Search for Trace Species in Europa's Exosphere

Abstract:

Understanding the present surface composition of Europa provides critical information about its potential habitability. From Earth we currently have very few tools at our disposal to measure Europa’s composition. Two notable exceptions are Keck NIR spectroscopy (cf Brown and Hand 2013) and Hubble Space Telescope UV spectroscopy (cf Cunningham et al. 2015). In March 2015 we obtained 5 orbits of deep UV spectroscopy of Europa using the Cosmic Origins Spectrograph covering wavelength range 1170–1760 Å. The purpose of the observations was to detect trace species in Europa’s exosphere, which is generated by charged particle sputtering of Europa’s water ice surface. The composition of the exosphere therefore provides an indirect measurement of surface composition. Furthermore, if active plumes are present, the composition of the exosphere may also reflect the composition of Europa’s subsurface water reservoir. Of particular interest in the observed wavelength range are multiplets of atomic chlorine, because chlorine is predicted to be a major constituent of Europa’s ocean (Kargel et al. 2000), and Na and K chlorides are expected to be major constituents of the icy shell (Zolotov and Shock 2001; Zolotov and Kargel 2009). The present situation at Europa is analogous to that at Io in the late 1990s, when chlorine ions were first detected in the plasma near Io (Kuppers and Schneider 2000), motivating searches for atomic chlorine and chlorine-bearing species that were subsequently detected in Io’s atmosphere (Lellouch et al. 2003, Feaga et al. 2004). Galileo plasma measurements have detected chlorine ions near Europa (Volwerk et al. 2001), which has motivated the present search for chlorine in Europa’s exosphere. We will present the new COS spectra of Europa and discuss the implications of the trace species that have been detected in these data.