  Silicon tetrafluoride (SiF4) is observed in terrestrial volcanic gases and is
predicted to be the major F - bearing species in low temperature volcanic gases
on Io (Schaefer and Fegley, 2005b). SiF4 gas is also a potential indicator of
silica-rich crust on Io. We used F/S ratios in terrestrial and extraterrestrial
basalts, and gas/lava enrichment factors for F and S measured at terrestrial
volcanoes to calculate equilibrium SiF4/SO2 ratios in volcanic gases on Io. We
conclude that SiF4 can be produced at levels comparable to the observed
NaCl/SO2 gas ratio. We also considered potential loss processes for SiF4 in
volcanic plumes and in Io's atmosphere including ion-molecule reactions,
electron chemistry, photochemistry, reactions with the major atmospheric
constituents, and condensation. Photochemical destruction (tchem ~ 266 days)
and/or condensation as Na2SiF6 (s) appear to be the major sinks for SiF4. We
recommend searching for SiF4 with infrared spectroscopy using its 9.7 micron
band as done on Earth.
