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It’s a moon… and an ice ball filled with water?
With a large crater carved from its surface, Saturn’s 250-mile-wide moon Mimas bears more than a passing resemblance to the Death Star from “Star Wars.” (When the Millennium Falcon first encounters the Death Star, Obi-Wan Kenobi ominously says: “This is not the moon. It’s a space station.”)
For eight years now, scientists have thought that Mimas, a seemingly hard frozen ball of ice, may be hiding a secret: an ocean flowing 14 to 20 miles below the surface.
In recent years, such oceanic worlds — Europe on Jupiter and Enceladus on Saturn, two names – jumped to the top of the charts for scientists considering places in the solar system where life could arise. A NASA spacecraft Juno, This year it will cross Europa for a closer look, and another mission, the Europa Clipper, will arrive there in 2030 on a special mission.
But unlike other icy moons known to have sub-ice oceans, Mimas has a surface within it that presents no signs of cracking or melting that might suggest sloshiness. It also increased scientific credibility that the interior of a moon as small as Mimas could be warm enough to keep an ocean frozen.
A planetary scientist who thought the idea of a Mimas ocean was unlikely, now finds thermodynamics plausible.
“I changed my mind quite recently,” said Alyssa Rhoden, a geophysics expert on icy satellites at the Southwest Research Institute in Boulder, Colo. Having an ocean for Mimas would really challenge our intuition about Mimas. And when I realized that, I thought scientists shouldn’t work like that. We can’t come to a conclusion without actually testing the hypothesis.”
Dr. Rhoden, together with Matthew Walker of the Planetary Sciences Institute in Tucson, Arizona, designed a computer simulation to explore the tidal forces of Saturn and Mimas. They found that the heat produced by the tides that would compress the Moon may be enough to sustain the hypothetical ocean.
Dr. “It works really well,” Rhoden said this week.
One of the keys to explaining the absence of cracks is that the ocean, if any, formed relatively recently. It can also be fixed in size or larger. When water freezes into ice, its volume expands and the upward pressure breaks the ice above.
Dr. “The ice crust cannot be thickening today,” Rhoden said. “So Mimas must either warm up or be stable.”
A Mimas ocean proposal comes from measurements made by NASA’s Cassini spacecraft, which orbited Saturn from 2004 to 2017. Mimas’ orbit is tidally locked with Saturn: The same side of the Moon is always facing the ringed planet, just as we only see one on Earth. The moon’s side of the Earth. But in 2014, scientists reported a larger-than-expected wobble in Mimas’ rotation. This indicated that the core of Mimas was either lying in the direction of Saturn or was an ocean.
“Although we suggested the ocean as a possibility, I personally am starting to lose hope that it might actually have an ocean,” said Radwan Tajeddine, lead author of the study. 2014 article published in the journal Science. “The surprising thing about this paper is that it shows that if you just use reasonable ice properties and apply a more complex model, you can actually have an ocean in it and survive.”
William McKinnon, professor of earth and planetary sciences at Washington University in St. Louis, remains skeptical. “My short answer is that it’s hard to believe,” he said in an email. “There’s nothing on the surface of Mimas that says ‘ocean’ or ‘high heat flux’, unlike Enceladus.”
The other possibility – a stretched solid interior – also remains plausible. Answers may have to await a future robotic probe to Saturn that can make more detailed measurements of Mimas.
Dr. “This is another piece of the puzzle,” Tajeddine said. “This article says the ocean is not a crazy idea.”
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