In the seven years Cassini has spent orbiting Saturn, the spacecraft has sent back mountains of data that has changed our view of the ringed planet and its moons. Saturn’s largest moon, Titan, has been a particular focus of attention because of its dense, complex atmosphere, its weather and its lakes and oceans.
Now it looks as if Titan is even stranger still. The 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.
Today, Rose-Marie Baland and buddies at the Royal Observatory of Belgium in Brussels, crunch some numbers to see whether a liquid model is compatible with the measured moment of inertia. “We assume the presence of a liquid water ocean beneath an ice shell and consider the gravitational and pressure torques arising between the different layers of the satellite,” they say.
Their conclusion is that 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.
It’s also worth pointing out that there is another explanation for Titan’s strange moment of inertia. The calculations assume that the moon’s orbit is in a steady state but it’s also possible that Titan’s orbit is changing, perhaps because it has undergone a recent shift due to some large object passing nearby, a comet or asteroid, for example.
So although Baland and co’s analysis is good evidence that Titan has a subsurface ocean it is not quite a slam dunk. There’s more life in this problem yet.
Ref: arxiv.org/abs/1104.2741: Titan’s Obliquity As Evidence For A Subsurface Ocean?
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