Radiolysis of sulfuric acid, sulfuric acid monohydrate, and sulfuric acid tetrahydrate and its relevance to Europa

摘要

We report laboratory studies on the 0.8MeV proton irradiation of ices composed of sulfuric acid (H_2SO_4), sulfuric acid monohydrate (H_2SO_4·H_2O), and sulfuric acid tetrahydrate (H_2SO_4·4H_2O) between 10 and 180K. Using infrared spectroscopy, we identify the main radiation products as H_2O, SO_2, (S_2O_3)_x, H_3O~+, HSO4-, and SO42-. At high radiation doses, we find that H_2SO_4 molecules are destroyed completely and that H_2SO_4·H_2O is formed on subsequent warming. This hydrate is significantly more stable to radiolytic destruction than pure H_2SO_4, falling to an equilibrium relative abundance of 50% of its original value on prolonged irradiation. Unlike either pure H_2SO_4 or H_2SO_4·H_2O, the loss of H_2SO_4·4H_2O exhibits a strong temperature dependence, as the tetrahydrate is essentially unchanged at the highest irradiation temperatures and completely destroyed at the lowest ones, which we speculate is due to a combination of radiolytic destruction and amorphization. Furthermore, at the lower temperatures it is clear that irradiation causes the tetrahydrate spectrum to transition to one that closely resembles the monohydrate spectrum. Extrapolating our results to Europa's surface, we speculate that the variations in SO_2 concentrations observed in the chaotic terrains are a result of radiation processing of lower hydration states of sulfuric acid and that the monohydrate will remain stable on the surface over geological times, while the tetrahydrate will remain stable in the warmer regions but be destroyed in the colder regions, unless it can be reformed by other processes, such as thermal reactions induced by diurnal cycling.