Theoretical Studies of Io's Atmosphere

Citation

Summers, Michael Earl (1985) Theoretical Studies of Io's Atmosphere. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/rsm8-qh33. https://resolver.caltech.edu/CaltechTHESIS:01302013-100511748

Abstract

A range of theoretical models of the compositional structure of Io's dayside atmosphere and ionosphere are developed. The dominant neutral gas, SO₂, is provided by sublimation of surface frost. Photochemical processes lead to the build up of O, S, SO, and O₂ as minor gasses near Io's surface while O becomes the dominant gas near the exobase. The vertical column density of O₂ in all models considered is less than 10¹⁴ cm^-2. The dayside ionosphere is formed as a result of ionization of neutral species by solar UV radiation. Charge exchange and rearrangement reactions are important for determining the ionic composition of the ionosphere. The dominant ion in the models considered is SO⁺. A number of charge exchange reactions are identified whose rates need to be better determined in order to refine the present model of the ionosphere. The best matches of the model ionospheres to that observed by the Pioneer 10 radio occultation experiment require atmospheric surface concentrations of SO₂ in the range of 2.5 x 10⁹ to 1 x 10¹¹ cm^-3, and an exospheric temperature in the range of 960 K to 1230 K. The ratio of the escape fluxes of O to S from the exobase is ≥ 2 in the models considered, while the models which allow surface deposition of minor constituents always have a total sulfur depositional rate greater than 1/2 of the total oxygen depositional rate, thus a surface enrichment of S relative to that predicted by a pure SO₂ surface. The depositional rate of this "excess" sulfur is in the range 100 m to 1 km thickness per billion years.

Atmospheric Na is provided by surface sputtering of SO₂ surface frost with Na impurities by MeV type magnetospheric ions. An upward flux of Na₂O of S x 10⁷ cm^-2 s^-1 leads to an escape flux of Na from the exobase of 1 x 10⁷ cm^-2 s^-1. The chemistry (ion and neutral) of Na species in the atmosphere has only minor effects on the major characteristics of the atmosphere and ionosphere.

It is generally accepted that Io is the source of S, O, Na, and K which, subsequent to ionization, form the constituents of the Io plasma torus. It is shown in chapter II that the escape of S and O from Io can be understood in terms of the photochemistry of a predominantly SO₂ atmosphere created by the high vapor pressure of SO₂. However, the vapor pressures of Na₂S, K₂S and other common compounds containing Na and K are negligible at the surface temperature of Io. In chapter III we propose that Na and K escape from Io in two stages. Atoms of Na and K (or molecules containing these atoms) are first sputtered into the atmosphere from the surface by high energy magnetospheric ions. Atmospheric sputtering by low energy corotating ions then removes these constituents (along with others present) out of Io's gravitational control. The estimated injection rates are sufficiently large to maintain the observed Na, K, and O clouds observed around Io.

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