  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.
