Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus (original) (raw)

Nature volume 459, pages 1098–1101 (2009)Cite this article

Abstract

Saturn's moon Enceladus emits plumes of water vapour and ice particles from fractures near its south pole1,2,3,4,5, suggesting the possibility of a subsurface ocean5,6,7. These plume particles are the dominant source of Saturn’s E ring7,8. A previous in situ analysis9 of these particles concluded that the minor organic or siliceous components, identified in many ice grains, could be evidence for interaction between Enceladus’ rocky core and liquid water9,10. It was not clear, however, whether the liquid is still present today or whether it has frozen. Here we report the identification of a population of E-ring grains that are rich in sodium salts (∼0.5–2% by mass), which can arise only if the plumes originate from liquid water. The abundance of various salt components in these particles, as well as the inferred basic pH, exhibit a compelling similarity to the predicted composition of a subsurface Enceladus ocean in contact with its rock core11. The plume vapour is expected to be free of atomic sodium. Thus, the absence of sodium from optical spectra12 is in good agreement with our results. In the E ring the upper limit for spectroscopy12 is insufficiently sensitive to detect the concentrations we found.

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Acknowledgements

We thank U. Beckmann, M. Burger, M. Burton, M. Gellert, E. Grün, J. K. Hillier, N. Schneider, F. Spahn, and M. Zolotov for discussions. We acknowledge the efforts of the Cassini team and JPL. The work has been supported by the DLR, the DFG and the Frontier programme of the University of Heidelberg.

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Authors and Affiliations

  1. Institut für Geowissenschaften, Universität Heidelberg, 69120 Heidelberg, Germany
    F. Postberg
  2. Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany
    F. Postberg, S. Kempf & R. Srama
  3. IGEP,Technische Universität Braunschweig, 38106 Braunschweig, Germany
    S. Kempf
  4. Nichtlineare Dynamik, Universität Potsdam, 14476 Potsdam–Golm, Germany
    J. Schmidt
  5. Department of Mathematics, University of Leicester, Leicester LEI 7RH, UK
    N. Brilliantov
  6. Department of Physics, Moscow State University, 119991 Moscow, Russia
    N. Brilliantov
  7. Institut für Physikalische Chemie, Universität Göttingen, 37077 Göttingen, Germany
    A. Beinsen & B. Abel
  8. Wilhelm-Oswald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, 04103 Leipzig, Germany
    B. Abel
  9. Max-Planck-Institut für Dynamik und Selbstorganisation, 37073 Göttingen, Germany
    U. Buck

Authors

  1. F. Postberg
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  2. S. Kempf
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  3. J. Schmidt
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  4. N. Brilliantov
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  5. A. Beinsen
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  6. B. Abel
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  7. U. Buck
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  8. R. Srama
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Corresponding author

Correspondence toF. Postberg.

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Postberg, F., Kempf, S., Schmidt, J. et al. Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus.Nature 459, 1098–1101 (2009). https://doi.org/10.1038/nature08046

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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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