Bad Astronomy

Mimas is an egg-shaped UFO

Mimas is a moon of Saturn, most notable because it has a whopping ginormous crater on it, making it look like the Death Star. But a few years ago I stumbled on the fact that Mimas isn’t spherical; it’s actually quite noticeably ovoid. A few years back I wrote about it on this blog, and a few commenters took me to task because they weren’t sure if the image posted really did make Mimas look elliptical.

Well, wonder no more.

Whoa. This image from the Cassini spacecraft orbiting Saturn was taken in January 2009. It’s unprocessed, meaning the cosmic rays and other flaws have not been cleaned up yet. They’re distracting, but try to ignore them. Instead notice two things: the “dark” part of Mimas is being lit by light from Saturn (the crescent is from sunlight), and Mimas isn’t even close to being a sphere.

You can see this again in this Cassini picture taken in October 2008:

In fact, Mimas is egg-shaped! From pole to pole, the diameter is 381.4 kilometers. The diameter going through Mimas and pointing right at Saturn is 414.8 km, and the diameter pointing along its orbit is 394.4 km.

That’s pretty far off of a sphere.

The diameter pointing to Saturn is the largest, just as you’d suspect. Saturn’s immense gravity means Mimas feels huge tides, stretching it out in a line pointing right at Saturn’s center. All big planets do that to their moons (including us). I’m not precisely sure why the other two diameters are so different (by about 3%) but it may have to do with the internal structure of the tiny moon. Interestingly, Mimas’s shape is roughly like a flying saucer… and Pan and Atlas, two more of Saturn’s moons, are far more obviously described that way. I wonder if the same forces are acting on all the moons, but it becomes obvious at some particular size?

Also, I think that Mimas may be the smallest body in the solar system that is nonetheless big enough to be almost spherical. Its gravity is just enough to form it into a ball instead of an irregular lump like smaller moons and asteroids.

And another thing: the crater Herschel sprawls across the Mimasian surface, the biggest crater with respect to its parent body known. In science, if one object has two weird things about it, chances are they’re related. You can’t be sure, but it does make me wonder what the heck is going on with this strange little world.