Energy Distributions for Desorption of Sodium and Potassium from Ice: The Na/K Ratio at Europa

R. E. Johnson (1) , François Leblanc (1, 2) , Yakshinskiy B. V. (3) , T. E. Madey (3)

1 Engineering Physics Program [Charlottesville]
2 SA - Service d'aéronomie
3 Department of Physics and Astronomy [Piscataway]

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.

Domaines: Astrophysique [astro-ph]

R. E. Johnson, François Leblanc, Yakshinskiy B. V., T. E. Madey. Energy Distributions for Desorption of Sodium and Potassium from Ice: The Na/K Ratio at Europa. Icarus, 2002, 156 (1), pp.136-142. ⟨10.1006/icar.2001.6763⟩. ⟨insu-02151952⟩