Hidden Ocean May Exist on Saturn's 'Death Star' Moon

Saturn’s moon Mimas might strike one as comparable to the Death Star owing to its resemblance, but it harbors secrets more intriguing than just its exterior. Beneath its distinctive, crater-marked surface, it could be hiding an immense ocean of liquid water.

A new analysis of data brought back by NASA’s Cassini probe shows a slight alteration in Mimas’ orbit, where it gets closest to Saturn, over a period of 13 years. This was revealed by Paris Observatory astronomer Valéry Lainey and his team in their report published in Nature on February 7. The shift in orbital dynamics and past wobbles of the moon link to a potential liquid interior. This is because Mimas’ internal content directly influences the gravitational interplay between the moon itself and Saturn.

The discovery is, in the words of uninvolved geologist Francis Nimmo from the University of California, Santa Cruz, “a very surprising result.” Mimas, he notes, "does not look like a moon with an ocean."

This isn't the first speculation about Mimas, Saturn’s smallest major moon with a diameter of only about 400 kilometers, potentially having liquid water. A 2014 study proposed slight fluctuations in the moon’s rotation could be because of a water reservoir beneath its icy exterior (SN: 10/16/14). Many researchers rejected this possibility, contending that Saturn’s gravitational pull would result in significant flexing of such a hidden ocean and cause large fractures to appear on the surface of the moon’s ice (SN: 2/28/17). Yet, no such fractures have been found.

The recent data analysis suggests that Mimas might possess an ice shell approximately 20 to 30 kilometers thick, under which lies a 70-kilometer-deep ocean, and finally a solid rocky core. The question then arises, if Mimas does have such an ocean, why are there no visible surface cracks? The answer might lay in how recently the ocean was formed. Lainey's team at the Paris Observatory proposes the ocean was possibly formed between 5 to 50 million years ago, a period so short in geological terms that it wouldn't allow time for major disruptions to the moon’s exterior.

Skeptics in the scientific community, like Alyssa Rhoden, a planetary scientist from the Southwest Research Institute at Boulder, Colo., have had their doubts alleviated by this compelling evidence. “I was the most skeptical of Mimas having an ocean,” she says. “But you really have to go where the data takes you, and it seems like we’re getting a new ocean world.”

Nimmo, however, remains unconvinced. If an ocean formed at the same time human beings were exploring the planets, our understanding of the solar system would need to be considered very fortunate, he says. Despite there not being enough time for the supposed ocean to crack the surface, Nimmo thinks there should be visible signs of contraction because water occupies less space than ice, which means an ocean formed recently would leave noticeable voids under Mimas’ crust. No evidence of such voids has been found, he notes.

If the presence of an ocean on Mimas is eventually confirmed, it would suggest that there may be hidden seas beyond our own solar system, in places such as the moons of Uranus. Lainey suggests that possibility of the ocean being geologically young is exciting, as a future probe could potentially drill through the icy surface and allow us to observe fresh liquid water interacting with a rocky core.

"It’s really the place you want to look if you want to look at the beginning of conditions for life,” he says. As to whether life exists on Mimas, Lainey adds, “nobody knows.”