Mimas Wobbles Due to Ocean or a Weird Core

Meet Mimas, the moon with the wobbly orbit around Saturn that might be hiding a weird core or a subsurface ocean. Courtesy NASA/JPL-Caltech/Space Science Institute

Okay, so this is kind of cool news. Astronomers studying the Saturnian moon Mimas used Cassini images of it to figure out how much it wobbles as it orbits Saturn. Turns out, it jiggles quite a bit, and that has set off a flurry of speculation about what would cause it to do that. Picture a top the shape of a Star Wars Death Star (which is kind of what Mimas looks like) wending its way around Saturn. As it does, its spin is off-kilter. And that means something’s a little odd inside Mimas.

What are the possibilities?  For one, since the wobble is about double what scientists expected it to be, whatever it is that throws the moon off as it spins has to be massive. That could mean an ocean (which would easily affect how the moon spins) or an oddly shaped core. One of the scientists suggested that if it’s the core, then it would have to be nearly football-shaped to do the trick.

How could a core get to be oblong instead of round in a world as old as Mimas? (It dates back to the earliest epochs of the solar system and is about 4 billion years old.)  One school of thought says that the core may have frozen into an oblong shape long ago, thus preserving some hint of its early history.

If Mimas is hiding an ocean beneath its cratered surface, then a little bit of math tells us that a liquid water ocean would be hidden at least 24 kilometers beneath the crust. There also needs to be some mechanism to keep the water liquid. Mimas long ago lost all the heat from its formation and its core is likely cool as well.

So, what would keep things warm enough to sustain liquid?  It turns out that tidal flexing — that is, the squeezing and contracting due to Saturn’s strong gravitational pull that Mimas undergoes as it orbits the planet — could keep things warm enough through friction to do the trick. Mimas undergoes a fairly elongated (think of it as egg-shaped) orbit around Saturn, and so at different times at its orbit it encounters changes in the gravitational pull.  This slight deviation in its orbit causes the point on Mimas’ surface that faces Saturn to vary a bit over time. If you could watch Mimas from Saturn, you’d see that wobble and notice how small areas of the surface limb shift just enough to become visible. This effect is called libration. Our own Moon has the same  motion.

So, which is it: football-shaped core or liquid ocean?  Further analysis leans toward an ocean, since models of an oddly shaped core seem to result in a different-looking Mimas than the one we really have out there. As usual, more data will help tell the story, and the Cassini Solstice mission can be counted on to crank out more images of Mimas as it pursues its wobby path around the Saturn. Stay tuned!

What’s Up and Happening in our Skies

Now that we’ve all enjoyed (if we could last week) the lunar eclipse, there’s a partial solar eclipse coming up next week that will be visible to at least some observers around the world on October 23rd. It’s partial because the Sun will only be partially blocked, which means that it’s not an eclipse you can watch with the naked eye. In fact, it is REQUIRED that you wear eye protection or use special projection methods to observe this eclipse. Although it should go without saying, I’m going to say it anyway: NEVER LOOK AT THE SUN WITHOUT EYE PROTECTION!  Practice safe solar viewing. Use eclipse glasses, welder’s glass, or use pinhole projection to see this event.

It begins at 3:38 p.m. EDT (7:38 PM UT). It sets eclipsed for most viewers.  If you live in North America or the extreme eastern parts of Asia, you have a shot at observing the eclipse. For more information, check out, Eclipsewise, and’s pages on eclipses.

October is a good month for sky viewing. For most of the world, the weather isn’t too bad, and clear skies give you a chance to do some exploring. I’ve got a little video up at that explains some of the sky sights you can seek out. I talk about the planet Mars, low in the western sky after sunset, along with Saturn, Jupiter in the early morning, and a possible glimpse of Mercury (also in the predawn hour). There’s also the Orionid Meteor shower, and while you’re waiting for it to send some meteors our way, you can search out the Andromeda Galaxy and Perseus Double Cluster (for northern hemisphere explorers) and the Magellanic Clouds and the globular cluster 47 Tucanae in the southern hemisphere skies. Check it all out in Our Night Sky.