Cassini Ion and Neutral Mass Spectrometer: Enceladus Plume Composition and Structure

• Published in: Science (American Association for the Advancement of Science) Vol. 311; no. 5766; pp. 1419 - 1422
• Main Authors: Waite, J. Hunter, Combi, Michael R, Ip, Wing-Huen, Cravens, Thomas E, McNutt, Ralph L, Kasprzak, Wayne, Yelle, Roger, Luhmann, Janet, Niemann, Hasso, Gell, David, Magee, Brian, Fletcher, Greg, Lunine, Jonathan, Tseng, Wei-Ling
• Format: Journal Article
• Language: English
• Published: United States American Association for the Advancement of Science 10-03-2006; The American Association for the Advancement of Science
• Subjects: Ammonia - analysis; Atmosphere; Carbon Dioxide - analysis; Density; Evolution, Planetary; Extraterrestrial Environment - chemistry; Gas density; Ionization; Mass Spectrometry; Methane - analysis; Molecules; Moons; Outgassing; Plumes; Saturn; Saturnian satellites; Scientific Concepts; Spacecraft; Stripes; Water - analysis; Water vapor
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.1121290

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.