Isotopic evidence for long-lived volcanism and mass-loss at Io

Io’s volcanic activity is driven by tidal heating due to its Laplace resonance with Europa and Ganymede, yet rapid resurfacing erases any signatures of its geological history. The isotopic composition of Io's volatile inventory reflects its outgassing and atmospheric mass loss history, providing an alternative avenue for exploring its evolution. We measured 34S/32S =0.060±0.004 in gaseous SO2 and SO, and 37Cl/35Cl=0.40±0.02 in gaseous NaCl and KCl, in Io’s atmosphere. The highly enriched 34S/32S relative to average Solar System values (δ34SVCDT=+360±90‰) indicates that Io has lost 94-99% of its available sulfur. Together with the similarly enriched chlorine isotopic signature (δ37ClSMOC =+250±60‰), this indicates that Io has been vigorously volcanically active over its entire history, and possibly experienced higher outgassing and mass loss rates in the past.