SO 2 ‐rock interaction on Io: Reaction under highly oxidizing conditions

Laboratory simulations have been carried out to test the possibility that interactions of SO 2 and silicates can produce Na‐S compounds which can account for the observed surface enrichment of Na relative to Si on Io. A wide variety of silicate compositions were heated under oxidizing conditions with a SO 2 /O 2 = 1 gas phase at a mid‐level crustal temperature for Io (1123K). All experiments produced sulfate deposits on the silicate surfaces. The nature of the sulfate changed systematically with the silicate Ca/Na ratio, with mixtures of CaSO 4 and Na‐rich sulfate formed by basaltic compositions having higher Ca/Na but only alkali‐rich sulfates formed for more granitic (low Ca/Na) compositions. For crystalline albite and an albite‐orthoclase eutectic glass composition, K and Al‐rich sulfates were formed. Assuming that burial of SO 2 to the temperatures of our experiments is plausible on Io and that somewhat less oxidizing conditions do not qualitatively change the results, SO 2 ‐rock interactions producing Na‐rich sulfates could account for the Io surface enrichment in Na. Observations on minor elements, such as K or Ca, in the atomic cloud or in magnetospheric ion spectra could be used to identify sulfates as Io surface phases and, conceivably, even define Ionian crustal magma types. Many analyses show significant S excesses which are best interpreted as due to the presence of bisulfate (HSO 4 .) components, and NaHSO 4 deserves consideration as an Io surface mineral. The possibility of elemental S on Io can be regarded as a totally separate issue from the problem of the surface Na enrichment.