Saturn's moon Enceladus is a relatively small body, only a bit over 500km across. That's not big enough to have retained much heat from its formation, nor to have a huge cache of radioactive material that can provide heat. Yet all indications are that the moon has an extensive under-surface ocean, which fuels geysers near the moon's south pole. Thermal imaging suggests that there are Gigawatts' worth of heat being released in the area around the geysers.

All of which should be unsustainable. Most of the heat inside Enceladus must be produced by tidal forces, which deform the moon over the course of its orbit, creating internal friction. And there's no indication that these can generate sufficient heat. This implies that the geysers, and the E-ring of Saturn that they create, are a very temporary phenomenon, and we're lucky to have sent Cassini there while the geysers were active. But that may not be the case. Some scientists are now suggesting that Enceladus may be relatively young, and a separate study is saying that the geysers may be stable for up to a million years.

The new study is based on an attempt to create a physical model of Enceladus' plumes. These originate in a series of fissures known as the tiger stripes, shown on the left side of the moon in the image above. Together, these fissures add up to roughly 500km of active venting.

The authors model these stripes as a series of rectangular slots, each 120km long, that go through the surface of a 35km-deep icy shell, with liquid water below. The whole system is subjected to tidal stresses, which regularly raise the pressure exerted by the water below and change the width of the slots themselves.

The key feature to get the model to work is the width of the slots (or at least the width they adopt when free of tidal stresses). If they're less than a half-meter across, they'll rapidly freeze shut. Wider than 2.5m, and ice starts to freeze out on the walls, narrowing them back down again.

At this width, the water becomes very turbulent as the pressure and width of the slots change. This both releases heat through friction and prevents any of the slots from icing over. If water within the slots gets cool enough, the mixing will cause it to drop into the warmer ocean below, ensuring that the water in the slots maintains a relatively stable temperature.

The model successfully recreates a key aspect of the geysers' behavior: the peak of erupted material occurs about five hours after tidal forces draw the tiger stripes to their maximum width. In the model, this occurs simply because the turbulent flow slows down the refilling of the slots with water as the tidal forces shift. Previously, this behavior was unexplained.

One thing it doesn't do successfully, however, is capture the five-fold difference in the amount of material that's erupting at different points in the orbital cycle. But the authors suggest a number of ways this might occur, such as the existence of narrow choke points in the slots or interactions with particles that have already been ejected. In fact, they're hoping that this first attempt serves as the basis for elaborating a more physically realistic model of Enceladus.

But the key feature of this model may be its longevity. In it, the power balances out, in part because the heat needed for ice melting is offset by force generated as the local surface subsides. This can keep things stable for periods of at least a million years, according to the authors' calculations. Which may mean that Cassini wasn't just lucky to see the spectacular displays at Enceladus.

PNAS, 2016. DOI: 10.1073/pnas.1520507113 (About DOIs).