1. Planetary Geodynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
2. CRSR, Cornell University, Ithaca, New York 14853, USA
3. Raytheon, Woburn, Massachusetts 01801, USA
4. Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA
5. Institute for Geophysics and Planetary Physics, Scripps Institution of Oceanography, La Jolla, California 92093, USA

Correspondence to: T. A. Hurford¹ Correspondence and requests for materials should be addressed to T.A.H. (Email: hurfordt@core2.gsfc.nasa.gov).

In 2005, plumes were detected near the south polar region of Enceladus¹, a small icy satellite of Saturn. Observations of the south pole revealed large rifts in the crust, informally called 'tiger stripes', which exhibit higher temperatures than the surrounding terrain and are probably sources of the observed eruptions². Models of the ultimate interior source for the eruptions are under consideration^{1, 3, 4, 5}. Other models of an expanding plume⁶ require eruptions from discrete sources, as well as less voluminous eruptions from a more extended source, to match the observations. No physical mechanism that matches the observations has been identified to control these eruptions. Here we report a mechanism in which temporal variations in tidal stress open and close the tiger-stripe rifts, governing the timing of eruptions. During each orbit, every portion of each tiger stripe rift spends about half the time in tension, which allows the rift to open, exposing volatiles, and allowing eruptions. In a complementary process, periodic shear stress along the rifts also generates heat along their lengths^{7, 8, 9}, which has the capacity to enhance eruptions. Plume activity is expected to vary periodically, affecting the injection of material into Saturn's E ring¹⁰ and its formation, evolution and structure. Moreover, the stresses controlling eruptions imply that Enceladus' icy shell behaves as a thin elastic layer, perhaps only a few tens of kilometres thick.

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