Chemical and morphological evolution of a silicate surface under low-energy ion irradiation

International audienceAims. Olivine surfaces have been subjected to low-energy ion irradn. with H+, He+ and Ar+ at energies within the keV range in order to simulate the effects of energetic gas-grain interactions within shocked regions of the interstellar medium. Methods. The induced modifications in the chem. compn. and the bonding configuration of the upper and the near surface regions were monitored in situ by XPS. The assocd. morphol. evolution of the samples was studied by at. force microscopy (AFM). Results. Results show that the surface chem. evolves during irradn. with a noticeable Mg enrichment relative to Si. This evolution is interpreted as coming from magnesium atom diffusion driven by the elec. field caused by the pos. ion implantation. The iron valence state is also strongly affected by irradn., with redn. occurring at relatively high fluxes. However, at low fluxes the iron is found to oxidise from Fe2+ to Fe3+ due to a charge transfer between the incident pos. ions and the iron in the sample. The at. force microscopy results show that the surface roughness tends to increase with irradn. and that this roughness influences the surface chem. reactivity of the grains, as shown by the enhanced formation of carbonate on the surfaces when they are exposed to a CO2 atmosphere. The implications for the evolution of interstellar dust include an enhanced dust catalytic activity. These effects would arise from modification under irradn. of the surface reactivity and an increase in the available grain surface area