NOTE: Volcanic Production of Sulfur Monoxide (SO) on Io

nic gases exsolved from magmas (Symonds et al. 1994). Thus, Ionian volcanic gases exsolved from magmas during eruption may also chemically We use thermochemical equilibrium calculations to show equilibrate (see below). that SO is expected in volcanic gases erupted on Io, and derive We modeled volcanic gas chemistry on Io in the 1000–2000 K range the range of temperatures, pressures, and elemental composithat covers the eruption temperatures of many terrestrial and lunar silicate tions that provide the observed SO/SO2 ratio of 3–10% in Io’s magmas. We used 1400 K as a nominal temperature because it is a typical atmosphere. Our predictions could be tested during the Galileo eruption temperature for basaltic magmas on Earth, such as at Kilauea (Symonds et al. 1994). The total pressure and bulk elemental composition Europa Mission (GEM), by Earth-based and Hubble Space of the volcanic gases are also inputs to our modeling. Although the Telescope observations in the millimeter and UV regions, or pressures at which Ionian volcanic gases exsolve from magmas are unby a mass spectrometer and/or infrared spectrometer on an Io known, pressures up to p100 bars in volcanic conduits on Io appear Volcanic Observer mission.  1998 Academic Press plausible depending on the depth of the volcanic source region. We note that Voyager observations indicate a pressure of p102 bars in an observed plume (Pearl et al. 1979), but we are modeling volcanic gas chemistry Sulfur monoxide is the second most abundant gas observed in the inside volcanic conduits and not in the plumes. We used pressures of atmosphere of Jupiter’s moon Io (Lellouch et al. 1996). Earth-based, 102 to 100 bars in our modeling; intermediate pressures in this range hemisphere-average millimeter wave-observations give SO/SO2 ratios reproduce the observed SO/SO2 ratio at observed hot spot temperatures. of 3–10% by volume. The observed abundance is consistent with SO We considered the chemistry of sulfur and oxygen, because these are production by SO2 photolysis (e.g., Kumar 1982, 1985; Summers and the two major elements observed in Io’s atmosphere, are the major Strobel 1996; Wong and Johnson 1996). On the other hand, Io’s low components of Io’s torus, and are generally considered to be the most pressure (p102 bar), patchy SO2 atmosphere is at least partially due to abundant elements in the nonsilicate portions of Io’s surface (Lellouch volcanic outgassing (e.g., Lellouch 1996; Spencer and Schneider 1996). 1996; Spencer and Schneider 1996). Observed O/S atomic ratios range Gaseous SO2 has been observed in volcanic plumes and over volcanically from p0.3 (deposits around the Pele volcano (Spencer et al. 1997a)) to active regions (Pearl et al. 1979; Sartoretti et al. 1994). Here we show p2, which is a typical value for the atmosphere (Lellouch et al. 1996) that high temperature volcanic gases exsolved from magmas can provide and a maximum value for the torus (Hall et al. 1994). We used O/S the observed SO abundance in Io’s atmosphere. atomic ratios from 102 to 3 (i.e., essentially from pure sulfur to SO3 gas). Io’s surface topography and temperatures of the hot spots, which range A nominal O/S atomic ratio of unity, which is an average value for up to 1700 K (Veeder et al. 1994; Blaney et al. 1995; McEwen et al. 1997; the torus, was chosen for some computations. Ideal gas thermochemical Spencer et al. 1997b; Stansberry et al. 1997), are evidence for predomiequilibrium calculations were done using our existing codes (Fegley and nantly silicate volcanism. The observed hot spot temperatures are plausiLodders 1994; Fegley et al. 1997). Thermodynamic data for S1–S8 , S2O, bly only lower limits to the actual temperatures because of the limited SO, SO2 , SO3 , O, O2 , O3 , and solid sulfur were taken from Gurvich spatial resolution of the infrared observations and the rapid formation et al. (1989–1994). of cooler crust (Howell 1997) on the lava flows. Nevertheless, at least Eruption velocities of Ionian volcanic gases have been estimated as some of the hot spot temperatures suggest the eruption of high tempera0.2 km s2 in the underground conduit (Kieffer 1984). Typical eruption ture silicate magmas such as basalts and even komatiites (Veeder et al. times (terupt) from volcanic source regions at 30 to 50 km depth in the 1994; Blaney et al. 1995; Spencer et al. 1997b). On Earth, eruption temperlithosphere and in a partially molten asthenosphere (Spencer and Schneider 1996) are thus 150–250 s. atures $900 K are high enough for thermochemical equilibrium in volca-