No sodium in the vapour plumes of Enceladus (original) (raw)

Nature volume 459, pages 1102–1104 (2009)Cite this article

Abstract

The discovery of water vapour and ice particles erupting from Saturn’s moon Enceladus fuelled speculation that an internal ocean was the source1,2,3. Alternatively, the source might be ice warmed, melted or crushed by tectonic motions4. Sodium chloride (that is, salt) is expected to be present in a long-lived ocean in contact with a rocky core. Here we report a ground-based spectroscopic search for atomic sodium near Enceladus that places an upper limit on the mixing ratio in the vapour plumes orders of magnitude below the expected ocean salinity5. The low sodium content of escaping vapour, together with the small fraction of salt-bearing particles6, argues against a situation in which a near-surface geyser is fuelled by a salty ocean through cracks in the crust1. The lack of observable sodium in the vapour is consistent with a wide variety of alternative eruption sources, including a deep ocean6, a freshwater reservoir, or ice. The existing data may be insufficient to distinguish between these hypotheses.

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Figure 1: Observing geometry and corresponding sodium D-line spectrum of Enceladus and nearby E ring.

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Figure 2: Determining the 4 × 10 -7 upper limit for direct sodium ejection into a south-directed plume.

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Figure 3: Determining the 7 × 10 -6 upper limit for a molecular source, with panels displaying the same information as in Fig. 2 .

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Acknowledgements

Some of the data presented here were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the financial support of the W.M. Keck Foundation. We also acknowledge the Anglo-Australian Telescope and its staff. This work was supported by the National Science Foundation’s Planetary Astronomy Program and the NASA Postdoctoral Program. This paper has benefited from discussions with M. Zolotov, J. Spencer, C. Porco, T. Johnson, A. Ingersoll, W. McKinnon, C. Mackay, F. Postberg, J. Schmidt, S. Kempf and R. Pappalardo.

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Authors and Affiliations

  1. Laboratory for Atmospheric & Space Physics, University of Colorado, Boulder, Colorado 80309, USA ,
    Nicholas M. Schneider
  2. University of Maryland, Baltimore County, and NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, USA ,
    Matthew H. Burger
  3. Institute for Astronomy, University of Hawaii, Honolulu, Hawaii 96822, USA ,
    Emily L. Schaller
  4. Geological & Planetary Sciences, Caltech, Pasadena, California 91125, USA ,
    Michael E. Brown
  5. Engineering Physics, University of Virginia, Charlottesville, Virginia 22904, USA ,
    Robert E. Johnson
  6. Hydrology & Water Resources, University of Arizona, Tucson, Arizona 85721, USA ,
    Jeffrey S. Kargel
  7. Space & Atmospheric Physics, Imperial College, London SW7 2AZ, UK
    Michele K. Dougherty
  8. Physics & Astronomy, University College, London WC1E 6BT, UK
    Nicholas A. Achilleos

Authors

  1. Nicholas M. Schneider
  2. Matthew H. Burger
  3. Emily L. Schaller
  4. Michael E. Brown
  5. Robert E. Johnson
  6. Jeffrey S. Kargel
  7. Michele K. Dougherty
  8. Nicholas A. Achilleos

Corresponding author

Correspondence toNicholas M. Schneider.

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Schneider, N., Burger, M., Schaller, E. et al. No sodium in the vapour plumes of Enceladus.Nature 459, 1102–1104 (2009). https://doi.org/10.1038/nature08070

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Editorial Summary

An ocean on Enceladus ocean: the sodium test

Images from the Cassini spacecraft showed erupting plumes of water vapour and ice particles on Saturn's moon Enceladus, prompting speculation a subsurface ocean might be acting as a source of liquid water. Two groups this week report evidence relevant to the search for this subsurface ocean. The results, at first sight contradictory, leave the ocean a possibility, though still a hypothetical one. Postberg et al. used the Cassini Cosmic Dust Analyser to determine the chemical composition of ice grains in Saturn's E-ring, which consists largely of material from Enceladus. They find a population of E-ring grains rich in sodium salts, which should be possible only if the plumes originate from liquid water. Schneider et al. used Earth-based spectroscopic telescopes to search for sodium emission in the gas plumes erupting from Enceladus and found none. This is inconsistent with a direct supply from a salty ocean and suggests alternative eruption sources such as a deep ocean, a freshwater reservoir or ice. Or if there is a salty reservoir of water, some process not yet determined must be preventing the sodium from escaping into space.

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