Saturn’s moon Mimas might harbor a hidden ocean

An uncanny resemblance to the Death Star might not be the only intriguing thing about Saturn’s moon Mimas. It could also harbor a vast ocean of liquid water beneath its pockmarked exterior.

A new look at data from NASA’s Cassini probe reveals that the point in Mimas’ orbit where it comes closest to Saturn changed slightly over 13 years, researchers report February 7 in Nature. Because Mimas’ internal composition affects the gravitational dance between the moon and its planet, these orbital dynamics, along with some previously seen moon wobbles, point to a liquid interior, astronomer Valéry Lainey of the Paris Observatory and colleagues say.

“It’s a very surprising result,” says Francis Nimmo, a geologist at the University of California, Santa Cruz who was not involved in the findings. “When you look at Mimas, it does not look like a moon with an ocean.”

This is not the first time that liquid water has been suspected on tiny Mimas, which at 400 kilometers across is the smallest of Saturn’s major moons. A 2014 study suggested that slight wobbles in the satellite’s rotation could be explained by a watery reservoir under its frozen shell (SN: 10/16/14). Other researchers pooh-poohed this possibility, maintaining that Saturn’s gravity would flex a hidden ocean so much that large cracks would eventually appear in the moon’s surface ice (SN: 2/28/17). Such fractures have not been seen.

The new calculations suggest that Mimas has an ice shell roughly 20 to 30 kilometers thick, followed by a 70-kilometer-deep ocean and a solid rocky core. To account for the lack of surface cracks, Lainey and colleagues believe the ocean formed between 5 million and 50 million years ago, a geological eyeblink that wouldn’t provide enough time for major alterations to the moon’s exterior.

The accumulating evidence has converted some prior naysayers, such as Alyssa Rhoden, a planetary scientist at the Southwest Research Institute in Boulder, Colo. “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 isn’t entirely convinced. The presence of an ocean at the same epoch as humanity’s exploration of the planets “would mean that we had to be seeing the solar system at an extremely special time,” he says. And even if there hasn’t been enough time for the putative ocean to fracture the surface, Nimmo notes, there should still be signs of contraction: Water takes up less space than ice, so a recently formed ocean would have created voids beneath Mimas’ crust that left visible scars. There are no hints of that, he says.

If the ocean hypothesis bears out, it would bolster the prospect that hidden seas abound in the outer solar system, such as the moons of Uranus. The possibility that the ocean is so young, geologically speaking, also excites Lainey, because some future probe might drill through the icy exterior and 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. “Whether there is life there, nobody knows.”