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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
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