Using Io’s sulfur isotope cycle to understand the history of tidal heating

Stable isotope fractionation of sulfur offers a window into the history of tidal heating of Io, which is difficult to constrain because Io’s dynamic atmosphere and high resurfacing rates leave it with a young surface. Using literature constraints on relevant isotopic fractionations, spectroscopic constraints on the 34S/32S ratio of SO2 in the current atmosphere of Io, and a numerical model we constructed to describe the fluxes in Io’s sulfur cycle, we place constraints on the rates and isotopic fractionation factors for the processes that move sulfur between reservoirs and model the evolution of sulfur isotopes in these reservoirs over time. Gravitational stratification of SO2 in the upper atmosphere, which leads to a decrease in 34S/32S with increasing altitude, is the main cause of isotopic fractionation for sulfur lost to space. Efficient recycling of the residue of atmospheric escape into the crustal and mantle sulfur reservoirs is required to explain the magnitude of 34S/32S enrichment in the modern atmosphere. We hypothesize this occurs due to burial of SO2 surface frosts and reaction of SO2 with crustal rocks, which founder into the mantle and/or mix with mantle-derived magmas as they pass through the crust. This interpretation leads to the prediction that plumes of magmatic SO2 freshly vented to the atmosphere will be lower in 34S/32S than the ambient atmosphere, yet still significantly enriched compared to solar-system average sulfur. Future observations of the variations in atmospheric 34S/32S with time and location could be used to infer the average mantle melting rate and hence infer whether the current rate of tidal heating is anomalous compared to Io’s long-term average.