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There's something strange going on below the surface of Saturn's Death Star-looking moon Mimas, a new study suggests.

Mimas' rotation and its orbit around Saturn make the moon look like it's rocking and back forth and oscillating similar to the way a pendulum swings. That rocking motion is called libration. Using images of the moon captured by the Cassini spacecraft, Radwan Tajeddine, a research associate at Cornell University, discovered that the satellite's libration was much more exaggerated in one spot than predicted.

"We're very excited about this measurement because it may indicate much about the satellite's insides," Tajeddine said in a statement. "Nature is essentially allowing us to do the same thing that a child does when she shakes a wrapped gift in hopes of figuring out what's hidden inside." [See photos of Mimas, Saturn's Death Star Moon]

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The prominent crater on the surface of Mimas, one of Saturn's moons, makes it look like the Death Star from the "Star Wars" saga. NASA / JPL-Caltech

Tajeddine and his colleagues tested five different models of what Mimas might look like below the surface to see which one could explain the exaggerated rocking. They quickly ruled out the possibility that Mimas has a uniform interior, an interior with two different layers or an abnormal mass under the moon's 88-mile-wide (142-kilometer-wide) crater that makes it look like the Death Star from the "Star Wars" franchise.

However, either of the last two models could explain Mimas' extreme libration. One idea is that the moon has an elongated, oval-shaped core. This elongation might have happened as the moon formed under the push and pull of Saturn's rings.

The teeter-tottering could also come from a subsurface ocean, similar to the one on Jupiter's moon Europa. But Tajeddine thinks the subsurface ocean is an unlikely explanation. Astronomers have not observed any evidence of liquid water on Mimas, unlike some of Saturn's other moons.

The new research was published this week in the journal Science.

— Kelly Dickerson, Space.com

This is a condensed version of a report from Space.com. Read the full report. Follow Kelly Dickerson on Twitter. Follow Space.com on Twitter, Facebook or Google+.