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

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.

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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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