Cascading parallel fractures on Enceladus

Abstract

Active eruptions from the south polar region of Saturn’s ~500-km-diameter moon Enceladus are concentrated along a series of lineaments known as the ‘tiger stripes’1,2, thought to be partially open fissures that connect to the liquid water ocean beneath the ice shell3,4. To date, no study simultaneously explains why the tiger stripes should be located only at the south pole, why there are multiple approximately parallel and regularly spaced fractures, what accounts for their spacing of about 35 km, and why similarly active fissures have not been observed on other icy bodies. Here we propose that secular cooling, which leads to a thickening of the ice shell and building of global tensile stresses5,6, causes the first fracture to form at one of the poles, where the ice shell is thinnest owing to tidal heating7. The tensile stresses are thereby relieved, preventing a similar failure at the opposite pole. The steadily erupting water ice loads the flanks of the open fissure, causing bending in the surrounding elastic plate and further tensile failure in bands parallel to the first fracture—a process that may be unique to Enceladus, where the gravity is too weak for compressive stresses to prevent fracture propagation through the thin ice shell. The sequence of fissures then cascades outwards until the loading becomes too weak or the background shell thickness becomes too great to permit through-going fractures.

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Fig. 1
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Data availability

All required data are available in the published literature as indicated.

Code availability

The computer code required to carry out the calculations discussed herein is available on request from the corresponding author.

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Acknowledgements

This work was made possible by the NASA/ESA Cassini mission to Saturn and, in particular, the work of the Imaging Science Subsystem team. D.J.H. was funded in part by the Miller Institute for Basic Research in Science at the University of California Berkeley and in part by the Carnegie Institution for Science in Washington DC. D.J.H. and M.M. acknowledge support from the Center for Integrative Planetary Science (CIPS) at the University of California Berkeley. M.M. was supported in part by NASA Solar System Workings grant 80NSSC19K0557. M.L.R. was supported in part by NSF DMS-1624776. We thank the CIDER working group, supported by NSF EAR-1135452, for early discussions that contributed to parts of this work. We thank M. Bland, R. Citron, J. Jordan, S. Kattenhorn, E. Kite and T. Mittal for helpful discussions.

Author information

M.M. developed the analytical equations for the secular cooling-induced tangential stresses in the ice shell. M.L.R. developed the analytical and boundary element models for crack penetration for the first fracture and carried out the related calculations. M.L.R. computed the relationship between turbulent dissipation in the fissure and the crack opening angle. D.J.H. proposed the mechanism of forming subsequent parallel fractures due to bending stresses and carried out the related calculations. D.J.H. drafted the manuscript with input from M.M. and M.L.R. All authors discussed the analysis and reviewed and commented on the manuscript.

Correspondence to Douglas J. Hemingway.

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Peer review information Nature Astronomy thanks Gaël Choblet and An Yin for their contribution to the peer review of this work.

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Hemingway, D.J., Rudolph, M.L. & Manga, M. Cascading parallel fractures on Enceladus. Nat Astron (2019). https://doi.org/10.1038/s41550-019-0958-x

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