Exospheric signatures of alkali abundances in Europa's regolith

Abstract

International audienceThe sodium and potassium exospheres of Europa are described as well as the variations of the Na/K ratio from Europa's surface up to about 20 Europa radii. Observations carried out by M. E. Brown (2001) are used to constrain the Na and K source terms. We find that an average source rates ratio of 17 ± 2 at the surface and loss rates ratio of 26 ± 2 reproduce well the observations. These values are consistent with the conclusions by R. E. Johnson et al. (2002) on its most probable origin. Moreover, we here show that a local increase of the surface Na/K ratio by a factor 2 in a 20° × 30° wide region should induce an observable exospheric signature between 80 and 300 km above the surface. Therefore simultaneous observations of sodium and potassium from an Europa orbiter would allow one to derive important properties of the surface and subsurface of the Galilean moon

Aqueous fluid composition in CI chondritic materials: Chemical equilibrium assessments in closed systems

“…The upper dotted lines show the range of Na/K ratios in icy grains emitted from Enceladus' (Postberg et al, 2009). The lower dotted lines correspond to the exosphere of Europa that may characterize the moon's subsurface ice and an ocean (Cipriani et al, 2009). These data may indicate lower temperature of aqueous processes on Enceladus than on Europa.…”

“…However, a presence of sulfate species on the surface of Europa (McCord et al, 1999;Carlson et al, 2009) together with the low Na/K ratio in its exosphere (26 ± 2, Brown, 2001;Johnson et al, 2002;Cipriani et al, 2009) may indicate sub-oceanic hydrothermal processes (Fig. 9).…”

Towards a Global Unified Model of Europa’s Tenuous Atmosphere

“…The improved model suggests that the production and brightness of the Na emissions at Europa depend on various factors such as the local time in Europa's orbit and the moon's position with respect to the plasma sheet. Cipriani et al (2008Cipriani et al ( , 2009 updated the model to account for non-uniform Na density distribution on the icy surface and for ejection of Na atoms by solar UV photons, and then applied it also to potassium using the Na/K ratio as measured by Brown (2001). Sputtered K has a lower scale height than that of Na, and therefore Cipriani et al (2008Cipriani et al ( , 2009 identified three regions with different Na/K values.…”

“…Cipriani et al (2008Cipriani et al ( , 2009 updated the model to account for non-uniform Na density distribution on the icy surface and for ejection of Na atoms by solar UV photons, and then applied it also to potassium using the Na/K ratio as measured by Brown (2001). Sputtered K has a lower scale height than that of Na, and therefore Cipriani et al (2008Cipriani et al ( , 2009 identified three regions with different Na/K values. These are: a) altitudes above 3 R E where Na/K = 20-25, b) altitudes between 1.5 and 2 R E where Na/K = 7, and c) a transition region between (a) and (b).…”

Composition and Detection of Europa’s Sputter-induced Atmosphere

“…Leblanc et al (2002Leblanc et al ( , 2005 also predicted a globally averaged sodium concentration at the surface equal to ~0.01, and suggested that the observed sodium was sputtered from ice and not directly from a mineral or a salt. Recent modeling has improved on this description of the variability and on the Na/K ratios (Cipriani et al, 2008(Cipriani et al, , 2009.…”