Mimas, the "Death Star" moon of Saturn, may be hiding a subsurface ocean beneath its battered and pummeled exterior, some astronomers suggests.
Like most moons, Mimas, with its resemblance to the Death Star of the Star Wars movies, wobbles in its orbit under the influence of its parent planet.
That's unremarkable; what is remarkable is that the wobble is much larger than anyone expected to measure, Cornell University astronomer Radwan Tajeddine says.
"In physical terms, the back-and-forth wobble should produce about 3 kilometers of surface displacement," he says. "Instead we observed an unexpected 6 kilometers of surface displacement."
Analyzing data gathered by NASA's Cassini spacecraft circling Saturn and its many moons, Tajeddine and his colleagues decided there had to be something unusual about the interior of Mimas that was causing the larger-than-expected wobble.
"We're very excited about this measurement because it may indicate much about the satellite's insides," Tajeddine says. "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."
Two theories have come to the forefront after computer modeling of the moon's orbital sway; an irregularly-shaped core -- something like a football -- or a global ocean just 15 to 20 miles beneath Mimas' icy crust.
Neither theory is without its problems, the scientists acknowledge.
If there's an irregularly shaped core rather than a spherical one, there should be telltale signs visible on the surface of the 250-mile-wide moon. No such features have been observed.
If there's a subsurface ocean, that would require a source of interior heat to keep it liquid, but the moon's surface again displays no signs of such heating.
Rather than internal heat, a possible mechanism creating a subsurface ocean might be variations in the gravitational effect Saturn has on Mimas during the moons eccentric orbit around the ringed planet; stronger when the moon is closer, weaker when it's further away.
"This phenomenon will create friction inside the moon, which will induce heating that will melt the ice and form an ocean," Tajeddine says. "The ocean will sustain as long as the orbit is eccentric."
If a subsurface ocean does indeed exist beneath the icy covering of Mimas, it would put the tiny moon on a short list of moons and planets in the solar system that might be amenable to the existence of life.
The Mimas study has been published in the journal Science.
