The atmosphere and ionosphere of Io

Abstract

A variety of models for Io's atmosphere, ionosphere, surface, and environment are developed and discussed in the context of recent observational data. The sodium emission detected by Brown appears to require a collisional excitation process in Io's atmosphere, and the extended sodium emission measured by Trafton et al. may require scattering of the planetary radiation by an extended sodium cloud. The sodium is presumably present initially in bound form on Io's surface and may be released by the sputtering mechanism suggested by Matson et al. The ionosphere detected by the radio occultation experiment on Pioneer 10 could be attributed to photoionization of atmospheric sodium if Io's atmosphere could sustain significant vertical motions, of order 1 s^(-1) directed up during the day, down at night. Vertical motions of this magnitude could be driven by condensation of atmospheric NH_3. The total density of gas at Io's surface appears to lie in the range 10^(10)-10^(12) molecules cm^(-3). Corpuscular ionization could play an additional role for the Ionosphere. In this case the satellite should exhibit an exceedingly bright, ~ 10 kR, airglow at Lɑ. The incomplete hydrogen torus observed by Judge and Carlson in the vicinity of Io requires a large supply of hydrogen from the satellite's atmosphere. The escape flux should be of order 10^(11) cm^(-2) s^(-1) and could be maintained by photolysis of atmospheric NH_3. The observed geometry of the hydrogen torus appears to require a surprisingly short lifetime, ~ 10^5 s, for neutral hydrogen near Io's orbit, and may indicate the presence of a large flux, ~ 10^9 cm ^(-2) s^(-1), of low-energy protons in Jupiter's magnetosphere. Implications of the hydrogen torus for the energy and mass balance of Jupiter's magnetosphere are discussed briefly, and observational programs are identified which might illuminate present uncertainties in our understanding of Io.

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