Several moons of the outer solar system, once thought solid and dull, host internal oceans. Now one more may be added to that list.
Until now Saturn's innermost round moon, Mimas, had two claims to fame. Measuring less than 400 kilometers across, it is the smallest body in the solar system whose gravity has pulled it into a nearly spherical shape. Its second claim to fame is its huge crater, Herschel, that makes it look like the Death Star.
Like most moons, Mimas is “locked” with the same face always pointing towards its planet. However, like our own moon, the orientation shifts ever so slightly as a result of its orbit being elliptical rather than a perfect circle, a process known as libration.
"Observing libration can provide useful insights about what is going on inside a body," says Cornell's Dr. Radwan Tajeddine. One libration turned out to be 6 km, twice as large as anticipated.
"In this case, it is telling us that this cratered little moon may be more complex than we thought," says Tajeddine. He compares his work to, “A child [shaking] a wrapped gift in hopes of figuring out what’s hidden inside.”
In Science, Tajeddine presents two possible explanations. Either Mimas is liquid inside, or it has an elongated core.
Tajeddine admits neither explanation is entirely satisfactory and he didn't expect anything like this when he examined Mimas' orbit. An oddly shaped core would be expected to slowly relax towards roundness, and Mimas' four billion year life offers plenty of time to become spherical. But an ocean is equally strange – an object this small would have long ago lost all its heat of formation, and radioactivity could not have kept it warm enough to sustain liquid water. Mimas' orbit is also too circular for tidal flexing to provide the heat required to melt the ice within. Furthermore, oceans on moons such as Enceladus and Europa lead to the erasure of craters over at least part of the surface.
One possible explanation is that Mimas' orbit was recently more stretched out, providing a powerful source of heat that has not entirely dissapated. A similar explanation has been proposed for Enceladus, which seems to be emitting more heat than its current orbit should produce.
Tajeddine hopes more data will resolve the question. In the process, some light may be shed on Mimas' strange heat pattern and how it survived the collision that created Hershcel, the largest crater in the solar system relative to the size of the solar body it impacted. An impact that great could easily have destroyed the moon entirely.