Mimas, Saturn’s ‘Death Star’ moon, could harbor a secret ocean

Scientists say Saturn's "Death Star" moon, Mimas, has a strange wobble that could indicate it has an elongated core -- or that it's hiding a subsurface ocean.
Scientists say Saturn’s “Death Star” moon, Mimas, has a strange wobble that could indicate it has an elongated core -- or that it’s hiding a subsurface ocean.

When it comes to Saturn’s moons, water-squirting Enceladus and the hydrocarbon lakes of Titan typically steal the spotlight. But now, scientists think that lesser-known Mimas may be harboring a strange secret of its own. Scientists who studied the ‘Death Star’ moon with NASA’S Cassini spacecraft have discovered a weird wobble in its motions that could mean one of two things: Either Mimas has an oddly elongated core, or it’s hiding an ocean inside its icy body.

The findings, described in the journal Science, shed new light on a mysterious but often-overlooked moon that could hold clues to its early formation.

Mimas is an icy moon that’s 246 miles wide and whose most distinctive feature is the 88-mile-wide Herschel crater – a giant Cyclops-like indentation that makes the moon resemble the planet-obliterating superweapon from “Star Wars” that’s known as the Death Star.

For a paper led by Radwan Tajeddine of Cornell University, an international team set out to study this lesser-known Saturnian satellite. Using a method called stereo-photogrammetry, the researchers built a precise 3D computer model of hundreds of reference points on the moon’s surface with the help of Cassini photographs taken at different times and from different angles.


But the scientists noticed something strange – the moon seemed to be wobbling, or “librating,” about twice as much as they expected. After going through several different explanations, they settled on two main possibilities. Either this round moon has a football-shaped core that’s causing the wobble, or there’s a liquid water ocean underneath the icy surface.

If Mimas holds an ocean, it joins an elite group of moons (including Enceladus as well as Jupiter’s moon Europa) that potentially hold liquid water – which is key for the search for other worlds besides Earth with life-hosting potential. If it holds a misshapen core that hasn’t pulled itself into a more spherical shape after more than 4 billion years of existence, then it could serve as a fascinating ‘fossil’ whose shape offers clues about its formation.

“In any case,” the study authors wrote, “the measurement of the physical forced librations using Cassini ISS images shows surprising evidence that Mimas is more complex than we thought.”

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