Mysterious wobbling of strange moon.

• Author: Braffman-Miller, Judith
• Position: Science & Technology

MYSTERIOUS, frozen Mimas is a mid-sized moon of the ringed gas-giant planet Saturn, as well as the 21st largest moon in our solar system. Named for Mimas, son of Gaia in Greek mythology, it is the smallest known astronomical body to be round in shape as a result of self-gravitation. When observed from certain angles, Mimas bears a resemblance to the "Death Star"--a fictional space station portrayed in the movie "Star Wars Episode IV: A New Hope."

In the Oct. 17, 2014, issue of Science, Radwan Tajeddine, research associate in astronomy at Cornell University, and his colleagues describe new discoveries indicating that this icy moon's frozen surface hides either a rugby ball-shaped rocky core or a churning subsurface ocean of liquid water. Their study made use of instruments aboard the Cassini-Huygens spacecraft measuring the wobbles of this very distant moon.

"After carefully examining Mimas, we found it librates--that is, it subtly wobbles--around the moon's polar axis. In physical terms, the back-and-forth wobble should produce about three kilometers of surface displacement. Instead, we observed an unexpected six kilometers of surface displacement," explains Tajeddine. "We're very excited about this measurement because it may indicate much about the satellite's insides. Nature is essentially allowing us to do the same thing that a child does when she shakes a wrapped gift in hopes of figuring out what's hidden inside."

Mimas was discovered by the German-born astronomer William Herschel on Sept. 17, 1789. He wrote about his discovery: "The great light of my forty-foot telescope was so useful that on the 17th of September, 1789, I remarked the seventh satellite, then situated at its greatest western elongation."

The names of all seven of the then-known moons of Saturn--Mimas included--were proposed by William Herschel's polymath son John in his 1847 publication, Results of Astronomical Observations Made at the Cape of Good Hope. John Herschel named the moons after the giant Titans of Greek mythology, because Saturn (which bears the Roman name of Kronos), was the leader of the Titans and mythic ruler of the world.

With a diameter of 246 miles, and a surface area that is only a bit less than the land area of Spain, Mimas possesses a veiy low density of 1.15 grams per cubic centimeter. This suggests that it is made up primarily of water ice with only trace quantities of rock. Because of the tidal forces affecting it, Mimas obviously is...