Mimas is a small moon of Saturn, often compared to the Death Star from Star Wars, thanks to its large, distinct crater. Scientists have long believed that Mimas is an inert ball of ice because of its heavily cratered surface. Icy worlds with oceans are usually smooth, since changes in their surface ice pave over craters, or cracked. Tidal forces stretch and relax these moons, which both cracks the surface ice and heats the moons' insides, sustaining internal oceans.

But NASA's Cassini mission, which orbited and studied Saturn for over a decade, detected an unexpected oscillation in Mimas's rotation. As it spins on its axis, Mimas wobbles slightly. Such oscillations may point to an ocean deep beneath the moon's ice, according to a new analysis.

"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," Alyssa Rhoden, a geophysicist who led the analysis at the Southwest Research Institute, said in a press release.

Rhoden's finding was published online in the journal Icarus this week, in a paper co-authored with Matthew Walker of the Planetary Science Institute in Arizona.

It's still no guarantee of a secret Mimas ocean. Researchers need to investigate the moon for further evidence.

"The work doesn't prove that there is a subsurface ocean on Mimas, but it does show that an ocean is perfectly consistent with the available data and our understanding of the physics, and the authors are appropriately cautious about this," Michael Bland, a space scientist who studies icy worlds at the US Geological Survey, and previously spoke with Rhoden about the research, told Insider in an email. "I think the study also opens as many questions as it answers."