NASA's conclusion is based on the extent to which Titan changes shape as it orbits Saturn. Titan elongates with proximity to the gas giant, and reverts to a more spherical form when farther away. But it is the extent of this "bulging" that has surprised NASA scientists.
According to their calculations, Cassini scientists claim that, were Titan to be composed solely of rock, the bulges would be 3 feet (1 meter) tall. But according to the Cassini data, the bulges are in fact 30 feet (about 10 meters) in height.
"Cassini's detection of large tides on Titan leads to the almost inescapable conclusion that there is a hidden ocean at depth," said Luciano less, a member of the Cassini team at the Sapienza University of Rome. The ocean is not necessarily deep since even a relatively thin liquid layer between the ice shell and the planet's mantle would be sufficient to allow the bulging (or solid tides).
The method used to calculate the bulging is in fact rather indirect, and relies on observation of Cassini itself rather than Titan. Between 2006 and 2011 Cassini undertook six close flybys of Titan, and it's the close monitoring of changes in Cassini's acceleration, reported back to NASA's Deep Space Network (DSN), that allow these conclusions to be drawn.
"We were making ultra-sensitive measurements, and thankfully Cassini and the DSN were able to maintain a very stable link," said Sami Asmar of the Cassini team at NASA's Jet Propulsion Laboratory.
It's thought that the liquid ocean, if it contains ammonia, could explain the presence of methane in Titan's atmosphere, as ammonia-water at the surface could free up methane contained in the ice crust. "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," explained Jonathan Lunine, another Cassini team member, based at Cornell University.
However, NASA claims that a liquid water ocean can not necessarily be interpreted as an indicator of the presence of life as it's thought that this would require water to be in contact with rock. Titan's ocean bed may instead be composed of ice.
The team's findings appeared in Science Magazine on Thursday in a report titled The Tides of Titan.