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Science 10 March 2006:
Vol. 311. no. 5766, pp. 1419 - 1422
DOI: 10.1126/science.1121290
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Reports

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

J. Hunter Waite, Jr.,1 Michael R. Combi,1 Wing-Huen Ip,2 Thomas E. Cravens,3 Ralph L. McNutt, Jr.,4 Wayne Kasprzak,5 Roger Yelle,6 Janet Luhmann,7 Hasso Niemann,5 David Gell,1 Brian Magee,1 Greg Fletcher,1 Jonathan Lunine,6,8 Wei-Ling Tseng2

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.

1 Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, MI 48109, USA.
2 Institutes of Astronomy and Space Science, National Central University, Chung Li 32054, Taiwan.
3 Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA.
4 Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA.
5 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
6 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA.
7 Space Science Laboratory, University of California, Berkeley, CA 94720, USA.
8 Istituto di Fisica dello Spazio Interplanetario, Via del Fosso del Cavaliere 100, 00133 Rome, Italy.

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