Enceladus, one of Saturn's moons, was placed firmly on planetary scientists' research agenda after the intriguing insights of the Cassini spacecraft in 2005. The diameter of this little moon is 505 km — no larger than the distance between Madrid and Barcelona or Houston and New Orleans — but Enceladus beats most of the other Saturnian moons in terms of activity: plumes of water vapour that have been compared to the Old Faithful geyser at Yellowstone are expelled from four 'tiger-stripes'; long trenches that cross the moon's south pole.

One attractive explanation for the variability of these water vapour eruptions links them to the variation in magnitude of tidal stresses at the moon's surface as it travels along its eccentric orbit. According to this idea, the tiger stripes open under the tension experienced in some parts of the orbit and close when compressed elsewhere, thus regulating the expulsion of water.

Credit: NASA/JPL/SPACE SCIENCE INSTITUTE

But Enceladean cryovolcanism is not so straightforward, report Candice Hansen, of the Jet Propulsion Laboratory, and colleagues (Nature 456, 477–479, 2008). The researchers took advantage of the occultation of a star by the water plume on 24 October 2007 to estimate the density of the jets, thereby providing a measurement subsequent to the 2005 Cassini fly-by. Contrary to the tidal stress model, which predicted a weaker plume, the water column density in the 2007 plume was about twice as large as that observed in 2005.

Although the mechanism proposed originally does not therefore appear to hold, it may just need refining: not only does the magnitude of the tidal stresses vary throughout Enceladus' eccentric orbit, but the longitude of the tidal bulge also oscillates. A more sophisticated model that accounts for this additional effect may better explain when, and how vigorously, Enceladus expels its water jets.