To the naked eye Mimas is a rather unspectacular moon. Apart from an enormous crater on the surface that makes it look a bit like the Death Star from Star Wars, it looks like a giant, round rock. According to a new study, however, there is more going on beneath the surface.

Researchers working with NASA’s Casini mission have noticed a wobble in the orbit of Mimas that suggests that it either has a large ocean beneath the surface or that the moon’s core is football shaped. After four million years, Midas should have settled into a spherical shape, a football shaped core would be a record, frozen in time, of the moon’s formation.

“The data suggest that something is not right, so to speak, inside Mimas. The amount of wobble we measured is double what was predicted,” said Radwan Tajeddine in a statement. Tajeddine is a Cassini research associate at Cornell University, Ithaca, New York, and lead author of a paper on Mimas published in the October 17 edition of the journal Science.

Two of Saturn’s other moons, Enceladus and Titan, are “ocean worlds” as are several moons of Jupiter. Because of the unstable oceans, however, those moons show signs of geologic activity, Mimas does not.

Like our own moon, Mimas is locked in “spin-orbit resonance” which means that it always shows the same face to its planet. This occurs because it takes the same time to rotate on its axis as it does to orbit Saturn. Mimas’ orbit is slightly stretched out so that it forms an ellipse rather than a circle. The deviation causes the area that faces the planet to wobble slightly over time. This effect, called “libration” provides important clues to researchers.

“Observing libration can provide useful insights about what is going on inside a body. In this case, it is telling us that this cratered little moon may be more complex than we thought,” said Tajeddine.

What exactly is going on beneath Mimas’ surface is likely to remain a mystery for some time. Neither the ocean, nor the football scenario seem particularly likely. According to Tajeddine and his French and Belgian co-authors Mimas is too small to have retained internal heat and should not be able to support a liquid ocean, but if its core were football shaped the researchers would expect the surface to have a different appearance.

It is possible that there is a third explanation, but no one knows what that might be yet.