New evidence that Saturn’s smallest moon Mimas harbours an ocean

Scientists have found new evidence that Mimas, the smallest and innermost of Saturn's major moons, harbours an internal ocean. Numerical simulations of the Herschel impact basin, a large crater and the most distinguishing feature on the moon's surface, revealed that the basin’s structure and the lack of tectonics on the heavily cratered moon are compatible with a thinning ice shell and geologically young ocean.

"Mimas seemed like an unlikely candidate, with its icy, heavily cratered surface marked by one giant impact crater that makes the small moon look much like the Death Star from Star Wars. If Mimas has an ocean, it represents a new class of small, ‘stealth’ ocean worlds with surfaces that do not betray the ocean’s existence," said Southwest Research Institute scientist Dr Alyssa Rhoden, a specialist in the geophysics of icy satellites and the second author of a new Geophysical Research Letters paper on the subject.

Dr Rhoden and Purdue graduate student Adeene Denton teamed up to better understand how a heavily cratered moon like Mimas could possess an internal ocean.

Denton, who is now a postdoctoral researcher at the University of Arizona, used simulation software to study the formation of the Hershel impact basin on Mimas and it indicated that the moon's ice shell had to be at least 55 km thick at the time of the impact. However, current observations and models suggest the present-day ice shell is no more than 30 km thick if it contains an ocean. This suggests that an interior ocean may have been warming and expanding since the impact.

Denton also found that the impact models that included an interior ocean produced a more accurate shape of the basin.

"We found that Herschel could not have formed in an ice shell at the present-day thickness without obliterating the ice shell at the impact site. If Mimas has an ocean today, the ice shell has been thinning since the formation of Herschel, which could also explain the lack of fractures on Mimas. If Mimas is an emerging ocean world, that places important constraints on the formation, evolution and habitability of all of the mid-sized moons of Saturn," Denton said in a statement.