Sulfuric acid production on Europa: The Radiolysis of sulfur in water ice

Europa's surface is chemically altered by radiolysis from energetic charged particle bombardment. It has been suggested that hydrated sulfuric acid (H2SO4 . nH(2)O) is a major surface species and is part of a radiolytic sulfur cycle, where a dynamic equilibrium exists between continuous production and destruction of sulfur polymers S-x, sulfur dioxide SO2, hydrogen sulfide H2S, and H2SO4 . nH(2)O. We measured the rate of sulfate anion production for cyclo-octal sulfur grains in frozen water at temperatures, energies, and dose rates appropriate for Europa using energetic electrons. The measured rate is G(Mixture) (SO42-) = f(Sulfur) (r(0)/r)beta G(1) molecules (100 eV)(-1), where f(Sulfur) is the sulfur weight fraction, r is the grain radius, r(0) = 50 mum, beta approximate to 1.9, and G(1) = 0.4 +/- 0.1. Equilibrium column densities N are derived for Europa's surface and follow the ordering N(H2SO4) much greater than N(S) > N(SO2) > N(H2S). The lifetime of a sulfur atom on Europa's surface for radiolysis to H2SO4 is tau(-S) = 120(r/r(0))(beta) years. Rapid radiolytic processing hides the identity of the original source of the sulfurous material, but Iogenic plasma ion implantation and an acidic or salty ocean are candidate sources. Sulfate salts, if present, would be decomposed in <3800 years and be rapidly assimilated into the sulfur cycle. (C) 2002 Elsevier Science (USA).