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

J. Hunter Waite Jr., Michael R. Combi, Wing-Huen Ip, Thomas E. Cravens, Ralph L. McNutt Jr., Wayne Kasprzak, Roger Yelle, Janet Luhmann, Hasso Niemann, David Gell, Brian Magee, Greg Fletcher, Jonathan Lunine and Wei-Ling Tseng
Science
New Series, Vol. 311, No. 5766 (Mar. 10, 2006), pp. 1419-1422
Stable URL: http://www.jstor.org/stable/3845770
Page Count: 4
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Cassini Ion and Neutral Mass Spectrometer: Enceladus Plume Composition and Structure
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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.

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