NASA's Cassini spacecraft has come up with strng evidence that Saturn's moon Titan is hiding a layer of liquid water under its ice shell.
As the spacecraft made its flyby, researchers saw a large amount of squeezing and stretching as the moon orbited Saturn. But if Titan were composed entirely of stiff rock, they deduced, the bulges caused by the gravitational attraction of Saturn would be just three feet in height - not the 30 feet that Cassini observed.
"Cassini's detection of large tides on Titan leads to the almost inescapable conclusion that there is a hidden ocean at depth," says Luciano Iess of the Sapienza University in Rome.
"The search for water is an important goal in solar system exploration, and now we've spotted another place where it is abundant."
Titan takes only 16 days to orbit Saturn, and the scientists were able to study the moon's shape at different parts of its orbit. Because Titan isn't spherical but slightly elongated like a football, its long axis grew when it was closer to Saturn. Eight days later, when Titan was farther from Saturn, it became more nearly round.
Scientists weren't sure Cassini would be able to detect the bulges caused by Saturn's pull on Titan - but succeeded by measuring variations in the gravitational pull of Titan using data returned to NASA's Deep Space Network (DSN).
"We were making ultrasensitive measurements, and thankfully Cassini and the DSN were able to maintain a very stable link," says Sami Asmar of NASA's Jet Propulsion Laboratory.
"The tides on Titan pulled up by Saturn aren't huge compared to the pull the biggest planet, Jupiter, has on some of its moons. But, short of being able to drill on Titan's surface, the gravity measurements provide the best data we have of Titan's internal structure."
Because Titan's surface is mostly made of water ice, it's a reasonable assumption that the underground ocean is too. It wouldn't have to be huge or deep to create these tides.
Depending on what's under the ocean, it could be an encouraging indicator for life. Scientists think life is more likely to arise when liquid water is in contact with rock, and so far it's impossible to tell whether the ocean rests on rock or ice.
But there are implications for the mystery of methane replenishment on Titan.
"The presence of a liquid water layer in Titan is important because we want to understand how methane is stored in Titan's interior and how it may outgas to the surface," says Jonathan Lunine of Cornell University.
"This is important because everything that is unique about Titan derives from the presence of abundant methane, yet the methane in the atmosphere is unstable and will be destroyed on geologically short timescales."