This mosaic of Saturn's moon Mimas was created from images taken by NASA's Cassini spacecraft during its closest flyby of the moon on Feb. 13, 2010. (NASA/JPL-Caltech/Space Science Institute)

Planets aren't always filled with molten rock, and guessing the composition of distant planets can be a tricky business. But scientists now believe that Saturn's moon Mimas has one of two things under its icy, rocky surface: a core shaped like a football or a liquid ocean.

In a study published Friday in Science, researchers analyzed Mimas's movements as it orbited Saturn. By looking at images from NASA's Cassini satellite, they determined how much Mimas was wobbling.

The wobble was a lot wonkier than expected, leaving the research team puzzled. But analyzing the shakes helped researchers guess what might be going on inside.

"It's a bit like how you can figure out if an egg is hardboiled or raw just by spinning it. By watching the rotation, you can gain information on what's inside," Radwan Tajeddine, Cornell University astrophysicist and lead author of the study, told Popular Mechanics.

Either possibility would be pretty cool: If Mimas's wobbling is caused by an elongated core inside the round planet, it might mean that the core is a sort of fossilized remnant of the planet's original shape, kept oblong while the outer layers rounded out over the past 4 billion years or so.

A liquid ocean sitting about 15 miles under the surface would also account for the planet's jiggle. It's an intriguing possibility (though there are other so-called ocean worlds, like the Saturn moons Enceladus and Titan) because Mimas seems to cold and barren to have liquid water inside of it.

With Mimas too cold to support liquid water, some geological quirk would have to be keeping it from freezing. And the lack of ice geysers and other hints at the surface makes it difficult to say whether Tajeddine's hypothesis is correct.

Another model may be proposed that makes more sense -- and it's possible that further data from Cassini could unlock the mysteries of this misunderstood moon.