NASA researchers determined Saturn's closest moon, Mimas, has an an ocean hidden beneath its chilly surface.
The findings were made by using the Cassini spacecraft to measure Mimas' wobble, NASA reported. The icy moon is about 250 miles (400 kilometers) in diameter and has a relatively smooth outer surface.
"After carefully examining Mimas, we found it librates -- that is, it subtly wobbles -- around the moon's polar axis," Radwan Tajeddine, Cornell research associate in astronomy and lead author of the article. "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."
The team used a technique called stereo-photogrammetry to interpret the images taken by the spacecraft. The method looked at Cassini images taken from different angles and at different times to build 3-D computer models of the object. The team determined the moon does not rotate smoothly, but rather rocks back and forth in its orbit.
The amount of "to-and-fro" motion suggest the moon's interior is also not uniform, such as an asymmetrical rock or ocean. The researchers believe an ocean is most likely the culprit, and is located underneath an icy crust that could be as much as 18 miles (30 kilometers) thick. The moon itself is believed to have been formed by a gradual buildup of ring particles or directly from Saturn's primordial gas nebula.
"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," Tajeddine said.
The findings were published Oct. 17 in the journal Science.
