Does the Saturn moon hide a 'fossil' core or an ocean?

A new study focused on the interior of Saturn's icy moon Mimas suggests that its icy surface cloaks either a rugby ball-shaped rocky core or a sloshing sub-surface ocean.

Researchers used numerous images of Mimas taken by NASA's Cassini mission to determine how much the moon wobbles as it orbits Saturn.

They evaluated several possible models for how its interior might be arranged, finding two possibilities that fit their data.

"The data suggest that something is not right, so to speak, inside Mimas," said Radwan Tajeddine, a Cassini research associate at the Cornell University, New York, in the US.

"The amount of wobble we measured is double what was predicted," added Tajeddine.

Either possibility for the interior of Mimas would be interesting, Tajeddine said, as the moon's heavily cratered outward appearance does not suggest anything unusual lies beneath its surface.

Because Mimas formed more than four billion years ago, scientists would expect its core to have relaxed into a more or less spherical shape by now.

So if Mimas' core is oblong in shape, it likely represents a record of the moon's formation, frozen in time, the astronomers added.

The team used a technique called stereo-photogrammetry to interpret images taken by the Cassini Imaging Science Subsystem to measure the libration (perceived oscillating motion of orbiting bodies relative to each other, such as the motion of the Moon relative to Earth).

Mimas is about 400 km in diameter, and its possible internal global ocean is located under an icy crust ranging in thickness between 25 and 30 km.

The study appeared in the journal Science.