molecular clouds or in the protosolar nebula is much higher than predicted by current models. Molecular clouds and protosolar nebula chemistries involve a strong interaction between the gas phase and the solid phase consisting in icy grains. Such grains are made of a silicate nucleus on which a frozen volatile layer grows due to the formation of ice by surface chemistry and exchanges with the gas phase. This icy layer is highly porous and uneven. We investigate the effect of surface non-planarity and porosity on the chemistry on such icy grain. With a simple model we estimate the effects on atomic and molecular exchanges between the solid and gas phases. These results suggest that on uneven or porous surface, compared to a planar surface, hydrogen is more likely to diffuse out of the grain than heavier species like O or O2. We estimate the impact of this enhanced hydrogen escape and of the resulting enrichment of the grains in radiolytic oxygen on the composition of the icy grain mantle and by extension on the composition of comets.