By the Light of a Wobbly Moon

Mimas Wobbles Due to Ocean or a Weird Core

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!