* Astronomy

Silicon Tetrafluoride on Io
Authors: Laura Schaefer, Bruce Fegley Jr

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.
SiF4 gas is also a potential indicator of silica-rich crust on Io.

Researchers have used Fluorine/Sulphur ratios in terrestrial and extraterrestrial basalts, and gas/lava enrichment factors for Fluorine and Sulphur measured at terrestrial volcanoes to calculate equilibrium SiF4/SO2 ratios in volcanic gases on Io.
The researchers conclude that SiF4 can be produced at levels comparable to the observed NaCl/SO2 gas ratio.
They 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.
They recommend searching for SiF4 with infrared spectroscopy using its 9.7 micron band as done on Earth.

4 HF (g) + SiO2 (silica) = SiF4 (g) + 2 H2O (g)

While Io is bone-dry they therefore do not expect HF gas to be present, alkali fluoride gases may react with pure silica to give SiF4 via reactions analogous to the one above. They therefore performed equilibrium calculations modelling the reaction of pure silica with volcanic gases containing O, S, Na, K, Li, Rb, Cs, Cl, F, Br, and I.

These calculations covered a range of temperatures from 500 - 2000 K and pressures from 10-8 to 10+2 bars.
They found that SiF4 is only important at low temperatures.
For a nominal pressure of 0.01 bars, SiF4 is the most abundant fluorine compound at temperatures below ~ 800 K.

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