Physics
Scientific paper
Apr 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996icar..120..290s&link_type=abstract
Icarus, Volume 120, Issue 2, pp. 290-316.
Physics
49
Scientific paper
We present an updated model for the photochemistry of Io's atmosphere and ionosphere and use this model to investigate the sensitivity of the chemical structure to vertical transport rates. SO_2 is assumed to be the dominant atmospheric gas, with minor molecular sodium species such as Na_2S or Na_2O released by sputtering or venting from the surface. Photochemical products include SO, O_2, S, O, Na, NaO, NaS, and Na_2. We consider both ``thick'' and ``thin'' SO_2 atmospheres that encompass the range allowed by recent HST and millimeter-wave observations, and evaluate the possibility that O_2 and/or SO may be significant minor dayside constituents and therefore likely dominant nightside gases. The fast reaction between S and O_2 limits the column abundance of O_2 to ~10^4 less than that calculated by Kumar (J. Geophys. Res. 87, 1677-1684, 1982; 89(A9), 7399-7406, 1984) for a pure sulfur/oxygen atmosphere. If a significant source of NaO_2 or Na_2O were supplied by the surface and mixed rapidly upward, then oxygen liberated in the chemical reactions which also liberate free Na would provide an additional source of O_2. Fast eddy mixing will enhance the transport of molecular sodium species to the exobase, in addition to increasing the vertical transport rate of ions. Ions produced in the atmosphere will be accelerated by the reduced corotation electric field penetrating the atmosphere. These ions experience collisions with the neutral gas, leading to enhanced vertical ion diffusion. The dominant ion, Na^+, is lost primarily by charge exchange with Na_2O and/or Na_2S in the lower atmosphere and by diffusion through the ionopause in the upper atmosphere. The atmospheric column abundance of SO, O_2, and the upper atmosphere escape rates of Na, S, O, and molecular sodium species are all strong functions of the eddy mixing rate. Most atmospheric escape, including that of molecular sodium species, probably occurs from the low density ``background'' SO_2 atmosphere, while a localized high density ``volcanic'' SO_2 atmosphere can yield an ionosphere consistent with that detected by the Pioneer 10 spacecraft.
Strobel Darrell F.
Summers Michael E.
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