Isotopic evidence of long-lived volcanism on Io

TL;DR

This study uses isotopic analysis of Io's atmosphere to provide evidence that Io has experienced long-term, sustained volcanic activity, with significant volatile element loss over its history.

Contribution

It presents the first isotopic measurements of sulfur and chlorine in Io's atmosphere, revealing extensive volcanic outgassing and long-term activity.

Findings

Io's sulfur isotopic ratio is highly enriched, indicating 94-99% sulfur loss.

Chlorine isotopic ratio is similarly enriched, suggesting significant outgassing.

Io has been volcanically active throughout most of its history.

Abstract

Jupiter's moon Io hosts extensive volcanism driven by tidal heating. The isotopic composition of Io's inventory of volatile elements, including sulfur and chlorine, reflects its outgassing and mass loss history and provides an avenue for exploring its evolution. We used millimeter observations of Io's atmosphere to measure sulfur isotopes in gaseous SO2 and SO, and chlorine isotopes in gaseous NaCl and KCl. We find $^{34}$S/$^{32}$S=0.0595$\pm$0.0038 ($\delta^{34}$S=+347$\pm$86 per mille), 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 of $^{37}$Cl/$^{35}$Cl=0.403$\pm$0.028 ($\delta^{37}$Cl=+263$\pm$88 per mille) shows chlorine is similarly enriched. These measurements 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.