Changes in the spin rate of Saturn's moon Titan suggest an ocean of liquid water lies beneath its icy surface, a new study reports. The finding bolsters the possibility that the moon might foster life.

Titan's low density suggests it is composed of a combination of water and rock. During the moon's early days, heat from its formation and the decay of radioactive material should have melted much of this water to create an ocean.

Much of the ocean would have since frozen. But scientists suspect a liquid layer up to 300 kilometres thick persists beneath an ice crust, probably aided by ammonia, which acts as an antifreeze.

Hard evidence for such an ocean has been difficult to come by, however. Apparent radio echoes observed by the Huygens probe as it landed on the moon's surface in 2005 might be due to radio waves reflecting off the top of an ocean. But it's possible they're simply an instrument error caused by motion of the lander's parachute.

Now, slight variations in Titan's rotation rate detected by the Cassini spacecraft have provided new evidence for an ocean, say Ralph Lorenz of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, US and colleagues. Watch an animation showing Titan's ocean.

Faster spin

Titan is close enough to Saturn that Saturn's gravity should distort the moon's shape, causing internal friction that slows down its rotation. This slowing continues until the moon rotates exactly once per orbit, keeping one face forever locked on the planet.

But when Cassini's radar tracked surface features on the moon, scientists found evidence that Titan rocks slightly due to tiny shifts in its rotation rate. Currently, Titan spins an extra 0.36° over the course of a year beyond what it would if it were in perfect sync with the moon. The moon's rotation rate also appears to be slowly increasing.

The rocking effect was actually predicted in 2005 as a result of changes in the direction of winds in Titan's massive atmosphere over the course of its 29.5-year orbit around the Sun. A similar effect is known to vary Earth's rotation rate.

Liquid layer

Importantly, the rocking is easier to produce if Titan's surface is simply a shell that floats on top of an ocean. The rotation of the relatively lightweight shell would be easily disturbed by changing winds. Alternatively, it would be much more difficult for winds to change Titan's rotation if the moon were solid from surface to core.

The observations weigh "in favour of a liquid layer, but it is not a definitive proof", Gabriel Tobie of the Université de Nantes in France, told New Scientist.

Tobie and Christophe Sotin of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, US, say changes in the orientation of the moon's rotation axis over thousands of years could also explain the observed shift.

But Bryan Stiles of JPL, who is a member of Lorenz's team, disagrees. He says the existing data "is sufficient to distinguish between movement of the pole and changes in spin rate" and that the team has indeed detected changes in spin rate.

Quick changes

Tobie says further observations could help settle the matter. If the spin rate changes on a timescale of a few years, that would bolster the case that the variations are produced by seasonal wind changes, rather than very long-term changes in the direction of Titan's rotation axis.

If Titan has an ocean beneath its surface, could life be present there? Possibly. Tobie says Titan may have provided especially good conditions for the development of life. Early in its history, liquid water may have been exposed to the surface, allowing complex carbon-containing molecules from the atmosphere to mix with the water.

"Organic [carbon-based] chemistry and warm water provide very good conditions for life to arise," Tobie says, although he adds that it might have been difficult for this life to survive after the ocean was cut off from the atmosphere by ice.

Other icy moons, including Jupiter's Europa and Saturn's Enceladus, may also harbour liquid water, making them potentially prime locations for life in the solar system.

Cassini: Mission to Saturn - Learn more in our continually updated special report.

Journal reference: Science (DOI: 10.1126/science.1151639)

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