title: Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa's water vapor aurora.
abstract: We report far-ultraviolet observations of Jupiter's moon Europa taken by Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope (HST) in January and February 2014 to test the hypothesis that the discovery of a water vapor aurora in December 2012 by local hydrogen (H) and oxygen (O) emissions with the STIS originated from plume activity possibly correlated with Europa's distance from Jupiter through tidal stress variations. The 2014 observations were scheduled with Europa near the apocenter similar to the orbital position of its previous detection. Tensile stresses on south polar fractures are expected to be highest in this orbital phase, potentially maximizing the probability for plume activity. No local H and O emissions were detected in the new STIS images. In the south polar region where the emission surpluses were observed in 2012, the brightnesses are sufficiently low in the 2014 images to be consistent with any H2O abundance from (0-5)×10(15) cm(-2). Large high-latitude plumes should have been detectable by the STIS, independent of the observing conditions and geometry. Because electron excitation of water vapor remains the only viable explanation for the 2012 detection, the new observations indicate that although the same orbital position of Europa for plume activity may be a necessary condition, it is not a sufficient condition. However, the December 2012 detection of coincident HI Lyman-α and OI 1304-Å emission surpluses in an ∼200-km high region well separated above Europa's limb is a firm result and not invalidated by our 2014 STIS observations.
authors.affiliation: Southwest Research Institute, San Antonio, TX 78238; School of Electrical Engineering, Royal Institute of Technology, 100 44 Stockholm, Sweden; lorenzr@kth.se.
authors.forename: Lorenz
authors.identifier: http://orcid.org/0000-0003-0554-4691
authors.initials: L
authors.lastname: Roth
authors.affiliation: Southwest Research Institute, San Antonio, TX 78238;
authors.forename: Kurt D
authors.initials: KD
authors.lastname: Retherford
authors.affiliation: Institute of Geophysics and Meteorology, University of Cologne, 50969 Cologne, Germany;
authors.forename: Joachim
authors.initials: J
authors.lastname: Saur
authors.affiliation: Department of Earth and Planetary Science, The Johns Hopkins University, Baltimore, MD 21218; Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218;
authors.forename: Darrell F
authors.initials: DF
authors.lastname: Strobel
authors.affiliation: Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218;
authors.forename: Paul D
authors.initials: PD
authors.lastname: Feldman
authors.affiliation: NASA Marshall Space Flight Center, Huntsville, AL 35811; and.
authors.forename: Melissa A
authors.initials: MA
authors.lastname: McGrath
authors.affiliation: Department of Earth and Planetary Sciences, University of California, Santa Cruz, CA 95064.
authors.forename: Francis
authors.initials: F
authors.lastname: Nimmo
mesh_terms: D018559:Exobiology / Q000379:methods; D005118:Extraterrestrial Environment; D006859:Hydrogen / Q000032:analysis / Q000737:chemistry; D007053:Ice; D018538:Jupiter; D010100:Oxygen / Q000032:analysis / Q000737:chemistry; D013227:Steam; D061351:Telescopes; D014867:Water / Q000032:analysis* / Q000737:chemistry
chemical_list: D007053:Ice; D013227:Steam; D014867:Water; D006859:Hydrogen; D010100:Oxygen
keywords: Europa; Hubble Space Telescope; Jupiter; aurora; water vapor plumes