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Dr. Julianne I. Moses
Recent Research

Io's Atmosphere: Direct Volcanic Outgassing
or SO2 Frost Sublimation?

Jupiter's satellite Io has a thin, patchy atmosphere composed mostly of sulfur dioxide. Atmospheric constituents can be seen at ultraviolet and microwave wavelengths, and the atmosphere has been seen “glowing” in eclipse or nighttime images at visible wavelengths (see Figure 1). The three main proposed mechanisms for generating an atmosphere on Io include frost sublimation, surface sputtering, and active volcanism. Although the relative role of each mechanism is not well understood, both SO2 frost sublimation and active volcanism appear to play a role (surface sputtering is only important if atmospheric densities drop to very low values). Wong and Smyth (2000, Icarus 146, p. 60) summarize the evidence for a sublimation-driven component. The role of active volcanic outgassing is suggested by the apparent patchiness of SO2 vapor across Io's surface, the observed red-shifted and broadened shape of SO and SO2 at millimeter wavelengths, and the presence of Na, K, and Cl (i.e., non-sulfur or oxygen components) in the Io plasma torus and neutral clouds (see Moses et al. 2002, Icarus 156, p. 76 for further evidence and references). To determine how active volcanism might affect the standard picture of sulfur dioxide photochemistry on Io, Mikhail Zolotov, Bruce Fegley, and I have developed a one-dimensional atmospheric model in which a variety of sulfur-, oxygen-, sodium-, chlorine-, and potassium-bearing volatiles are volcanically outgassed at Io's surface and then evolve due to photolysis, chemical kinetics, and diffusion. Although Io's low-density atmosphere is complex, highly dynamic, and not well represented by one-dimensional hydrostatic-equilibrium models, our models give useful first-order predictions of the relative abundances of different potential atmospheric species and for determining the importance of photochemical processing of the volcanic gases. Thermochemical equilibrium calculations (Zolotov and Fegley 1999, Icarus 141, p. 40 and Fegley and Zolotov 2000, Icarus 148, p. 193) in combination with recent observations of gases in the Pele plume (Spencer et al. 2000, Science 288, p. 1208 and McGrath et al. 2000, Icarus 146, p. 476) are used to help constrain the composition and physical properties of the exsolved volcanic vapors. Both the observations and equilibrium models suggest that S2 may be a common gas emitted in volcanic eruptions on Io. If so, our photochemical models indicate that the composition of Io's atmosphere could differ significantly from the case of an atmosphere in equilibrium with SO2 frost.

The major differences as they relate to oxygen and sulfur species are an increased abundance of S, S2, S3, S4, SO, and S2O and a decreased abundance of O and O2 in the Pele-type volcanic models as compared with frost sublimation models (see Figure 2). One observational test of the significance of active volcanoes in maintaining Io's atmosphere would be the simultaneous monitoring of SO and SO2 at ultraviolet or microwave wavelengths. We predict that the SO/SO2 ratio will be spatially and temporally variable as volcanic activity fluctuates. Many of the interesting volcanic species in our model (e.g., S2, S3, S4, S2O) are short lived and will be rapidly destroyed on Io once volcanic plumes shut off; condensation of these species near the source vent is likely. The diffuse red deposits associated with active volcanic centers on Io may be caused by S4 radicals that are created and temporarily preserved when sulfur vapor (predominantly S2) condenses around the volcanic vent. Condensation of SO across the surface and, in particular, in the polar regions might also affect surface spectral properties. We predict that the S/O ratio in the torus and neutral clouds might be correlated with volcanic activity — during periods when volcanic outgassing of S2 is prevalent, we would expect the escape of sulfur to be enhanced relative to that of oxygen, and the S/O ratio in the torus and neutral clouds would be correspondingly enhanced.

We also find that NaCl, Na, Cl, KCl, and K will be the dominant alkali and chlorine gases in atmospheres generated from Pele-like plume eruptions on Io. Although the relative abundances of these species depend on uncertain model parameters and initial conditions, these five species remain dominant for a wide variety of realistic conditions. Other sodium and chlorine molecules such as NaS, NaO, Na2, NaS2, NaO2, NaOS, NaSO2, SCl, ClO, Cl2, S2Cl, and SO2Cl2 will be only minor constituents in the ionian atmosphere because of their low volcanic emission rates and their efficient photochemical destruction mechanisms. Our modeling has implications for the general appearance, properties, and variability of the neutral sodium clouds and jets observed near Io. The neutral NaCl molecules present at high altitudes generated by active volcanoes might provide the NaX+ ion needed to help explain the morphology of the high velocity sodium “stream” feature observed near Io. The recent microwave detection of NaCl vapor in Io's atmosphere by Lellouch et al. 2002, Int. Astron. Union Circ. 7803) provides support for the hypothesis that active volcanoes play a role in maintaining the atmosphere and that NaCl may be the unidentified “NaX” molecule needed to explain the “stream” feature; however, the inferred NaCl/SO2 ratio of ~0.1% suggests that volcanoes are not the only atmospheric source.

Note: much of this material was cannibalized from Moses et al. (2002a, Icarus 156, p. 76) and Moses et al. (2002b, Icarus 156, p. 107). Further information about Io's atmosphere can be found within these papers and references therein.

Last updated
April 4, 2008