That’s No Space Station

4 May 2014

Saturn's moon Mimas. NASA/JPL
Saturn’s moon Mimas.

Mimas is a small moon of Saturn, about 400 km in diameter. It’s surface is dominated by Herschel crater, which gives the moon a passing resemblance to a fictional space station. For this reason it is often used in memes and jokes, but Mimas is actually an interesting puzzle.

Mimas has a very low density, not much higher than water. It would seem that Mimas is composed almost entirely of water ice. It is quite close to Saturn, orbiting just outside the planet’s rings. So close that it is tidally locked with Saturn, so that only one side faces the planet. It also seems to be entirely frozen solid. There is no evidence of any geyser activity, and its surface is heavily cratered unlike other moons like Enceladus. And therein lies the mystery.

Enceladus is more distant from Saturn, but does exhibit geysers and ice flows. Thus Enceladus maintains liquid water in its mantle. This is thought to be due to tidal heating, where Saturn’s gravity stretches and compresses the moon slightly, which keeps it warm. This effect is similar to squeezing a lump of clay in your hands, which causes it to get warm. Mimas is closer to Saturn, and its orbit is more elliptical. Mimas is also oblate (it’s wider at its equator than it is from pole to pole), so it should experience tidal heating even more than Enceladus.

So why is it frozen solid? We aren’t sure. In fact some have called this puzzle the Mimas test, since any model that can explain the liquid water in Enceladus must also explain why Mimas is frozen solid.