Supply of trans-Europa Neutral Torus by the Surface-bounded Atmosphere of Europa

Magnetospheric ion sputtering is an important source of neutrals at the icy Galilean satellites [1-3]. The Cassini Magnetosphere Imaging Instrument (MIMI) has detected [4] a neutral cloud located at Europa's orbit and likely consisting of sputtering products. A collisional Monte Carlo model of Europa's surface-bounded atmosphere in which the thermal and sputtering sources of H2O molecules and their molecular fragments are accounted for was developed [5]. Dissociation and ionization of H2O and O2 by magnetospheric electrons, solar UV radiation and photoelectron impact, and collisional ejection from the atmosphere by the low energy plasma are taken into account. The surface-bounded O2 atmosphere of Europa originates from a balance between radiolysis of the satellite icy surface by the solar UV radiation and high-energy magnetospheric plasma and the collisional ejection from the surface-bounded atmosphere by the low-energy plasma. The enhanced oxygen production rate found earlier [6] is used along with comparable H2O ejection rate. Calculations show that the atmospheric chemical composition is determined by both the water and oxygen photochemistry in the near-surface atmospheric region, escape of suprathermal neutrals into the Jovian system, and the adsorption-desorption exchange by radiolytic water products with the satellite surface. It is shown, as was the case for the sodium atmosphere [7], that the oxygen and hydrogen loss from Europa's atmosphere supplies its gas torus [8]. The calculated oxygen supply rates are in the range (6-10)x10²⁷ atoms per second depending on the ratio of thermal (evaporation) and nonthermal (sputtering) source rates. Using an oxygen ionization rate of 2.0x10^-6 s^-1 the estimate of the total number N of neutrals in the torus is (3.0 - 5.0)x10³³ O atoms. This should be doubled to take into account the escape of atomic and molecular hydrogen. This eatimate approaches that inferred from observations [4] (4.5 - 9.0)x10³³ atoms. Therefore, the collisional Monte Carlo model of Europa's surface-bounded atmosphere may be able to provide the required supply of neutrals to the trans-Europa gas torus in the inner Jupiter's magnetosphere.

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