Astronomers may have discovered yet another subsurface ocean in our solar system, and its location may surprise you. The team from France's Observatoire de Paris analyzed orbital data from Saturn's moon Mimas, which is famous for its resemblance to the Death Star in Star Wars. It found that a global ocean is the only explanation for its unusual wobble. This confirms a previous analysis and could make Mimas a very appealing target for future exploration.
To make this discovery, the researchers returned to the greatest storehouse of data we have on Saturn and its moons: the Cassini-Huygens mission. Cassini orbited Saturn for more than 13 years, measuring the orbit of Mimas as it zipped around the gas giant. The team focused on small oscillations in Mimas' orbit known as librations. They compared observations with mathematical predictions of the planet's orbital characteristics. The Observatoire de Paris says the only thing that explained the difference between models and reality was a subsurface ocean.
The discovery of an ocean inside Mimas is surprising for a few reasons. For one, it's a small moon with a diameter of just 250 miles (about 400 kilometers)—in fact, it's the smallest known object that was shaped into a sphere by its own gravity. That means it doesn't retain internal heat for very long. The surface is also pockmarked with craters, including the second largest on any moon, which gives it the distinctive Death Star appearance. It does not, as far as we are aware, fire planet-destroying lasers. Most ocean worlds, like its fellow Saturnian moon Enceladus and Jupiter's Europa, show extensive surface remodeling and even geysers. That made their oceans easier to spot and is why Europa's ocean is the first to get attention with the upcoming Europa Clipper mission.
Further analysis suggests the ocean is global in nature, occupying the entire space between the icy shell and the moon's solid core. The data even revealed the thickness of the ice layer to be between 20 and 30 kilometers. That's in the same neighborhood as Enceladus and Europa. Mimas orbits near Saturn, so the team also modeled the tidal effects of the gas giant's enormous gravity. This helped the team establish that Mimas' ocean is probably very young, on the order of 5 to 15 million years old.
This fledgling ocean might greatly interest planetary scientists, even though it would be hard to inspect the water. Enceladus and Europa, with their fissures and geysers, offer more opportunities for study, but Mimas could be a unique and important place in the solar system. The research has been published in the journal Nature.
