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Evidence comes from careful observations of Titan's orbit and rotation. This indicates that Titan has an orbit similar to our Moon's: it always presents the same face towards Saturn and its axis of rotation tilts by about 0.3 degrees.
Together, these data allow astronomers to work out Titan's moment of inertia and this throws up something interesting. The numbers indicate that Titan's moment of inertia can only be explained if it is a solid body that is denser near the surface than it is at its centre.
That's just plain weird--unthinkable really, given what we know about how planets and moons form.
But there is another explanation, however: that Titan isn't solid at all.
Titan's moment of inertia could well be explained by the presence of liquid ocean beneath an icy shell.
The chemistry of the ocean is an important factor in calculating its depth and how thickly it can be covered in ice. Baland and co assume that it must consist of water. That seems a curious assumption given that Titan's atmosphere is packed full of methane and other hydrocarbons.
Astronomers have long known that methane is quickly broken down by sunlight. So Titan's ought to have long since have disappeared...unless it is being refilled from an internal reservoir. A huge underground ocean of methane, perhaps?
A methane ocean would require Baland and co to look at their calculations again to see what the mechanical and thermodynamic relationship would be between methane ice and liquid. So there could be some interesting calculations ahead.
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