By studying the way Saturn's moon Titan squeezes and stretches as it orbits the ringed planet, researchers using data from NASA's Cassini orbiter have determined that a layer of liquid water likely exists beneath the moon's frozen exterior.
Titan is the largest of Saturn's 53 known moons and is larger than the planet Mercury. Since 2004, the Cassini mission has shown that below Titan's thick, hazy atmosphere, complex meteorological processes are taking place, endowing the moon with lakes and rivers of methane, and windblown dunes that shift over time.
Now, in a paper that appears in the advance online version of the journal Science, a team of scientists—including Caltech planetary scientist David Stevenson—describes bulges, or solid "tides," produced by Saturn's gravitational tug on Titan. The measurements, gathered over the course of six close flybys of Titan between 2006 and 2011, have helped reveal the moon's internal structure.
From the data, the scientists determined that the moon could not be made entirely of solid rock. If it were, Saturn's gravitational pull would produce tides only about three feet (one meter) in height. Instead, the team measured solid tides that were more like 30 feet (10 meters) in height—suggesting a liquid layer at depth. And since the moon's surface is mostly water ice, that layer is most likely liquid water.
"We have long suspected that Titan, like Jupiter's moons Ganymede and Callisto, has a subsurface water-rich ocean," says Stevenson, the Marvin L. Goldberger Professor of Planetary Science. "Our results now show that there is very likely to be such an ocean."
In a NASA press release, the lead author of the paper, Luciano Iess of the Sapienza University of Rome, Italy, adds, "The search for water is an important goal in solar system exploration, and now we've spotted another place where it is abundant."