Published online 3 October 2011 | Nature | doi:10.1038/news.2011.569

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Millions of years of snowfall on Enceladus boost promise of subsurface ocean.

A 100-metre thick layer of snow has muted many of the surface features in this area of Enceladus. NASA/JPL / Paul Schenk

Jets of water vapour and ice shooting from the south pole of Saturn's moon Enceladus have been active for up to 100 million years, boosting the odds that the moon harbours a liquid ocean beneath its icy surface, a study suggests. If the existence of such an ocean is confirmed, Enceladus will become one of the most promising places in the Solar System in which to search for signs of extraterrestrial life.

Paul Schenk, a planetary scientist at the Lunar and Planetary Institute in Houston, Texas, and his colleagues based their findings on high-resolution images of Enceladus recorded by the Cassini spacecraft, combined with a model of jet activity. Schenk described the study at a joint meeting of the European Planetary Science Congress and the American Astronomical Society's Division for Planetary Sciences1 in Nantes, France, on 3 October.

The Cassini images depict a blanket of snow-like material that has all but erased old surface fractures and rounded the sharp edges of canyon rims on some parts of Enceladus, much as a snowfall on Earth mutes the shapes of street kerbs and potholes.

The blanketing material is in two strips running from south to north on opposite sides of the moon, and its bluish colour is a perfect match for the ice particles from the plumes that are predicted to fall as snow on Enceladus. This is only a fraction of the plume material; most of the ice goes to feed the outermost of Saturn's rings, the E ring, as described by Sascha Kempf, a planetary scientist at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, and Jürgen Schmidt, a researcher in celestial mechanics at the University of Potsdam, Germany, in Icarus last year2.

The fine powder of ice crystals falls on Enceladus at a wimpy average rate of less than one thousandth of a millimetre per year, according to Kempf and Schmidt's calculations. Yet when Schenk and his colleagues examined high-resolution stereo images, they found that in one region near the moon's south pole, the blanket of snow is about 100 metres thick. That means that the plumes must have been active for tens of millions of years.

Liquid assets

Before Schenk's study, researchers interested in the lifetime of the jets had only a lower limit of hundreds of years, based on the extent of the E ring and the distribution of ice particles within it, says Frank Postberg, a planetary scientist at the University of Stuttgart in Germany, who has modelled how the Enceladus jets feed the E ring. "This is the first solid evidence for long-lived plume activity on Enceladus," he says.

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The extended lifetime of the jets, which shoot out of warm vents at the south polar region, indicates that the heat source driving them must have also been around for a while. "A long-lived heat source could make it easier to maintain a liquid layer inside the moon", from which water could be forced out through cracks in the surface ice as plumes, says Schenk. Kempf, Schmidt and their colleagues have previously reported evidence for a salty ocean beneath the moon's surface3,4.

John Spencer, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, says that the deep snow deposits don't necessarily mean that there is an ocean inside the moon, but "a long history of activity does make long-term persistence of water more likely, and that has to be good news for any hypothetical organisms out there".