That’s no space station: It really is a small moon, even though it looks like the Death Star. Saturn’s moon Mimas is most remarkable for a huge crater, the result of an impact that must have nearly shattered it. But it’s weird in other respects, too: Mimas has a strange pattern of warming that makes it look almost like Pac-Man, and it’s slightly non-spherical. As it orbits Saturn, it rocks back and forth slightly; while that’s something other moons do as well, leave it to Mimas to do it weirdly.
The rocking motion, technically known as “libration,” provides clues to Mimas’s interior. In a study published last week, researchers used Cassini probe data to determine that the moon is wobbling about twice as much as it should, based on theoretical models. That could mean several things, but the authors propose that either Mimas has a vast ocean under its icy surface, or beneath its mostly spherical exterior there’s a lumpy core of rock, like an irregular asteroid.
Mimas is the smallest of Saturn’s regular satellites. “Regular” means that it’s spherical or nearly so under its own gravity. Objects more massive than a certain amount are more spherical than not, while punier specimens are lumpy and irregular. Due to the action of gravity, scientists think regular objects like the larger moons and planets are regular all the way through: If the surface is spherical, the core should be ball-shaped, too.
That’s certainly true for objects where we have data. Earthquakes go deep below the surface bounce off Earth’s core. Scientists used them to determine the size and shape of both the solid inner core and molten outer core, even though those are far deeper than anything we could ever dig to. For other worlds, we usually have to rely on other data: fluctuations in gravity, or the gentle rocking motion known as libration.
(I’m speaking mostly about rocky moons and planets here, though the Dawn probe determined that the asteroid Vesta is very planet-like in structure, despite not being spherical. By contrast, the interior of big planets like Jupiter still hold some mysteries, such as whether they have rocky cores. The Juno mission heading to Jupiter now may help clarify that.)
Mimas always presents the same face to Saturn, just as the Moon does to Earth. However, Mimas is much smaller: only about 392 kilometers (244 miles) in diameter, compared with the Moon’s 3,500-kilometer (2,200 miles) diameter. It’s also relatively much closer to Saturn, which itself of course is a far larger planet than Earth.
However, both the Moon and Mimas experience libration wobbling slightly and bringing part of the “far side” into view. The authors of the new study used data from the Cassini probe orbiting Saturn to measure the libration of Mimas. They compared what they found to a model of the moon, assuming it to be regular and solid.
But things didn’t match: Mimas had roughly twice the amount of libration as it should according to the theoretical calculation. When that happens, it means the assumptions of the model are not correct. The researchers considered several possibilities. Maybe Mimas has two distinct layers that disturb the internal regularity, or maybe the impact that made the huge Death Star crater—known officially as Herschel—rearranged the mass inside the moon. Those two options didn’t seem to work any better than the initial assumption.
The other two ideas they tested worked much better. Maybe Mimas is solid, but its core is irregular, like the shape of many asteroids. This model seemed to work, though as the authors pointed out, it’s uncertain what effect a non-spherical core would have on the mostly spherical surface. However, others have proposed that some moons, like Saturn’s Enceladus, also could have lumpy cores, so this idea isn’t new.
The final guess is also eminently reasonable: Mimas might have a partly liquid interior. A number of moons in the Solar System have global oceans beneath surfaces of solid ice or ice mixed with other materials. Those include Europa, Ganymede (orbiting Jupiter), Titan, and Enceladus (orbiting Saturn); other candidates have been suggested, as well. To see how a global ocean could affect libration, take two bottles, fill one with sand and the other with water, then spin them. This isn’t a perfect model—sand is much denser, for example—but you can see how the water’s motion makes a difference in how the bottle spins.
The authors are cautious about the ocean idea, though, as Mimas doesn’t have ice volcanos like Enceladus or the kind of tectonics recently seen on Europa. However, they don’t rule it out: Saturn’s tidal force, the difference in gravity between the near and far side of the moon, could be enough to keep an internal ocean liquid.
Whichever answer is correct, it’s fascinating that a moon as small as Mimas can still be surprising. It may look like the Death Star, but this little moon can show us as incredible secrets as any Jedi master.