You’ll forgive Mimas, one of Saturn’s smaller ice moons, for seeming a little dull. It doesn’t have the obvious enticements of its siblings, such as Enceladus, with its water-spewing fissures, or Titan, with its thick methane atmosphere. Studies of Mimas, thought to be geologically dead, have been few and far between. “We thought it was the most boring satellite,” says Radwan Tajeddine, a planetary scientist at Cornell University.
But the moon may hold a special secret of its own. A stronger than expected rotational wobble points to one of two intriguing scenarios: Mimas either has an irregularly shaped core or has an ocean buried underneath its icy surface. “Something else has to be going on inside,” says Tajeddine, who publishes a study with his colleagues online today in Science. Other scientists say it’s unlikely Mimas has such an interesting interior and think the wobble can be explained more simply.
Tajeddine and his team relied on pictures taken by NASA’s Cassini spacecraft, which has been exploring the Saturn system since 2004. They built a 3D model of the moon and found that it rotates with an extra wobble, like a misshapen top spinning slightly askew. Because Mimas is nearly spherical, the wobble hinted that something lumpy, or perhaps sloshy, lay beneath the surface. The scientists tested several models of the moon’s interior to see what might give rise to the observed wobble.