Energy Distributions for Desorption of Sodium and Potassium from Ice: The Na/K Ratio at Europa |
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| Authors: | R.E. JohnsonF. Leblanc B.V. YakshinskiyT.E. Madey |
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| Affiliation: | a Engineering Physics, University of Virginia, Charlottesville, Virginia, 22903, f1rej@virginia.eduf1b Engineering Physics, University of Virginia, Charlottesville, Virginia, 22903c Service d'Aéronomie du, CNRS, Vierrières-Le-Buisson, 91371, Franced Department of Physics and Astronomy and Laboratory for Surface Modification, Rutgers University, Piscataway, New Jersey, 08854 |
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| Abstract: | The sputtering and decomposition of the surface of Europa by fast ions and electrons lead to the production of an atomosphere containing sodium and potassium atoms. Here time-of-flight energy distributions are measured for Na and K sputtered from a vapor-deposited ice by 200-eV electrons. These data are then used in a Monte Carlo simulation for alkalis in Europa's atmosphere. Na/K ratios versus distance from Europa are calculated and compared to the recent observations in the range 6 to 18 Europan radii from the surface. Normalizing to the observations, the Na/K ratio for the loss rates is ∼27 and the ratio for the average surface source rates is ∼20. These ratios are very different from the Na/K ratio at Io and are larger than the Na/K ratio suggested for Europa's putative subsurface ocean, consistent with fractionation on freezing and upwelling of ocean material. |
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| Keywords: | Europa |
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