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."
The scientists used computer simulations of different scenarios to look for something that would produce the observed wobble. One idea was that the remnants of an asteroid are buried beneath the giant Herschel impact crater, which gives Mimas its uncanny resemblance to the Death Star from Star Wars. The dense asteroid leftovers would make Mimas more massive on one side than the other, creating a wobble through an imbalance in the pull of Saturn's powerful gravity.
But that imbalance would have permanently reoriented Mimas, with the crater itself pointed more directly to Saturn, which isn't the case. So that idea was rejected.
An elongated, rocky core could create a wobble in a similar way but 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.
If there really is a subsurface sea on Mimas, it would hardly be the first. Planetary scientists are convinced that Jupiter's moons Europa, Ganymede, and Callisto have their own underground oceans. So does Saturn's moon Enceladus, and perhaps Saturn's Titan does as well.
Tajeddine and his colleagues hasten to make clear that they have not actually proved Mimas has an ocean. "They've shown that this moon isn't behaving the way it's supposed to," says Cornell planetary scientist Joseph Burns, who wasn't involved in the research, "and they've come up with some plausible explanations."
Scientists think that if Mimas does have a buried sea, however, it must be made of liquid water, which is one of the key factors needed for life. That doesn't mean life necessarily exists here, of course. But along with the subsurface water that has been found on other icy moons in the solar system, it at least raises the possibility.
"Oceans trapped beneath ice shells," says Jet Propulsion Laboratory astrobiologist Kevin Hand, a National Geographic emerging explorer, "may harbor the greatest volume of real estate in our solar system and beyond."
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