I. INTRODUCTION We present an updated model for the photochemistry of Io's
The structure of Io's SO 2 atmosphere has long been a
atmosphere and ionosphere and use this model to investigate the sensitivity of the chemical structure to vertical transport subject of considerable debate, especially with regard to rates. SO 2 is assumed to be the dominant atmospheric gas, with the surface pressure and its distribution over the surface. minor molecular sodium species such as Na 2 S or Na 2 O released The initial Pioneer 10 radio occultation experimental deby sputtering or venting from the surface. Photochemical prodtection of dayside and nightside ionospheres (Kliore et al., ucts include SO, O 2 , S, O, Na, NaO, NaS, and Na 2 . We consider 1974) seemed to suggest that the atmosphere was in fact both ''thick'' and ''thin'' SO 2 atmospheres that encompass the global in nature but with possible strong day/night variarange allowed by recent HST and millimeter-wave observations, tions in surface pressure (Brown and Yung, 1976). The and evaluate the possibility that O 2 and/or SO may be significant Voyager encounters and the IRIS detection of gaseous minor dayside constituents and therefore likely dominant nightside gases. The fast reaction between S and O 2 limits the SO 2 on Io's dayside (Pearl et al. 1979) and the identification column abundance of O 2 to Θ10 4 less than that calculated by of SO 2 surface frosts and volcanoes perhaps made the issue Kumar (J. Geophys. Res. 87, 1677-1684, 1982; 89(A9), 7399of the atmospheric surface pressure even more difficult to 7406, 1984) for a pure sulfur/oxygen atmosphere. If a significant resolve. The vapor pressure of SO 2 exhibits such a strong source of NaO 2 or Na 2 O were supplied by the surface and mixed dependence on temperature that atmospheric SO 2 gas in rapidly upward, then oxygen liberated in the chemical reactions equilibrium with surface frosts could lead to a ''buffered'' which also liberate free Na would provide an additional source atmosphere with as much as a factor of 10 7 difference in of O 2 . Fast eddy mixing will enhance the transport of molecular surface pressure between the day and night hemispheres. sodium species to the exobase, in addition to increasing the vertical transport rate of ions. Ions produced in the atmosphere Thus Io's dayside SO 2 atmosphere might be collisionally will be accelerated by the reduced corotation electric field penedominated while Io's nightside SO 2 atmosphere may be trating the atmosphere. These ions experience collisions with essentially exospheric in nature (see Kumar and Hunten the neutral gas, leading to enhanced vertical ion diffusion. The 1982, Fanale et al. 1982 for post-Voyager reviews). This dominant ion, Na Ψ , is lost primarily by charge exchange with picture would be considerably modified if a non-condens-Na 2 O and/or Na 2 S in the lower atmosphere and by diffusion ing gas such as O 2 were present (Kumar and Hunten 1982, through the ionopause in the upper atmosphere. The atmo-Ingersoll et al. 1985). This has motivated two classes of spheric column abundance of SO, O 2 , and the upper atmosphere escape rates of Na, S, O, and molecular sodium species are all models for the chemical structure of Io's atmosphere, i.e., strong functions of the eddy mixing rate. Most atmospheric the aeronomical models such as those of Kumar (1979, escape, including that of molecular sodium species, probably 1980, 1984 , Summers 1985) ) and the low-density gas models occurs from the low density ''background'' SO 2 atmosphere, of Fanale et al. (1982) and Matson and Nash (1983).
while a localized high density ''volcanic'' SO 2 atmosphere can The ''thickness'' of the atmosphere has important impliyield an ionosphere consistent with that detected by the Pioneer cations for atmospheric stability and thus for escape pro-10 spacecraft.