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 KCI, O(Cl)(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)/CI = 1. As
the (Naf IC)/CI ratio decreases below unity, which is possible for
lower temperature volcanic vents, the major Cl gases change to Cl-2,
Cl, S2Cl, and SCl2. 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 Na2S
(a higher temperatures and lower O/S ratios), Na2SO4 (at lower temperatures
and higher O/S ratios), and NaCl and KCI. Under some conditions,
Na2SO4 and Na2S 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 CI in the plasma torus are sputtering from solid Na2S/Na2SO4/chloride
surface condensates and ionization of alkali chloride and monatomic
alkali gases that could be present in volcanic plumes and Io's volcanic
atmosphere. (C) Academic Press.
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