Chemistry of Sodium, Potassium, and Chlorine in Volcanic Gases on Io

Abstract

We used thermochemical equilibrium calculations in the O–S–Na–K–Cl–H system to model the speciation of volcanic gases emitted from high-temperature (1000–2000 K) silicate magmas on Io. The effects of temperature, pressure, and bulk composition of the gases are explored. The bulk compositions are based on atomic ratios observed in Io's plasma torus and extended atmosphere, and from chondritic abundances. The results show that chlorides of Na and K are the major Cl gases, NaCl, Na, and (NaCl) 2 are the major Na gases, and KCl, (KCl) 2, and K are the major K gases for systems with (Na+K)/Cl>1. The abundances of Na, K, and Cl gases change dramatically at (Na+K)/Cl=1. As the (Na+K)/Cl ratio decreases below unity, which is possible for lower temperature volcanic vents, the major Cl gases change to Cl 2, Cl, S 2Cl, and SCl 2. The results show that abundances of H-bearing gases are insignificant for plausible hydrogen abundances in Io. Higher temperatures and lower pressures increase the abundances of monatomic Na and K. Sodium, K, and Cl compounds condense as Na 2S (at higher temperatures and lower O/S ratios), Na 2SO 4 (at lower temperatures and higher O/S ratios), and NaCl and KCl. Under some conditions, Na 2SO 4 and Na 2S condense simultaneously. These condensates can form coatings on silicate ash particles in the vicinity of volcanic vents. Condensation temperatures decrease as pressure decreases, and condensation is not favored by low-pressure volcanic vents. Silicate magmas, especially alkaline ultrabasic magmas may be important sources of S, alkalis, and Cl on Io. Our predictions agree with spectral absorption features indicating that sodium sulfate and/or sodium sulfide may be present in red deposits on Io's surface. The two major sources of Na, K, and Cl in the plasma torus are sputtering from solid Na 2S/Na 2SO 4/chloride surface condensates and ionization of alkali chloride and monatomic alkali gases that could be present in volcanic plumes and Io's volcanic atmosphere.