Titan’s tides point to a hidden ocean
4 July 2012
Earth is often known as the blue planet, because it is the only world in our Solar System which is largely covered by water. However, a number of satellites, including Saturn’s giant moon Titan, are thought to have oceans beneath their surfaces. The clue to the existence of Titan’s subsurface ocean comes from the discovery of large tides.
Anyone who visits the coast will be familiar with tides - the rise and fall in sea level twice each day. Less obvious are the tides which cause our planet's crust to rise and fall a few tens of centimetres. These tides are caused by the combined gravitational pull of the Moon and Sun on the Earth. Tides on Titan are created in a similar way. However, this time it is the pull of Saturn which causes the moon to change shape during its 16 day orbit of the planet.
Titan's shape-shifting has been detected by carefully tracking the path of the Cassini spacecraft when it flies close to the moon. Differences in the way that Titan tugged on Cassini showed that the moon’s gravitational pull – and therefore its shape – had altered very slightly. At its furthest point, when Saturn's pull is weaker, Titan is almost spherical, like a football. When the satellite is closest to Saturn, it is stretched so that it becomes more like a rugby ball.
The resulting tides cause Titan’s surface to rise and fall by more than 10 metres. However, this variation is much larger than would be expected if the moon was solid all the way through. This indicates that it has a subsurface ocean, possibly 250 km deep, beneath an icy shell some 50 km thick.