Powering Enceladus’ plumes

Astronomers in 2005 were astonished to find that Saturn’s tiny, chilly moon Enceladus expels giant plumes of water vapor from an array of cracks marking its southern hemisphere. Because Enceladus is so small, researchers reasoned that it ought to have lost any interior source of heat long ago, and so should be frozen solid (SN: 5/6/06, p. 282).

In the May 17 Nature, two teams trace the origin of the plumes to the action of Saturn’s gravity. Francis Nimmo of the University of California, Santa Cruz and his colleagues calculate that gravitational stresses, or tides, generated by Saturn make the sides of the cracks on Enceladus rub back and forth. The resulting friction creates enough heat to vaporize ice and to power the plumes, the researchers say.

A second team, which includes Terry Hurford of NASA’s Goddard Space Flight Center in Greenbelt, Md., gives a more specific calculation of how Saturn’s gravity opens and closes the cracks during each of the moon’s 1.3-Earth-day orbits about the planet.

Because Enceladus follows a slightly elongated orbit, the tidal forces at its surface vary in strength as the moon’s distance from Saturn changes. In addition, the location on Enceladus’ surface where the tides are strongest varies throughout each orbit.

These two effects, acting in concert, force open most of the cracks when Enceladus is farthest from the planet, and close most of them when the moon is closest, the team’s calculations indicate.

Assuming that water vapor shoots out as soon as a crack opens, researchers can use the model to predict when each fracture is likely to erupt.