Is there life beneath the ice? (Image: Dominic Fortes, Centre for Planetary Sciences, UCL/Birkbeck)

Alien hunters take note: a global water ocean potentially bigger than all those on Earth combined, is sloshing beneath Titan’s icy crust.

Combined with the cocktail of organic chemicals already known to exist on Titan, abundant water could make the moon prime real estate for life – though more work must be done to determine the exact state of the ocean.

Previous studies have painted Saturn’s largest moon as a trippy land, replete with lakes full of liquid methane and ethane. Theoretical models and measurements of the moon’s electric field have suggested there is liquid water beneath its icy surface, too, but the evidence was not conclusive.

Now Luciano Iess of Sapienza University of Rome in Italy and colleagues have analysed measurements of Titan’s gravity field taken by NASA’s Cassini spacecraft during its six fly-bys of the moon between 2006 and 2011.


Inescapable ocean

When the team looked at how Titan’s gravity – and shape – changes as it orbits Saturn, they found that gravitational tugs from the planet prompt Titan’s interior to deform in a way that suggests it has a tidal ocean beneath its icy crust – in a similar way that tugs from Earth’s moon cause tides in our seas.

The team calculated the liquid’s viscosity, and found it is consistent with water. “The conclusion that there is a liquid layer underneath the outer icy shell is almost inescapable,” says Iess.

The ocean probably covers most of Titan, the team’s calculations suggest. “This is likely to represent a habitat that is at least as large, if not larger, by volume than all the Earth’s oceans,” says Dominic Fortes, a planetary scientist at University College London, who was not involved in the study.

Fortes reckons that the ocean might contain organic molecules that are necessary for life. Titan’s surface is covered in hydrocarbons and there is growing evidence of subsurface hydrocarbon wells .

Rocky bottom

However, Jonathan Lunine, one of the study’s co-authors at Cornell University in Ithaca, New York, points out that the best current theories of how life forms require water to be in contact with a rocky surface.

Unlike on Earth, Titan’s ocean seems to completely surround the interior of the moon, so the only place this could happen is at the seabed. But Titan’s ocean may be so deep that the pressure at the bottom is sufficient for a layer of high-density ice to form between the liquid water and the moon’s rocky core, Lunine says, sealing off the water’s access to the rock.

Where does that leave Titan in the alien life stakes? Another of Saturn’s moons, Enceladus, and Jupiter’s moon Europa are thought to have sub-surface oceans too.

“Europa’s ocean is most likely to be underlain by rock, but unlike Titan we don’t know if there are organics in the Europan interior,” says Lunine. “Enceladus has organics and strong evidence for salty liquid water, but we don’t know how long-lived its pockets of liquid are. So Europa, Titan and Enceladus are all candidates [to host life] with different plusses and minuses.”

Journal reference: Science, DOI: 10.1126/science.1219631