NASA's Cassini probe, in orbit around Saturn, may have discovered evidence for a liquid ocean under the surface of Titan, Saturn's largest moon.

The data comes from radar observations of the surface that measure Titan's rotation and tell how it is oriented relative to the plane of its orbit - its axial tilt. According to a paper to be published in an upcoming issue of Astronomy and Astrophysics, the new data showed that the many of the planet's surface features were in the wrong place, sometimes off by as much as 30 kilometers (19 miles).

Titan always presents the same face toward Saturn, just like the Moon does to Earth. But in those situations one expects that the moon will be in the Cassini state, which means that the axial tilt will have a certain value. In Titan's case, the axial tilt was measured at 0.3 degrees. That seemed too high if one assumed Titan was a solid body.

A team at the Royal Observatory of Belgium in Brussels, led by Rose-Marie Baland, proposed that the tilt matches what one would see if Titan had a liquid ocean just under the surface. Given the measured mass of Titan and its density it looks like the ocean is made of water.

The researchers proposed a four-layer model for Titan, involving an icy shell, followed by the ocean, with a mantle of ice underneath that followed by core of rock and ice.

Most small worlds are solid all the way through because they cool fast enough that the insides don't stay molten. Titan is bigger than Mercury but still a good deal smaller than the Earth, which led many to believe it was probably a solid mass of ice and rock on the inside.

The presence of an ocean on Titan has important implications for the search for alien life. As far as anyone on Earth knows, liquid water is essential for life as we know it. If Baland and her team are correct there will be another place to look for it. Currently many scientists see Mars and Europa as two possible abodes for aliens (even if they are only bacteria) because both worlds have had liquid water in the past (Mars) or have it now (Europa, under the surface ice layer).

Baland and her co-authors say that there are some other possibilities besides a liquid ocean. One is the outer layers being denser than the interior. But that seems unlikely because one would ordinarily expect denser material to end up in the core. Another is that Titan was hit by something in the recent past. More observations will be needed to test those hypotheses.