Saturn's moon Mimas, long known for its uncanny resemblance to the Death Star, might harbour a secret far more intriguing than its appearance. New analysis of data gathered by NASA's Cassini spacecraft suggests the presence of a vast ocean of liquid water beneath the moon's pockmarked surface. Published in Nature, the study reveals a subtle shift in the point of Mimas' orbit where it comes closest to Saturn over a 13-year period. This seemingly minuscule change, combined with previously observed wobbles in Mimas' rotation, points towards a hidden liquid interior, according to astronomer Valéry Lainey of the Paris Observatory and his team. "It's a very surprising result," says Francis Nimmo, a geologist at the University of California, Santa Cruz, who was not involved in the research. "Looking at Mimas, it doesn't scream 'moon with an ocean.'" This isn't the first hint of hidden water on Mimas, the smallest of Saturn's major moons at just 400 kilometres across. A 2014 study suggested a watery reservoir under the frozen shell based on slight wobbles in its rotation. However, some researchers doubted this possibility, arguing that Saturn's gravity would cause extensive cracks in the moon's icy surface if it held a hidden ocean. No such cracks have been observed. The new calculations suggest a different scenario: Mimas might have an ice shell 20-30 kilometres thick, followed by a 70-kilometre-deep ocean and a solid rocky core. To explain the lack of surface cracks, Lainey and his team propose that the ocean formed relatively recently, between 5 and 50 million years ago, not giving enough time for major changes to the moon's exterior. This theory has convinced some previous naysayers, like Alyssa Rhoden, a planetary scientist at the Southwest Research Institute. "I was the most sceptical of Mimas having an ocean," she admits, "but you have to follow the data, and it seems we're looking at a new ocean world." However, Nimmo remains cautious. He argues that finding an ocean during humanity's exploration of the solar system "would imply we're observing the system at an exceptionally unique time." Even if the young ocean hasn't cracked the surface, he adds, signs of contraction should still be evident. As water takes up less space than ice, a recently formed ocean would have created voids beneath Mimas' crust, leaving visible scars – which haven't been observed. If confirmed, this hidden ocean would strengthen the possibility of similar bodies on other outer solar system moons, like those orbiting Uranus. The potential youth of the ocean also excites Lainey, as a future probe could potentially drill through the icy shell and directly observe the interaction of liquid water with the rocky core. "It's a prime location to investigate the potential beginnings of life, though whether life actually exists there, nobody knows," concludes Lainey.
