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Hansen, Candice J ¹ ; Esposito, Larry W ² ; Stewart, A Ian ³ ; Meinke, Bonnie K ⁴ ; Wallis, B ⁵ ; Colwell, Joshua E ⁶ ; Hendrix, Amanda R ⁷ ; , et al. ⁸

⁴ Presenting Author

¹ Jet Propulsion Laboratory/California Institute of Technology
² Laboratory for Atmospheric and Space Physics, University of Å·ÃÀ¿Ú±¬ÊÓƵ, Boulder
³ Laboratory for Atmospheric and Space Physics, University of Å·ÃÀ¿Ú±¬ÊÓƵ, Boulder
⁴ Laboratory for Atmospheric and Space Physics, University of Å·ÃÀ¿Ú±¬ÊÓƵ, Boulder
⁵ Jet Propulsion Laboratory/California Institute of Technology
⁶ Planetary Sciences Group, University of Central Florida
⁷ Jet Propulsion Laboratory/California Institute of Technology
⁸

In 2005, observations of Saturn’s moon Enceladus with the Cassini spacecraft revealed a plume of water vapor spewing from the moon’s south pole (Hansen et al., 2006). The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed an occultation of zeta Orionis by Enceladus’ plume in October 2007. This observation resulted in new data on the structure, composition, and water vapor flux of the plume. We detected four high-density gas jets in Enceladus’ water vapor plume that are consistent with the dust jets’ locations as observed in visible images (Spitale and Porco, 2007). Distinct separation of the two strongest absorption features due to the jets in the occultation profile constrains the ratio of the jets’ thermal velocity to vertical velocity to 0.65 +/- 0.1, which means the jets escape the ground at supersonic speeds. In addition, data from the 2007 observation sets a maximum water column density in the plume at ~3 x 10¹⁶ cm^-2, approximately twice the density observed by UVIS in 2005 at the same altitude in the plume (15 km). The comparison of densities in 2005 and 2007 does not agree with the model of tidally-driven compression and tension controlling plume activity-level by opening and closing fissures (Hurford et al., 2007). Another significant result from this observation is the composition of the plume, which reflects the geochemistry of Enceladus’ interior. UVIS constrained the Ion and Neutral Mass Spectrometer (INMS) detection of a species with a mass of 28 amu (Spencer et al, 2006), which could be N₂ or CO. UVIS did not observe absorption due to CO in 2005 or 2007, excluding 3% CO in the plume at the 2-sigma level.

Hansen, C. J. et al., 2006, Enceladus' water vapor plume: Science, v. 311, p. 1423–1425.

Hurford, T. A., Helfenstein, P., Hoppa, B. V., Greenberg, R. & Bills, B., 2007, Eruptions arising from tidally controlled periodic openings of rifts on Enceladus: Nature, v. 447, p. 292–294.

Spencer, J. R. et al., 2006, Cassini encounters Enceladus: Background and the discovery of a south polar hot spot: Science, v. 311, p. 1401–1405.

Spitale, J. N. & Porco, C. C., 2007, Association of the jets of Enceladus with the warmest regions on its south-polar fractures: Nature, v. 449, p. 695–697.