[LINK] "'Death Star' Moon May Hide a Buried Ocean"

[rfmcdonald]

National Geographic reports on findings from the Cassini probe suggesting Saturn's moon Mimas might have a subsurface ocean, too.

Saturn's moon Mimas, the smallest of the ringed planet's major satellites, may join the growing list of moons that hide an ocean of liquid water beneath their cratered surfaces, astronomers report Thursday in the journal Science.

That's one plausible interpretation, at least, of a rhythmic wobble Mimas displays as it orbits Saturn once every 23 hours or so, says study lead author Radwan Tajeddine, a planetary scientist at Cornell University. The other possibility, says Tajeddine, is that Mimas might be solid throughout but that its rocky core might not be spherical even though its icy outer layers clearly are. "Instead," he says, "the core might be elongated—shaped like a rugby ball."

Whatever the reason, the wobble Tajeddine and several co-authors discovered by carefully examining images from the Cassini space probe was unexpected. The scientists weren't surprised at the wobble itself, since many moons, including our own, oscillate slightly as they orbit. Mimas's shudder, however, is enormous for a moon just 250 miles or so in diameter. "We expected it would wobble by about three kilometers [1.8 miles] once every orbit," he says, "but it turned out to be twice that."

[. . .]

An elongated, rocky core could create a wobble [. . .] without affecting Mimas's orientation.

So could a subsurface ocean lying between a normal, spherical core of rock and a shell of ice perhaps 15 or 20 miles thick, say the paper's authors. "If you spin a raw egg and a hard-boiled egg, the boiled egg spins faster," says Tajeddine, and Mimas's wobble could also be related, in a slightly different way, to a partly fluid interior.