Disulfur monoxide and polysulfur oxides

Disulfur monoxide (S2O, arranged as bent SSO) is an unstable molecule at Io temperatures, decomposing to form SO2 and S3 (Steudel and Steudel, 2004). S3 and S2O can then react to form S5O and bimolecular decomposition of the latter produces SO2, S3, and S4. This decomposition of S2O occurs at 100 K or below (Blukis and Myers, 1965; Hapke and Graham, 1989) and explains the red color of S2O when deposited at cryogenic temperatures (Steudel and Steudel, 2004). Pure or nearly pure S2O concentrations can undergo radical-chain polymerization forming polysulfur oxides (PSO) - chains of sulfur atoms with occasional oxygen atoms attached to the side.

The fundamental SO stretch band of pure condensed S2O occurs at 1,165cm-1, the SO bend at 388 cm-1, and the SS stretch at 679 cm-1 (Blukis and Myers, 1965). The SO stretch bands for S5O and PSO are at 1,119 and 1,123 cm-1, respectively (Steudel, 2003c). The formation and mid- to near-infrared spectra of S2O and PSO are currently being studied in B. Schmitt's laboratory (Baklouti et al., 2004). Photolysis of S2O produces a cyclic isomer (Lo et al., 2002).

The original motivation for Hapke's (1989) suggestion for S2O and PSO on Io was to explain Io's near-ultraviolet reflectance spectrum that he showed was discrepant with S8(a) reflectance. It was later shown that reflectance by other sulfur allotropes was consistent with Io's spectrum (see above), so there is presently no need to invoke S2O or PSO on Io. Additionally, S2O is a minor constituent in the plumes (<1% molar, Zolotov and Fegley, 2000) and will be reduced even further by thermal decomposition reactions as discussed above, producing plume S5O that will be deposited on the surface and then photolyzed to SO2 and elemental sulfur (Steudel and Steudel, 2004). Conversely, plume sulfur monoxide (see above) may react to form S2O, so the monomers and PSO may be present as minor species with their signatures evident as overtones at ^2,330 cm-1 (4.29 ^m) and 2,246 cm-1 (4.45 ^m) respectively. These predicted positions straddle the strong band of SO2 and although no features attributable to S2O or PSO are evident, uncertainty in the positions and strengths preclude establishing upper limits. The cyclic form of S2O may be present on Io based on features at 800 and 580 cm-1 in the Voyager thermal emission spectrum that may match transitions of cyclic S2O (Lo et al., 2002).

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