After stringing together photos taken by NASA's 17-year-old Cassini probe, a team of astrophysicists came across a strange and unexpected discovery: something deep inside Saturn's moon Mimas is inexplicably causing the moon to wobble. That means there's a mystery tucked away inside the Death Star moon.

"This was a very surprising discovery for us," says Benoît Noyelles, an astrophysicist with the research team at the University of Namur, Belgium. "When we started, our team was convinced that Mimas was going to prove to be pretty boring."

A Mystery Arises

Astrophysicists are trying to figure out exactly how Saturn's moons were formed, and so Noyelles and his colleagues were tracking the eccentricities of Mimas's orbit, hoping their detailed measurements would tell them more about on the moon's core. "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," says Radwan Tajeddine, the astrophysicist at Cornell University who led the science team.

However, as they note in their study published today in the journal Science, the scientists were surprised to find that Mimas—one of Saturn's 62 moons, and the smallest round moon ever discovered— unaccountably sways back and forth toward its planet. This is the same type of wobbling that causes the supermoon phenomenon here on Earth, where our moon gets closer to the planet.

"My first thought was that this was some type of mistake, that I did something wrong in our experiment," Tajeddine says. That's because the wobbling is too strong to be caused by any irregularity on the moon's surface, and when the researchers checked and re-checked their work, they realized the source of the weirdness had to be something locked deep inside the moon.

Hidden Oceans or Rugby Ball Cores?

There are three possible explanations for Mimas's mysterious motion, Tajeddine says.

The first explanation, which initially seemed the most likely, was that Mimas's wobble is a byproduct of the moon's large crater, called Herschel—the thing that makes it look like the Death Star. The scientists thought that a chunk of unusually dense rock was hidden underneath Herschel's surface, probably caused by the impact that made the crater. But in all the scientists, the math just didn't add up. "The motion is just far too large to be due to the crater," Noyelles says.

Another explanation could be that while Mimas is basically round, the moon's core is bizarrely shaped. "We believe that if the core was elongated, shaped something like a Rugby ball, that could explain the wobbling we observe," Tajeddine says.

But this theory has got problems, too.

"To make such a shape, Mimas's could have been formed in Saturn's rings by the accretion of rock particles," Tajeddine says. At this distance from Saturn, the rock indeed would form a rugby ball shape. "And that core could have been conserved, like a fossil," he says, as Mimas moved farther away from Saturn and its surface rounded out. But when his team computed how that core would have grown into a moon, the result did not look like Mimas. "Mimas itself should be more elongated. And that makes us a bit suspicious," he says.

The final possibility—and by far the most exciting—is that Mimas, though, frozen, barren, and geologically inactive at the surface, is hiding a deep ocean. It's a theory that leaves more questions than it answers, but there's no denying the math. The scientists calculated that the sloshing of a sea somewhere between 15 to 18 miles under the crust of the 246-mile-wide moon would account for the mysterious wobbliness.

"The biggest problem with this theory is that Mimas is very cold," Noyelles says, "so how could the ocean keep from [freezing]? That's a key question that would need to be answered." One possible explanation: Changing tidal forces inside the moon fomented by the Mimas's elliptical orbit could cause a warming friction between the ocean and the crust.

Not everybody is buying that explanation, though.

"I think it's pretty unlikely there's an ocean down there," says Francis Nimmo, an astrophysicist at the University of California, Santa Cruz, who was not involved in the discovery. "And hypothetically, even if there were an ocean and it was being heated through Mimas's elliptical orbit, you would expect that the moon's orbit would be getting more and more circular over time," he says. (Nimmo admits, though, that scientists don't have a great picture of how the moon's orbit has changed over time.)

Nimmo also points to Enceladus, another of Saturn's moons. We know that Enceladus has an ocean, but unlike Mimas, Enceladus has clear geologic activity such as icy geysers. "And if you just look at Mimas, it looks like a [geologically] dead object, so it'd be very surprising if it had an ocean underneath," he says.

Whichever idea turns out to be true (and we're rooting for ocean), this finding vaults the small and underappreciated Mimas up the rankings of the solar system's most interesting objects. And new data could be coming. Tajeddine says that several measurements we could collect soon, such as data on Mimas's surface temperature and whether or not it has a magnetic field, could begin to unravel this mystery.

"Since the beginning of NASA's Cassini mission, Mimas was almost totally ignored, because it was thought to be just another boring [moon]," Tajeddine says. "But this is absolutely proof that Mimas deserves more attention."

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