We may be able to add another item to the growing list of bodies in our solar system that contain significant amounts of liquid. Jupiter's moon Europa has long been suspected of harboring a subsurface water ocean, but recent studies have suggested that a large part of the interior of its moon Io is also molten (although that's molten rock). Further out, Saturn's moon Titan appears to have a subsurface ocean that allows its surface to move somewhat independently of its core. Now, researchers have found evidence that Saturn's moon Enceladus needs to be added to this list.

Enceladus was a pretty obvious candidate for containing liquid, since the Cassini mission discovered that the small, icy moon shot dramatic geysers out of a series of fissures called the tiger stripes. Early models suggested these were released by a reservoir of liquid that is maintained in a molten state by the gravitational forces acting on the moon. Still, alternative models had suggested ways for jets of gas and icy particles might be liberated directly from water ice by the moon, so the geysers haven't been viewed as conclusive evidence for liquid water.

To help settle the dispute, the people running Cassini arranged a close flyby of the geysers. Very close. The orbiter skimmed within 21km of the moon's surface on one pass (that's possible due to the moon's very small mass), and passed through the periphery of one of the plumes itself. That allowed the orbiter's Cosmic Dust Analyser to capture some of the particles being released by Enceladus, and get a measure of their mass and composition.

These particles collect in Saturn's E Ring, which appears to be nearly pure water ice. But, closer to the moon, the particles were on average a bit larger, and rich in sodium and potassium salts (up to two percent salt). Although these salty particles account for only about five percent of the E-ring, during the flyby, levels reached over 40 percent of the plume. The presence of that much salt is rather significant, as the authors write, "A plume source dominated by salt-rich grains, as reported here, eliminates the possibility of significant contributions from dry, sodium-poor sources and severely constrains or rules out non-liquid models in their present form."

One of the problems for non-liquid explanations is how all that salt got there in the first place. It must have been dissolved by liquid water, but a slow freezing process like one would expect to happen during the moon's formation would have forced it out of solution and into the ice. Assuming it could get in the ice, then any form of sublimation—ice being heated directly into a gas—should cause sodium to appear in the E Ring in gaseous form. It doesn't. So, all indications are that there's a reservoir of salty, liquid water lurking under the surface of Enceladus.

Nature, 2011. DOI: 10.1038/nature10175 (About DOIs).

Listing image by Image: NASA