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

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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Figure 1: Spectra of E-ring particles with differing Na/H 2 O ratio.
Figure 2: Reproduction of CDA measurements in the laboratory by laser dispersion 19 of salt water.
Figure 3: Schematic of how liquid dispersion and condensation from vapour form ice particles with very different Na content.
Figure 4: Equilibrium concentrations of NaCl in the vapour phase above a subsurface Enceladus water reservoir.

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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.

Author information

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

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Correspondence to F. 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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