On Saturn's moons, some unexpected hints of water

Cassini flyby reveals water vapor in plume from Enceladus. A hidden ocean on Titan?

from the March 27, 2008 edition

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Some models had suggested that this might be the case. Still, finding evidence for it "is an amazing result," says Spencer.

Using Cassini's radar to map Titan's surface, a team led by Ralph Lorenz at the Johns Hopkins Applied Physics Laboratory plotted prominent surface features – dunes of organic "sand," lakes, river channels, and mountains. Surprisingly, during 19 passes between October 2005 and May 2007, they found that some 50 surface features had shifted from their initial positions by up to 19 miles.

The only way to explain the shift at the moment is through the presence of a liquid ocean between the crust and the core, with the crust's drift driven by winds in Titan's dense atmosphere.

If further flybys confirm the results, Titan would join Jupiter's moons Europa, Ganymede, and Callisto on the small, but apparently growing, list of moons in the solar system hosting oceans deep beneath their surfaces. The results appear in the current issue of the journal Science.

Why Titan would have a sustained ocean that hasn't frozen solid over the eons, given the icy temperatures, is another intriguing question, notes Jeffrey Kargel, a planetary scientist at the University of Arizona in Tucson.

He notes that Titan's heat source is different, and, because of the moon's size, much weaker, than Enceladus's. One explanation: Titan's ocean is laced with ammonia, which acts as a natural antifreeze.

Indeed, says Dr. Kargel, one of the features he was hoping to see in the data from Enceladus's plume was some sign of ammonia. It could imply that ammonia was a feature of the chunks of primitive planetary material that formed the moons.

Yet no ammonia has appeared in the Cassini data from Enceladus so far.

Others caution that it's still a bit early to attempt to extrapolate conclusions from one moon to another. "We're seeing so little of Enceladus," says Bruce Jakosky, a researcher at the University of Colorado at Boulder.

Returning with robots

Seven more flybys are planned through the end of 2010, including three more this year. Even so, he says, it will still be a long, hard slog to figure out what the moon is really revealing about itself, let alone what it may or may not reveal about any of its Saturnian siblings.

Which is why some researchers are keen to return with robots that can explore these moons' surfaces. Even without them, researchers anticipate more eyepopping moments during Cassini's remaining tour. "Surprises are coming fast and furious in this business," says Kargel.

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