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This Moon Isn’t A Death Star, But It Might Have An Ocean

When it comes to Saturn’s moons, Titan gets most of the extraterrestrial love. But there are plenty of other interesting little bodies orbiting the ringed planet. Take Mimas, the little moon that looks suspiciously like the Death Star. And adding to the intrigue, scientists suspect the moon might harbor a subsurface ocean of liquid water.

The tip off to the possible ocean is Mimas’ rhythmic wobble. A team of scientists led by Radwan Tajeddine, a planetary scientist at Cornell University, noticed in analyzing pictures from NASA’s Cassini spacecraft that the moon, which orbits around Saturn every 23 hours, wobbles around its axis.

This isn’t uncommon for a moon. Most moons, including our own, wobble just a little as they orbit. What was surprising was how much Mimas wobbles. The moon is small, just 250 miles in diameter. A moon that size is expected to wobble by less than 2 miles. Mimas’ wobble is twice that.

Moon wobble GIFThe slight wobble of our moon, visualized over the course of a month.

The scientists used computer simulations to try and figure out what could be making Mimas wobble so dramatically. One possibility pointed to the Herschel impact crater, the one that gives the moon to look of a fully operational battle station. Scientist played with the idea that remnants of an asteroid are buried under the crater, offsetting the moon’s center of gravity. But that doesn’t work. If that were the case, Mimas would have settled into a different orbit, likely with the crater pointing permanently towards Saturn.

Another idea was that Mimas has an oblong rocky core, making the moon slightly football-shaped and wobbly without affecting its orientation relative to Saturn.

A subsurface ocean could have the same perceived orbital effects as a football-shaped moon, especially if it’s lying between a spherical core and a shell of ice. A partially fluid core sloshing around inside the moon could account for Mimas’ irregular spin around its axis.

For the time being, the existence of Mimas’ subsurface ocean remains in the world of conjecture. But it’s not impossible. Planetary scientists have gathered significant evidence that Jupiter’s moons Europa, Ganymede, and Callisto, as well as Saturn’s moon Enceladus, all have subsurface oceans. Finding that Mimas should be on that list wouldn’t be that fantastical.

It would also give us a compelling reason to visit the moon; liquid water beneath the surface could harbor life. Maybe. And besides, the moon only looks like a Death Star.

Feature image from NASA/JPL

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