Аннотация
Observations of the to plasma torus and neutral clouds indicate that
the extended ionian atmosphere must contain sodium, potassium, and
chlorine in atomic and/or molecular form. Models that consider sublimation
of pure sulfur dioxide frost as the sole mechanism for generating
an atmosphere on to cannot explain the presence of alkali and halogen
species in the atmosphere-active volcanoes or surface sputtering
must also be considered, or the alkali and halide species must be
discharged along with the SO2 as the frost sublimates. To determine
how volcanic outgassing can affect the chemistry of to's atmosphere,
we have developed a one-dimensional photochemical model in which
active volcanoes release a rich suite of S-, O-, Na-, K-, and Cl-bearing
vapor and in which photolysis, chemical reactions, condensation,
and vertical eddy and molecular diffusion affect the subsequent evolution
of the volcanic gases. Observations of Pele plume constituents, along
with thermochemical equilibrium calculations of the composition of
volcanic gases exsolved from high-temperature silicate magmas on
to, are used to constrain the composition of the volcanic vapor.
We 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 to. Although the relative abundances of these species
will 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,
Na-2, NaS2, NaO2, NaOS, NaSO2, SCl, ClO, Cl-2, 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 to. The neutral NaCl molecules present at high altitudes
in atmospheres 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. (C) 2002 Elsevier Science (USA).
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