By Richard A. Lovett of Nature magazine
Evidence is growing in support of the idea that liquid water lies concealed beneath the surface of Saturn's icy moon Enceladus.
Some scientists hope that the characteristic plumes of ice crystals seen erupting from the moon's surface are geyser-like features fed by an underground water source, with all that would imply about Enceladus as a possible abode for life. Others argue that the crystals could be formed by "dry" processes such as the breakdown of clathrates (which combine ice and trapped gases such as methane) or from the sublimation of subsurface ice layers that never pass through a liquid phase in the transition from ice to gas.
Many of the latest findings, announced yesterday at a meeting of NASA's Enceladus Focus Group at the SETI Institute in Mountain View, Calif., reveal signs of liquid water.
One such sign is that the plumes seem to be made up of about equal parts ice and gas, says Andrew Ingersoll, a planetary scientist at the California Institute of Technology in Pasadena.
"It's hard to get solid-to-gas ratios of more than one per cent if all the particles are forming from a vapor," he said. "I think we should be thinking about the possibility that you throw out a big, heavy blob of liquid and it reaches the vacuum of space and explodes into a cloud of smaller particles."
Another sign that liquid water is present is the chemistry of the materials being emitted by the plumes. Frank Postberg, a physicist at Heidelberg University in Germany, has noted, for example, that ice crystals in Saturn's E ring that originated on Enceladus are rich in sodium, which wouldn't be possible if they had been emitted as a vapor that subsequently re-condensed into ice.
However, not all of the chemical evidence points to liquid water. Hunter Waite, a space scientist at the Southwest Research Institute in San Antonio, Texas, noted that the chemical composition of the plumes, as determined by the ion neutral mass spectrometer on the Cassini spacecraft, reveals some compounds whose presence isn't consistent with a liquid source. One such example is hydrogen cyanide, which, if it had ever met with liquid water, should have reacted with other compounds in it to produce other compounds that have not yet been found in plumes.
"There is no clear pattern here," said Waite. "There are some things that are consistent with solubility in water and other things that don't make sense in that regard."
One possibility, he said, is that chemicals in the plumes might come from multiple processes all happening at once.
Whatever their cause, it seems that the plumes have been active for a long time. Ice crystals falling back to the moon's surface have piled into drifts around 125 meters thick, according to Paul Schenk, a planetary scientist at the Lunar and Planetary Institute in Houston, Texas. The falling ice accumulates at a rate of about 1 millimeter every 1,500 years, he says, so the plumes must have been around for tens of millions of years.
Schenk based this conclusion on high-resolution photos that show a region just north of the geyser zone whose topography is muted into rounded contours indicative of deep snow.
"It's different from what you see in other areas that have been photographed at high resolution. Those had incredible detail everywhere you look. This is much smoother," says Schenk.
Other scientists at the meeting received this news with excitement. "This is a whole new order of constraints for how long the plumes have been active," says Postberg. Until now, he said, all that had been known was that they had been active for long enough for escaping ice grains to form Saturn's E ring--a process that would have taken between a few hundred and one thousand years.
"That's a big step from a thousand to ten million years," he said.
The source of the energy driving the plumes remains unknown. According to the latest estimates, the plumes and related hot spots are currently radiating 16 gigawatts of energy. But Enceladus receives only a fraction of that from the combination of radioactive decay and tidal flexing from Saturn's powerful gravity, said Francis Nimmo, a planetary scientist at the University of California, Santa Cruz.
As a result, most of the scientists at the meeting believe that, however many million years they may have been around for, the plumes don't operate continuously, instead turning on and off at intervals.
An even tougher question is whether Enceladus produces enough heat to maintain liquid water beneath its surface.
Nimmo doesn't think that there is enough heat to maintain a layer of water beneath the ice moonwide. But if all of the tidal heating were to be focused in one zone, there would be enough heat, he says, for a large regional ocean to exist indefinitely beneath the jets. And once such an ocean forms, he says, it concentrates tidal heating, and thereby becomes self-perpetuating.
This article is reproduced with permission from the magazine Nature. The article was first published on May 24, 2011.