%0 Journal Article %T Energy Distributions for Desorption of Sodium and Potassium from Ice: The Na/K Ratio at Europa %+ Engineering Physics Program [Charlottesville] %+ Service d'aéronomie (SA) %+ Department of Physics and Astronomy [Piscataway] %A Johnson, R. E. %A Leblanc, François %A B. V., Yakshinskiy %A Madey, T. E. %< avec comité de lecture %@ 0019-1035 %J Icarus %I Elsevier %V 156 %N 1 %P 136-142 %8 2002-03 %D 2002 %R 10.1006/icar.2001.6763 %Z Sciences of the Universe [physics]/Astrophysics [astro-ph]Journal articles %X 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. %G English %L insu-02151952 %U https://insu.hal.science/insu-02151952 %~ INSU %~ UPMC %~ CNRS %~ UVSQ %~ AERONOMIE %~ SORBONNE-UNIVERSITE %~ ALLIANCE-SU