Bad Astronomy

Do oceans turn under the face of Titan?

A startling result from the Cassini mission has just been announced: Titan, Saturn’s giant moon, may have an ocean of water and ammonia under its surface.

The Cassini probe does more than return hauntingly beautiful images. It is equipped with a sort of radar that allows it to map the topographical features of Saturn’s moons. This allows scientists to make accurate studies of the moons’ surfaces, including that of Titan. This in turn gives scientists an excellent set of landmarks, allowing them to study physical characteristics of the moons, including their rotation period.

Titan’s period is well-studied, and Cassini has visited Titan many times. Astronomers used the known landmarks and rotation period of Titan to predict what they will see with each visit… and they found landmarks like lakes and mountains that were far afield of where they were expected, as much as 30 kilometers (19 miles). A solid body will rotate as such, making these features very predictable. If the landmarks weren’t where they should be, then it must mean that Titan isn’t a solid body.

The more detailed story is that the crust, the surface layer of the moon must be decoupled, separate, from the interior. The only way for that to be is for there to exist a liquid layer between the surface and the core. Titan is far too cold and is comprised of the wrong material to have a hot mantle like the Earth does. Instead, scientists think it has an ocean 100 km below its frozen surface. Given the composition of the surface and the known density of Titan, they suspect it is made of liquid water and ammonia. The crust floats over this chilly liquid, and as winds blow on the surface the crust drifts, causing the predictions of landmark locations to be off.

The surface of Titan is loaded with what we call simple organic molecules: ethane, methane, and more. It is shocking, to say the least, to consider what would happen when you mix liquid water and organic compounds. Could there be life swimming deep under the surface of that planet-sized moon?

At the moment – and for the foreseeable future – there is no way to know. The ocean, if it exists, has not yet been confirmed, so we don’t want to put the cart before the horse. A lot of hard work lies ahead for those who study this distant world – seasonal variations in the positions of Titan’s landmarks would indicate that it is indeed the atmosphere blowing over the surface that is changing the rotation of the crust itself, and that in turn would give much credence to the idea of an underground ocean.

If this does pan out, then we will have to add yet another object in our solar system to the short list of worlds where liquid water can and does exist. And given the near-certainty of liquid water below the surface of Enceladus, Saturn will be able to proudly claim at least two them.