Other than resembling the fictional Star Wars space station Death Star, Mimas, one of Saturn's smallest moons, appear dull and not as exciting as the other natural satellites of the ringed planet. Planetary scientists even admitted that they viewed it as among the most boring satellites when it was first seen.

It appears, however, that there is more to Mimas, than being heavily cratered and geologically dead. It holds a secret that astronomers have just recently learned.

It all began when Radwan Tajeddine, a research associate from the Cornell University, and his colleagues took a closer look at the images that were taken by NASA's Cassini spacecraft, which has been orbiting Saturn since 2005. The scientists found that Mimas has a wobble that is twice as big for a moon believed to be made up of solid rock.

"After carefully examining Mimas, we found it librates - that is, it subtly wobbles - around the moon's polar axis," Tajeddine said. "In physical terms, the back-and-forth wobble should produce about 3 kilometers of surface displacement. Instead we observed an unexpected 6 kilometers of surface displacement."

The wobbling suggested that there may be something lumpy or sloshy lying beneath the moon so the scientists embarked on testing five different models of the moon's interior to find out what causes the exaggerated wobbling.

The researchers have ruled out that Mimas has a uniform interior and arrived at two possible scenarios. One is that the moon has an irregularly shaped rocky core that is shaped like a rugby ball and the other is that an ocean exists beneath the moon's icy shell.

"After considering various possible interior models of Mimas, we argue that the satellite has either a large nonhydrostatic interior, or a hydrostatic one with an internal ocean beneath a thick icy shell," Tajeddine and his colleagues reported in their study published in the journal Science on Oct. 17.

Tajeddine, however, said that of the two plausible explanations, he favors the theory of a subterranean sea on Mimas.

"When we saw this wobbling, the first thing we thought of was an ocean," Tajeddine said.

The research team has not proven that there is actually an ocean beneath the surface of Mimas but if a subterranean sea does exist there, Mimas joins a group of other moons that are believed to have underground oceans including the Europa, Ganymede and Callisto moons of planet Jupiter and Saturn's own Titan.

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