Europa's Atmosphere and Aurora: Recent Advances from HST-STIS and Plans for Plume Searches with JUICE-UVS

Space Telescope Imaging System (STIS) images of Europa's neutral oxygen 130.4 nm and 135.6 nm emissions contain a wealth of information about the molecular oxygen atmosphere, discovered using previous Hubble far-UV observations. Europa's magnetospheric plasma interaction generates auroral emissions, which exhibit a morphology that has been difficult to interpret. Recent observations in Nov. & Dec. 2012 allow a new understanding of how Jupiter's magnetic field orientation and relation to the plasma sheet control the emission variability, yet explanations for this general behavior, including the likely role of ocean-induced magnetic fields and possible local atmospheric density enhancements, remain incomplete (cf. Roth et al. this meeting). NASA's Ultraviolet Spectrograph (UVS) instrument contribution to the ESA-led Jupiter Icy Moons Explorer (JUICE) mission will obtain excellent imaging of these atmospheric and auroral emissions from Europa during two flybys currently planned, with the objective of investigating these and other unanswered questions. UVS's stellar occultation technique will be used to characterize Europa's atmosphere structure and composition and to also search for local enhancements created by plumes. This stellar occultation technique, demonstrated by Cassini-UVIS at Enceladus, has the benefit of being useful at relatively large distances (several 10's of Jupiter radii) as well as during the Europa flyby sequences (several 10's of Europa radii). A robust search for plumes is planned in JUICE's first year at Jupiter to provide a roughly 30-degree grid of global coverage, followed by focused targeting of likely plumes/active-regions during early and late stages of the flyby sequences. High spatial resolution limb imaging is also planned near closest approaches, which could directly image plume gases in a manner analogous with plume aurora imaging of Io. A UV spectrograph on the planned Europa Clipper mission could perform an even more robust search for such currently active geological sites with connectivity to subsurface liquid water.