Astronomers' mental image of Titan, the solar system's second-largest moon, used to be that of a vast swimming pool. But maybe they should have imagined a water bed instead.



Last year, researchers reported that radar mapping of Titan by the Cassini spacecraft had found a peculiar shift in landmarks on the moon's surface of up to 19 miles (30 kilometers) between October 2004 and May 2007.



Now investigators say the best explanation is a moon-wide underground ocean that disconnects Titan's icy crust from its rocky interior.



"We think the structure is about 100 kilometers of ice sitting atop a global layer of water … maybe hundreds of kilometers thick," says Cassini scientist Ralph Lorenz of Johns Hopkins University Applied Physics Laboratory in Laurel, Md.



If confirmed, Titan would be the fourth moon in the solar system thought to contain such an internal water ocean, joining Jupiter's satellites Ganymede, Callisto and Europa. Researchers believe that heat from radioactivity in a moon's core or gravitational squeezing may melt a layer of frozen water.



On Titan, Ganymede and Callisto, the liquid would become sandwiched between two different forms of ice, one that floats on water and one that sinks. Astronomers believe that of the four bodies, Europa has a larger and hotter core that directly borders its ocean, which lies beneath a thin layer of ice.



A hidden water layer would add to Titan's impressive resume: Larger in diameter than both Earth's moon and the planet Mercury, Titan is the only satellite in the solar system with a true atmosphere—a dense, rotating fog of nitrogen supporting hydrocarbon clouds made of methane and ethane.



For decades researchers suspected that its frosty surface temperature of around –290 degrees Fahrenheit (–180 degrees Celsius) would cause hydrocarbons to pool on its surface in a vast ocean. But during Cassini's first flyby in October 2004, its radar instruments detected no surface-spanning ocean, only methane lakes near the moon's north pole.



The shift in Titan's geologic features is strange because the moon is locked in a synchronous orbit around Saturn, meaning it always presents the same face to the planet. "It's a little bit improbable that Titan would be rotating asynchronously," Lorenz says.



Writing in Science, he and his colleagues instead connect the geologic displacement to models in which Titan's atmosphere pushes against mountains on the surface.



The exact thickness of the crust is an important component of the group's model of Titan but is not known precisely. Based on the dimensions of the Menrva impact crater, they estimate a thickness of about 60 miles (100 kilometers).



That would make the crust thinner than those of Ganymede or Callisto, where the oceans are thought to lie below as much as 125 miles (200 kilometers) of rock and ice. For Titan's presumed ocean to remain liquid at such a distance from the hot core, the researchers argue that it must contain ammonia.



There may also be other explanations for the observed shifting. In an editorial accompanying the report, planetary scientists Christophe Sotin of the Jet Propulsion Laboratory at the California Institute of Technology (Caltech), Gabriel Tobie of the University of Nantes in France, observe that a periodic wobble in Titan's rotation or, less likely, a recent asteroid impact could also explain the finding.



The ocean interpretation is still the most plausible one, according to David Stevenson, professor of planetary science at Caltech. "This is a perfectly natural thing to do in a water–ice dominated world, provided there is enough heat," he says.



What is less clear, he adds, is the ocean's depth. The movement of the crust likely depends on additional, poorly understood factors, such as seasonal weather patterns and gravitational attraction between the crust and the core, he says.



Luckily, the group's model is testable. It predicts a quickening of Titan's rotation rate in the coming year or two followed by a slowdown—something that can be measured on succeeding Cassini flybys.



As always, the possibility of water leads to talk of potential life. Researchers have speculated that Titan may have long ago harbored life or its building blocks, catalyzed by sunlight reacting with atmospheric carbon and hydrogen.



Experts have considered Europa a better candidate, however, because of the presumed contact between ocean and core, which would provide a steady supply of heat energy.



Lorenz and his colleagues note that Titan's ocean might be stirred instead by cryovolcanism or warmer (but still cold) water welling up from below. The addition of water, Lorenz says, makes Titan "astrobiologically very appealing."



Stevenson, for one, says he still sees Europa as a better bet for life. He agrees that Titan is an attractive natural laboratory for the kind of chemistry that would lead to life, but when it comes to energy sources, sizzling rock "is much better than ice."