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Jupiter’s moon Io hosts extensive volcanism, driven by tidal heating. The isotopic composition of Io’s inventory of volatile chemical elements, including sulfur and chlorine, reflects its outgassing and mass-loss history and thus records information about its evolution. We used submillimeter observations of Io’s atmosphere to measure sulfur isotopes in gaseous sulfur dioxide and sulfur monoxide, and chlorine isotopes in gaseous sodium chloride and potassium chloride. We find³⁴S/³²S = 0.0595 ± 0.0038 (equivalent to δ³⁴S = +347 ± 86‰), which is highly enriched compared to average Solar System values and indicates that Io has lost 94 to 99% of its available sulfur. Our measurement of³⁷Cl/³⁵Cl = 0.403 ± 0.028 (δ³⁷Cl = +263 ± 88‰) shows that chlorine is similarly enriched. These results indicate that Io has been volcanically active for most (or all) of its history, with potentially higher outgassing and mass-loss rates at earlier times. 

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 the³⁴S/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 SO₂in the upper atmosphere, leading to a decrease in³⁴S/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 the³⁴S/³²S enrichment magnitude measured in the modern atmosphere. We hypothesize this recycling occurs by SO₂surface frost burial and SO₂reaction with crustal rocks, which founder into the mantle and/or mix with mantle‐derived magmas as they ascend. Therefore, we predict that magmatic SO₂plumes vented from the mantle to the atmosphere will have lower³⁴S/³²S than the ambient atmosphere, yet are still significantly enriched compared to solar‐system average sulfur. Observations of atmospheric variations in³⁴S/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.