Using Io's Sulfur Isotope Cycle to Understand the History of Tidal Heating

Abstract

Stable isotope fractionation of sulfur offers a window into Io's tidal heating history, which is difficult to constrain because Io's dynamic atmosphere and high resurfacing rates leave it with a young surface. We constructed a numerical model to describe the fluxes in Io's sulfur cycle using literature constraints on rates and isotopic fractionations of relevant processes. Combining our numerical model with measurements of the ³⁴S/ ³²S ratio in Io's atmosphere, we constrain the rates for the processes that move sulfur between reservoirs and model the evolution of sulfur isotopes over time. Gravitational stratification of SO ₂ in the upper atmosphere, leading to a decrease in ³⁴S/ ³²S with increasing altitude, is the main cause of sulfur isotopic fractionation associated with loss to space. Efficient recycling of the atmospheric escape residue into the interior is required to explain the ³⁴S/ ³²S enrichment magnitude measured in the modern atmosphere. We hypothesize this recycling occurs by SO ₂ surface frost burial and SO ₂ reaction with crustal rocks, which founder into the mantle and/or mix with mantle‐derived magmas as they ascend. Therefore, we predict that magmatic SO ₂ plumes vented from the mantle to the atmosphere will have lower ³⁴S/ ³²S than the ambient atmosphere, yet are still significantly enriched compared to solar‐system average sulfur. Observations of atmospheric variations in ³⁴S/ ³²S with time and/or location could reveal the average mantle melting rate and hence whether the current tidal heating rate is anomalous compared to Io's long‐term average. Our modeling suggests that tides have heated Io for >1.6 Gyr if Io today is representative of past Io.

Plain Language Summary

Io is a moon of Jupiter and is the most volcanically active body in our solar system. Io is in an orbital resonance with two other large moons of Jupiter; Europa and Ganymede: every time Ganymede orbits Jupiter once, Europa orbits twice, and Io orbits four times. This situation causes tidal heating in Io (like how the Moon causes ocean tides on Earth), which causes the volcanism. We do not know how long this resonance has been occurring and whether what we observe today is “normal.” This is because the volcanism renews Io's surface all the time, leaving little trace of the past. We use the isotopes of sulfur as a tracer of tidal heating on Io because sulfur is released through volcanism, processed in the atmosphere, and recycled into the mantle. We build a numerical model to simulate the sulfur isotope cycle on Io. Recent measurements of the sulfur isotopic composition of Io's atmosphere allow us to constrain a likely evolution for Io over time. We find that tidal heating on Io has occurred for billions of years and that the variability of the sulfur isotopic composition of the atmosphere may indicate the average tidal heating rate on Io.

Key Points

• Rayleigh distillation, where gravitational stratification drives isotopic fractionation, is a good approximation for Io's sulfur isotope cycle

• Efficient mixing and recycling between Io's interior and atmosphere occurs by crustal sequestration, volcanic frost remobilization, and burial into the mantle

• The difference between mantle and crustal frost ³⁴S/ ³²S decreases as mantle melting rate increases; atmospheric variability could measure this