Evidence for sulfur dioxide, sulfur monoxide, and hydrogen sulfide in the Io exosphere

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Abstract
Molecules in the Io exosphere are ionized and accelerated in the Jovian magnetospheric electric field creating a distribution in velocity space that is unstable to the generation of magnetic fluctuations at the frequency of the gyrating ion. On the inbound portion of first pass by Io, Galileo detected strong waves centered on the SO2+ gyrofrequency, but on the outbound portion, waves near the SO+ gyrofrequency were seen, as well as a weak burst near the H2S+ gyrofrequency. On passes I24 and I25, waves at both the SO+ and SO2+ gyrofrequencies were present. On the I27 pass, additional components were also observed, close to mass 34, 35, and 37 amu, as would be expected if H2S+, 35Cl+, and 37Cl+ ions were present. These disparate observations on geometrically similar passes suggest that the atmosphere of Io is quite variable, possibly changing with solar zenith angle and perhaps changing in concert with the relative activity of different volcanic sources.