Saturnian moon Mimas: Crunchy on the outside, sub-surface ocean on the inside

Data from Cassini suggests hidden depths beneath crater-ridden body

Mimas, a moderate-sized moon orbiting Saturn, is likely to possess liquid oceans hidden under its icy surface, according to a research paper published this week.

New analysis of data beamed back to Earth from the Cassini spacecraft suggests that the sub-surface water world may be young relative to other similar oceans in the solar system.

A team led by Valery Lainey, an astronomer at France's Observatoire de Paris, performed a detailed analysis of Mimas's orbital motion based on data from Cassini, the NASA probe that hung around Saturn before sinking into its gaseous surface in 2017. Their conclusion that there is likely to be a sub-surface ocean on the moon was a surprise.

"The presence of these long-lived global oceans is generally betrayed by surface modification owing to internal dynamics. Hence, Mimas would be the most unlikely place to look for the presence of a global ocean. Here, from detailed analysis of Mimas's orbital motion based on Cassini data, with a particular focus on Mimas's periapsis drift, we show that its heavily cratered icy shell hides a global ocean, at a depth of 20-30 kilometres," the paper published in Nature this week said.

Mimas has slightly less land area than California, although we hear real estate is cheaper. It has a crater so large, relative to its size, that it looks like Death Star, the famed planet-destroying space station from the Star Wars movie franchise.

Adding Mimas to the list of ocean worlds present in the solar system changes the picture of what these moons can look like, argue Matija Ćuk, of the SETI Institute, and Alyssa Rose, of Colorado's Southwest Research Institute, in an accompanying article.

"The idea that relatively small, icy moons can harbor young oceans is inspiring, as is the possibility that transformational processes have occurred even in the most recent history of these moons. Lainey and colleagues' findings will motivate a thorough examination of midsized icy moons throughout the Solar System," they said.

Celestial sub-surface oceans are a tantalizing prospect for hunters of alien life. In 2017, data collected by Cassini showed that the plumes from Saturn's moon Enceladus are made up of water vapor, hydrogen, carbon dioxide, methane, and ammonia, suggesting complex chemical reactions. Later research found phosphates in the ring resulting from the same plumes. Data from the Galileo probe also showed Jupiter's Europa exhibits similar plumes.

The interest in sub-surface moon oceans has provoked space agencies to launch a new generation of hardware at the distant planets and their natural satellites. ESA's Juice probe began its eight-year mission to Jupiter in April, while NASA is set to launch the Europa Clipper to the same planet system in October next year and expects it to arrive in 2030. ®

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