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Timing of water plume eruptions on Enceladus explained by interior viscosity structure

Abstract

At the south pole of Saturn’s icy moon Enceladus, eruptions of water vapour and ice emanate from warm tectonic ridges1,2,3,4. Observations in the infrared5 and visible6 spectra have shown an orbital modulation of the plume brightness, which suggests that the eruption activity is influenced by tidal forces. However, the observed activity seems to be delayed by several hours with respect to predictions based on simple tidal models6,7. Here we simulate the viscoelastic tidal response of Enceladus with a full three-dimensional numerical model8,9 and show that the delay in eruption activity may be a natural consequence of the viscosity structure in the south-polar region and the size of the putative subsurface ocean. By systematically comparing simulations of variations in normal stress along faults with plume brightness data, we show that the observed activity is reproduced for two classes of interior models with contrasting thermal histories: a low-viscosity convective region above a polar sea extending about 45°–60° from the south pole at a depth below the surface as small as 30 km, or a convecting ice shell of 60–70 km in thickness above a global ocean. Our analysis further shows that the eruption activity is controlled by the average normal stress applied across the cracks, thus providing a constraint on the eruption mechanism.

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Figure 1: Interior structure and predicted tensile stress along the south-polar faults.
Figure 2: Predicted tidally induced activity along the south-polar faults.
Figure 3: Plume brightness variations and predicted fault activity.
Figure 4: Successful models for the NAS representation as a function of ice shell thickness, ocean angular width and minimum ice viscosity.

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Acknowledgements

The research leading to these results has received financial support from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013 Grant Agreement no. 259285) and from the Czech Science Foundation (project no. 14-04145S). C.P. acknowledges support from NASA CDAP, and F.N. from the CDAP-PS programme. The computations were carried out using CCIPL computational facilities (France) and IT4Innovations Centre (Excellence project CZ.1.05/1.1.00/02.0070, project Large Research, Development and Innovations Infrastructures no. LM2011033, Czech Republic).

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All authors contributed to the data analysis, the results discussion, and the paper writing. M.B. carried out the numerical calculations. O.Č. and M.B. contributed to the code development.

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Correspondence to Marie Běhounková.

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The authors declare no competing financial interests.

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Běhounková, M., Tobie, G., Čadek, O. et al. Timing of water plume eruptions on Enceladus explained by interior viscosity structure. Nature Geosci 8, 601–604 (2015). https://doi.org/10.1038/ngeo2475

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