The origin of sulfur-bearing species on the surfaces of icy satellites

Abstract

We present experimental results in order to understand the physico-chemical effects induced by fast ions irradiating sulfur bearing molecules. The experiments are relevant both to Solar System objects (icy satellites, comets, TNOs) and icy mantles on grains in the interstellar medium. Here we concentrate on the application to the Galilean moons that are exposed to high energetic particle fluxes in the jovian magnetosphere. In particular we show that: – Implantation of sulfur ions into water ice produces hydrated sulfuric acid with high efficiency providing an important contribution to the sulfur cycle on the surfaces of Europa and other jovian satellites. – Implantation of protons into sulfur dioxide produces mainly SO 3, polymers, and O 3 but not H–S bonds. The suggested hypothesis that H implantation could result in the formation of H 2SO 3 on Io is not confirmed by our experiments. – Irradiation of sulfur dioxide mainly produces SO 3 and sulfur polymers. – Irradiation and subsequent warming of H 2S induces the formation of a sulfurous residue that exhibits S–H bonds. – Water ice has been deposited on refractory sulfurous materials originating from SO 2 or H 2S irradiation. We looked to the formation of new species at the interface (ion mixing). We have not found evidence for the efficient synthesis of SO 2. In the case of residues rich in species containing S–H bonds, H 2S is re-formed.