1. Laboratory for Atmospheric & Space Physics, University of Colorado, Boulder, Colorado 80309, USA
2. University of Maryland, Baltimore County, and NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
3. Institute for Astronomy, University of Hawaii, Honolulu, Hawaii 96822, USA
4. Geological & Planetary Sciences, Caltech, Pasadena, California 91125, USA
5. Engineering Physics, University of Virginia, Charlottesville, Virginia 22904, USA
6. Hydrology & Water Resources, University of Arizona, Tucson, Arizona 85721, USA
7. Space & Atmospheric Physics, Imperial College, London SW7 2AZ, UK
8. Physics & Astronomy, University College, London WC1E 6BT, UK

Correspondence to: Nicholas M. Schneider¹ Correspondence and requests for materials should be addressed to N.S. (Email: nick.schneider@lasp.colorado.edu).

Abstract

The discovery of water vapour and ice particles erupting from Saturn's moon Enceladus fuelled speculation that an internal ocean was the source^{1, 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.

1. Laboratory for Atmospheric & Space Physics, University of Colorado, Boulder, Colorado 80309, USA
2. University of Maryland, Baltimore County, and NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
3. Institute for Astronomy, University of Hawaii, Honolulu, Hawaii 96822, USA
4. Geological & Planetary Sciences, Caltech, Pasadena, California 91125, USA
5. Engineering Physics, University of Virginia, Charlottesville, Virginia 22904, USA
6. Hydrology & Water Resources, University of Arizona, Tucson, Arizona 85721, USA
7. Space & Atmospheric Physics, Imperial College, London SW7 2AZ, UK
8. Physics & Astronomy, University College, London WC1E 6BT, UK

Correspondence to: Nicholas M. Schneider¹ Correspondence and requests for materials should be addressed to N.S. (Email: nick.schneider@lasp.colorado.edu).

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