Does Saturn’s Moon Mimas Have a Hidden Ocean?

Rebecca Jean T.
7 min readFeb 13, 2024

A study published in Nature on February 7th may have just found evidence that Saturn’s moon Mimas has an ocean under its surface. Researchers believe that if this distant world contains an ocean, it is hidden 12 to 19 miles beneath the surface and is relatively young.

A black and white image of Mimas, a round moon with many small craters on its surface. On the right, a large crater with a small raised portion in the center.
Image of Saturn’s moon Mimas taken by NASA’s Cassini space probe in 2010. The image features Mimas’ famous Herschel crater, which has given it the nickname “Death Star” for its uncanny resemblance to the Star Wars planet destroyer. Credit: NASA / JPL-Caltech / Space Science Institute.

Evidence in Mimas’ Rotation and Orbit

This is not the first time astronomers have suggested an underground ocean on Mimas. In 2014, a look at data from NASA’s Cassini probe showed that the moon has a wobble in its rotation. While a slight wobble is normal (our own Moon wobbles slightly during its orbit), the 2014 paper found that Mimas’ wobble was twice as much as it should have been. These findings were consistent with a subsurface ocean or an elongated core.

The new 2024 research paper doesn’t just account for wobbles in Mimas’ rotation; it also looks at its orbit. Analyzing Cassini data from flybys of Saturn and Mimas, the team found that Mimas’ orbit had shifted, with the point of Mimas’ closet approach to the planet changing by about 6 miles. Simulations of Mimas with an elongated core suggested that it should’ve shifted by 12 miles instead. This discrepancy, given Mimas small size, is notable enough to suggest that the moon’s wobble is most likely not due to an elongated core.

A (Relatively) Young Ocean

If Mimas has a subsurface ocean, it would have to be young from a geological standpoint. When we look at ocean moons like Jupiter’s Europa and Saturn’s Enceladus, we see smooth, icy surfaces. This is because their subsurface oceans occasionally send new material through geysers and other geological activity. This smooths the surface above, covering any evidence of impacts. Meanwhile, Mimas has a surface covered in pits and craters, including one very large mark that gives Mimas its “Death Star” nickname. A young ocean that hasn’t had time to affect the surface could explain why this moon doesn’t look like what we would expect from a water world.

The surface of Europa in color. Just over half the moon is visible, with a pale blue surface and dark orange streaks running all over it.
Image of Jupiter’s moon Europa taken by NASA’s Galileo space probe in the late 1990s. Europa is one of the four moons discovered by Galileo Galilei during his time studying Jupiter. Credit: NASA/JPL-Caltech/SETI Institute.

The team behind the latest paper suggests that Mimas’ ocean is likely less than 25 million years…

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Rebecca Jean T.

Published author on NASA’s Radio Jove project. Researching science topics to deliver to you in bite-sized stories.