That is a moon and an ice ball.
Mimas, a 250-mile-wide moon of Saturn, bears a resemblance to the Death Star in "Star Wars" because of its large crater carved out of its surface. Obi-Wan says: "That's no moon" when the Millennium Falcon first encounters the Death Star. It is a space station.
For eight years, scientists have been looking into the possibility of an ocean flowing 14 to 20 miles below the surface of Mimas, a pockmarked ball of ice frozen hard.
In the past few years, the ocean worlds of Jupiter and Enceladus have become top of the list for scientists who are considering places in the solar system where life could have arisen. This year, one NASA craft, Juno, will swoop past the moon for a closer look, and another craft, Europa Clipper, will arrive in 2030.
Unlike other icy moons that have under-ice oceans, Mimas has a surface that is not cracked or melting. It was shown that the interior of a moon could be warm enough to keep the ocean unfrozen.
A planetary scientist thought the idea of a Mimas ocean was unlikely, but now thinks it's plausible.
Rhoden is a specialist in the geophysics of icy moons at the Southwest Research Institute in Boulder, Colo. For Mimas to have an ocean would really challenge our intuition about him. I thought that that was not how scientists are supposed to work. We do not come to a conclusion without testing the hypothesis.
Dr. Rhoden, along with Matthew Walker of the Planetary Science Institute, created a computer simulation to explore the tidal forces of the planets. The heat generated by the tides could be enough to maintain the ocean.
Dr. Rhoden said this week that it works well.
The ocean formed relatively recently, one of the reasons for the lack of cracks. It could be either steady in size or getting bigger. Water expands in volume when it is frozen into ice, and the upward pressure causes the ice above to break.
Dr. Rhoden said that the ice shell couldn't be changed. It has to either be warm or stable.
The suggestion of a Mimas ocean was made by NASA's Cassini spacecraft. The same side of the moon always faces the ringed planet, just as we on Earth see only one side of Earth's moon. Scientists reported a bigger-than- expected wobble in Mimas's rotation. It was possible that the core of Mimas was stretched out in the direction of the moon or the ocean.
The lead author of the paper that was published in the journal Science said that he lost hope in the ocean. The paper shows that if you use reasonable ice properties and apply a more sophisticated model, you can have an ocean inside and survive.
William McKinnon is a professor of earth and planetary sciences at Washington University. He said in an email that it was hard to believe. There is no mention of the ocean or high heat flow on Mimas's surface.
The stretched solid interior is also plausible. There is a chance that a future robotic probe will be able to make more detailed measurements of Mimas.
Dr. Tajeddine said it was another piece in the puzzle. The paper says that an ocean is not crazy.
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