Saturn’s small moon Mimas is best known for the giant crater Herschel, which makes it look like the Death Star. However, a new study based on Cassini data shows that its interior is weird too. While Mimas presents the same face to Saturn due to tidal-locking (just like the Moon does with Earth), it rocks back and forth, a motion known as libration. The new observations indicate that Mimas is librating twice as much as it should if it were a solid body with a regular interior. That led the researchers to conclude it has either an irregularly shaped core, or a subsurface ocean like its fellow moons Enceladus and Titan, as I explained in my latest Daily Beast column:

Mimas is the smallest of Saturn’s regular satellites. “Regular” means that it’s spherical or nearly so under its own gravity. Objects more massive than a certain amount are more spherical than not, while punier specimens are lumpy and irregular. Due to the action of gravity, scientists think regular objects like the larger moons and planets are regular all the way through: If the surface is spherical, the core should be ball-shaped, too. [Read more…]