Mimas, a moon of Saturn dubbed the "Death Star" for its resemblance to the iconic space station of the Star Wars movies, has a serious case of the "wobbles," astronomers say, suggesting something strange is going on inside.
Such wobbles, known as libration, are common in moons under the sway of the gravity of their parent planets, but in the case of Mimas that wobble, centered around one spot on the moon, is much greater than would be expected, astronomers say.
"In physical terms, the back-and-forth wobble should produce about 3 kilometers of surface displacement," says Cornell University astronomer Radwan Tajeddine. "Instead we observed an unexpected 6 kilometers of surface displacement."
Something unusual about the hidden interior of the small moon, just 250 miles across, could be the cause, he says.
Analysis of data from NASA's Cassini spacecraft -- which has been investigating Saturn and its 62 moon moons -- combined with computer models suggests two possibilities, Tajeddine and his researcher colleagues report.
Either the moon's core is not spherical but more irregular, or -- more intriguingly -- the moon may possess a hidden liquid ocean beneath its frozen, icy surface, they suggest.
That possibility is exciting because it would add Mimas to the short list of moons and planets in our solar system on which the presence of liquid water might suggest they could harbor some form of life.
The thought of an irregular-shaped core has astronomers looking at the moon's enormous 80-mile-wide Herschel Crater, that gives Mimas its "Death Star" look, and considering if a giant geological deformity underneath it could account for the observed wobble.
However, a "huge mass anomaly" beneath the impact site would not account for the amount of movement observed, the researchers said.
Tajeddine says he favors the other possibility, a subterranean ocean that separated the moon's core from its frozen crust.
If that was the case, he says, "the shell can wobble more easily, because it's not attached to another mass."
One problem with that theory, the researchers acknowledge, is that a subsurface ocean would require a source of heat from the moon's interior heat to keep it liquid, but the surface of Mimas shows no signs of such heating visible on its icy crust.
Still, Tajeddine says, trying to figure out what's going on inside Mimas is a delightful scientific puzzle.
"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," he says.
