That’s a moon … and a waterlogged ice ball?
With a large crater carved out of its surface, Mimas, a 250-mile-wide moon of Saturn, bears more than a passing resemblance to the Death Star in “Star Wars.” (When the Millennium Falcon first encounters the Death Star, Obi-Wan Kenobi ominously says: “That’s no moon. It’s a space station.”)
For eight years, scientists have been considering that Mimas, seemingly a pockmarked ball of ice frozen hard, might be hiding a secret: an ocean flowing 14 to 20 miles below the surface.
In recent years, such ocean worlds — Europa at Jupiter and Enceladus at Saturn, to name two — have jumped to the top of the lists for scientists who are considering places in the solar system where life could have arisen. One NASA spacecraft, Juno, will swoop past Europa for a closer look this year and another mission, Europa Clipper, is to arrive for a dedicated mission there in 2030.
But unlike other icy moons known to possess under-ice oceans, Mimas has a surface that offers no hints of cracks or melting that might suggest sloshiness within. It also stretched scientific credulity that the interior of a moon as small as Mimas could be warm enough for an ocean to remain unfrozen.
A planetary scientist who thought the idea of a Mimas ocean was unlikely now finds the thermodynamics to be plausible.
“I did change my mind fairly recently,” said Alyssa Rhoden, a specialist in the geophysics of icy moons at the Southwest Research Institute in Boulder, Colo. “I was saying Mimas can’t have an ocean, but what I was really saying was, for Mimas to have an ocean would really challenge our intuition about Mimas. And when I realized that, I thought, well, that’s not how scientists are supposed to work. We don’t come to a conclusion without actually testing the hypothesis.”
Dr. Rhoden, along with Matthew Walker of the Planetary Science Institute, headquartered in Tucson, Ariz., devised a computer simulation to explore the tidal forces of Saturn and Mimas. They found that the heat generated by the tides, which would squeeze the moon, could be enough to maintain the hypothesized ocean.
“It works really beautifully,” Dr. Rhoden said this week.
One of the keys for explaining the lack of cracks is that the ocean, if it exists, formed relatively recently. It may also be either steady in size or getting bigger. When water freezes into ice, it expands in volume, and the upward pressure would fracture the ice above.
“The ice shell cannot be thickening today,” Dr. Rhoden said. “So Mimas has to either be warming or it has to be stable.”
The suggestion of a Mimas ocean comes from measurements by NASA’s Cassini spacecraft, which orbited Saturn from 2004 to 2017. Mimas’s orbit is tidally locked with Saturn: The same side of the moon always faces the ringed planet, just as we on Earth see only one side of Earth’s moon. But in 2014 scientists reported a bigger-than-expected wobble in Mimas’s rotation. That suggested either the core of Mimas was stretched out in the direction of Saturn or there was an ocean.
“Even though we suggested this, the ocean as a possibility, I personally started to lose hope that it might actually have an ocean,” said Radwan Tajeddine, the lead author of the 2014 paper, published in the journal Science. “What’s amazing about this paper is that it actually shows that if you just use reasonable ice properties and apply a more sophisticated model, you can actually have an ocean inside and surviving.”
William McKinnon, a professor of earth and planetary sciences at Washington University in St. Louis, remains skeptical. “My short answer is that this is hard to believe,” he said in an email. “There isn’t anything about Mimas’s surface that says ‘ocean’ or ‘high heat flow,’ unlike Enceladus.”
The other possibility — a stretched solid interior — also remains plausible. Answers might have to await a future robotic probe to Saturn that could make more detailed measurements of Mimas.
“It’s another piece in the puzzle,” Dr. Tajeddine said. “This paper says that an ocean is not a crazy idea.”
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