Sulfur and Carbon Cycles on the Icy Satellites

       The surfaces of the icy satellites in the outer Solar System are continuously modified by implantation of energetic ions and electrons from the magnetosphere. The study of the induced modifications is based on laboratory simulations aimed at investigating, among other effects, the non-thermal chemical reactions that lead to the formation of a large number of molecules, not present in the original target. In a typical experiment frozen molecules, pure (e.g. H₂O, NH₃, CO₂, SO₂ and many others) or mixtures are deposited at low temperature (10-150 K) and irradiated with the chosen ion beam. When the thickness of the target is greater than the ion penetration depth, ions are implanted in the target and if they are reactive (e.g., H⁺, Cⁿ⁺, Nⁿ⁺, Oⁿ⁺, Sⁿ⁺) induce all of the effects of any other ion, but in addition have a chance, being stopped in the target, to form new species containing the projectile. These experiments are of fundamental relevance to clarify the formation history of the molecular species in particular to reveal their endogenic or exogenic origin.

Some of the results obtained so far will be presented, in particular: (1) H₂O₂ is formed by ion bombardment of pure water ice and such a process is believed to be responsible for its presence in the ices of Europa and other satellites. (2) The experimental results relative to implantation of reactive ions indicate that some molecular species observed on icy planetary surfaces (e.g. CO₂ and hydrated sulfuric acid) could have been formed by implantation of reactive ions that populate the jovian (and others) magnetosphere and continuously bombard the surfaces of the embedded satellites. In some instances implantation of magnetospheric ions is the main mechanism to initiate the so called sulfur and carbon cycles on the icy surfaces. (3) Specific predictions can be done on the relationship between the fluxes of reactive ions and the formation of species on spatially resolved regions on the surface of Europa. This is of the greatest relevance to understand existing data from space missions and to program the future observations by JUICE.