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摘要
We present the first calculation of Europa's sputtering (ion erosion) rate as a function of position on Europa's surface. We find a global sputtering rate of 2 x 10(27) H2O s(-1), some of which leaves the surface in the form of O-2 and H-2. The calculated O-2 production rate is 1 x 10(26) O-2 s(-1), H-2 production is twice that value. The total sputtering rate (including all species) peaks at the trailing hemisphere apex and decreases to about 1/3rd of the peak value at the leading hemisphere apex. O-2 and H-2 sputtering, by contrast, is confined almost entirely to the trailing hemisphere. Most sputtering is done by energetic sulfur ions (100s of keV to MeV), but most of the O-2 and H-2 production is done by cold oxygen ions (temperature similar to 100 eV, total energy similar to 500 eV). As a part of the sputtering rate calculation we compared experimental sputtering yields with analytic estimates. We found that the experimental data are well approximated by the expressions of Fama et al. for ions with energies less than 100 keV (Fama, M., Shi, J., Baragiola, R.A., 2008. Sputtering of ice by low-energy ions. Surf Sci. 602, 156-161), while the expressions from Johnson et al. fit the data best at higher energies (Johnson, RE., Burger, M.H., Cassidy, T.A., Leblanc, F., Marconi, M., Smyth, W.H., 2009. Composition and Detection of Europa's Sputter-Induced Atmosphere, in: Pappalardo, RT., McKinnon, W.B., Khurana, K.K. (Eds.), Europa. University of Arizona Press, Tucson.). We compare the calculated sputtering rate with estimates of water ice regolith grain size as estimated from Galileo Near-Infrared Mapping Spectrometer (NIMS) data, and find that they are strongly correlated as previously suggested by Clark et al. (Clark, IN., Fanale, F.P., Zent, A.P., 1983. Frost grain size metamorphism: Implications for remote sensing of planetary surfaces. Icarus 56, 233-245.). The mechanism responsible for the sputtering rate/grain size link is uncertain. We also report a surface composition estimate using NIMS data from an area on the trailing hemisphere apex. We find a high abundance of sulfuric acid hydrate and radiation-resistant hydrated salts along with large water ice regolith grains, all of which are consistent with the high levels of magnetospheric bombardment at the trailing apex.
- 出版日期2013-3
