You’ll forgive Mimas, one of Saturn’s smaller ice moons, for seeming a little dull. It doesn’t have the obvious enticements of its siblings, such as Enceladus, with its water-spewing fissures, or Titan, with its thick methane atmosphere. Studies of Mimas, thought to be geologically dead, have been few and far between. “We thought it was the most boring satellite,” says Radwan Tajeddine, a planetary scientist at Cornell University.
But the moon may hold a special secret of its own. A stronger than expected rotational wobble points to one of two intriguing scenarios: Mimas either has an irregularly shaped core or has an ocean buried underneath its icy surface. “Something else has to be going on inside,” says Tajeddine, who publishes a study with his colleagues online today in Science. Other scientists say it’s unlikely Mimas has such an interesting interior and think the wobble can be explained more simply.
Tajeddine and his team relied on pictures taken by NASA’s Cassini spacecraft, which has been exploring the Saturn system since 2004. They built a 3D model of the moon and found that it rotates with an extra wobble, like a misshapen top spinning slightly askew. Because Mimas is nearly spherical, the wobble hinted that something lumpy, or perhaps sloshy, lay beneath the surface. The scientists tested several models of the moon’s interior to see what might give rise to the observed wobble.
In the end, just two scenarios could reproduce it. In one, a spherical shell of ice wraps around an irregular rocky core shaped like a rugby ball. If this scenario holds, Tajeddine says, the “fossilized” core could hold clues to Mimas’s formation. He says the core could have acquired an irregular shape by forming first, much closer to Saturn, where the planet’s strong tides could have stretched it. Then, the core could have migrated outward and accumulated ice, which, being more deformable, would settle into a sphere. But there still should have been a small sign of the irregular core protruding at the surface, like an elbow poking up against a sleeve, and the researchers found none.
That led them to favor the other scenario: an ocean underneath an ice cap 24 to 31 kilometers thick. If it is true, Mimas would join Enceladus, Titan, and Jupiter’s moons Europa, Ganymede, and Callisto as solar system moons suspected to harbor an ocean. One problem is that Mimas’s ancient, heavily cratered surface shows no signs that water has ever touched it—unlike, say, the freshly Zamboni-ed ice skating rink of Europa. Mimas is also too small to have enough heat-producing radioactive elements in a rocky core to maintain a liquid ocean. But Tajeddine says the gravitational tug of Saturn on Mimas’s eccentric orbit could produce enough tidal heating to maintain a liquid interior.
But not for very long, says William McKinnon, a planetary scientist at Washington University in St. Louis. If such an ocean were buried there, its tides would iron out Mimas’s orbital eccentricity within a few million years—eliminating the very thing that can keep the ocean liquid. That means that Cassini would have to be catching Mimas at an incredibly coincidental moment in its history, just after something put the moon into an eccentric orbit that in turn heated up and created the ocean. “I find it quite implausible,” McKinnon says. He says the rugby ball–shaped core scenario is more compelling, though that same result could be achieved with irregular gradations in the density of rock, ice, and pore space. He also suggests a scenario that the researchers did not consider, one in which the detected wobble could have been knocked into existence by something that has nothing to do with the moon’s interior: a comet impact.
Cassini made its closest pass to Mimas in 2010, when it flew within 10,000 kilometers of the moon. As part of its extended mission, the spacecraft in 2016 will fly within about 27,000 kilometers of the moon, the closest of a handful of encounters in 2016 and 2017 that could allow Tajeddine to refine his measurements of the wobble. However, these passes will not be close enough for the spacecraft to look for tiny variations in the moon’s gravity field or surface temperature—two methods that would help reveal what really lies within.