title: Cassini ion and neutral mass spectrometer: Enceladus plume composition and structure.
abstract: The Cassini spacecraft passed within 168.2 kilometers of the surface above the southern hemisphere at 19:55:22 universal time coordinated on 14 July 2005 during its closest approach to Enceladus. Before and after this time, a substantial atmospheric plume and coma were observed, detectable in the Ion and Neutral Mass Spectrometer (INMS) data set out to a distance of over 4000 kilometers from Enceladus. INMS data indicate that the atmospheric plume and coma are dominated by water, with significant amounts of carbon dioxide, an unidentified species with a mass-to-charge ratio of 28 daltons (either carbon monoxide or molecular nitrogen), and methane. Trace quantities (<1%) of acetylene and propane also appear to be present. Ammonia is present at a level that does not exceed 0.5%. The radial and angular distributions of the gas density near the closest approach, as well as other independent evidence, suggest a significant contribution to the plume from a source centered near the south polar cap, as distinct from a separately measured more uniform and possibly global source observed on the outbound leg of the flyby.
authors.affiliation: Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, MI 48109, USA.
authors.forename: J Hunter
authors.initials: JH
authors.lastname: Waite
authors.forename: Michael R
authors.initials: MR
authors.lastname: Combi
authors.forename: Wing-Huen
authors.initials: WH
authors.lastname: Ip
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Cravens
authors.forename: Ralph L
authors.initials: RL
authors.lastname: McNutt
authors.forename: Wayne
authors.initials: W
authors.lastname: Kasprzak
authors.forename: Roger
authors.initials: R
authors.lastname: Yelle
authors.forename: Janet
authors.initials: J
authors.lastname: Luhmann
authors.forename: Hasso
authors.initials: H
authors.lastname: Niemann
authors.forename: David
authors.initials: D
authors.lastname: Gell
authors.forename: Brian
authors.initials: B
authors.lastname: Magee
authors.forename: Greg
authors.initials: G
authors.lastname: Fletcher
authors.forename: Jonathan
authors.initials: J
authors.lastname: Lunine
authors.forename: Wei-Ling
authors.initials: WL
authors.lastname: Tseng
mesh_terms: D000641:Ammonia / Q000032:analysis; D001272:Atmosphere; D002245:Carbon Dioxide / Q000032:analysis; D019419:Evolution, Planetary; D005118:Extraterrestrial Environment / Q000737:chemistry; D013058:Mass Spectrometry; D008697:Methane / Q000032:analysis; D018539:Saturn; D018531:Spacecraft; D014867:Water / Q000032:analysis
chemical_list: D014867:Water; D002245:Carbon Dioxide; D000641:Ammonia; D008697:Methane