Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p23a1353c&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P23A-1353
Mathematics
Logic
0328 Exosphere, 5415 Erosion And Weathering, 5421 Interactions With Particles And Fields, 6055 Surfaces, 6221 Europa
Scientific paper
Sputtering of Europa's surface material by Jupiter's magnetospheric plasma results in a strong coupling between the moon's exosphere and its surface content (Johnson 2001). In particular, the presence of alkalis in Europa's exosphere (Brown and Hill 1996, Brown 2001, Brown 2004, Leblanc et al 2005; Cassidy et al. 2008), sputtered from potential surface salts embedded in the icy regolith, supports the hypothesis of a geologically young surface associated with a subsurface ocean. We have carried out test-particle simulations of the ejection of sodium and potassium atoms from the icy matrix of Europa, by both magnetospheric ions and electrons sputtering and desorption stimulated by UV solar photons (hereafter noted PSD). We show that a minimal surface source term of sodium of 3x106cm-2.s-1 is required to produce the average emission intensities observed at different positions of Europa's orbit around Jupiter, in good agreement with Leblanc et al 2002. We also obtain that PSD alone can not account for the emission brightness variations reported in Leblanc et al (2005), as suggested earlier. Instead we propose that a plasma transient corresponding to a global increase of the flux of particles impacting the surface by a factor of about 8 compared to the ambient energetic ions and electrons flux during 10 hours would be sufficient to explain the observed emission enhancement (Cipriani et al 2008). At altitudes typically lower than 500km, we observe that the surface content produces clear asymmetries of the exospheric density from trailing to leading hemispheres consistent with the Cassini observations of Europa in eclipse (Cassidy et al. 2008). We investigate those asymmetries and relate them to signatures of surface processes such as PSD, or surface inhomogeneities and density gradients of sodium. Of particular importance is the estimated Na/K ratio at Europa, whose estimated values close to 20 support the hypothesis of endogenic processes releasing sodium bearing materials at Europa's surface (Johnson, 2001). Our model of Europa's exosphere and surface content, constrained by previous observations, is also used to study variations of the Na/K ratio in Europa's exosphere with respect to surface inhomogeneities. This is carried out in order to test the feasibility of characterizing surface-composition from an orbiter.
Cassidy Thomas
Cipriani Fabio
Johnson Robert E.
LeBlanc Francis
Witasse Olivier
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