Saturn’s “Death Star Moon” Mimas Probably has an Ocean Too

A recent study published in Nature presents a groundbreaking discovery that Saturn’s moon, Mimas, commonly known as the “Death Star” moon due to its similarities with the iconic Star Wars space station, possesses an internal ocean underneath its rocky crust. This study was conducted by an international team of researchers and holds the potential to help planetary geologists better understand the conditions for a planetary body to possess an internal ocean, which could also possess the conditions for life as we know it. While Mimas was photographed on several occasions by NASA’s Cassini spacecraft, including a close flyby in February 2010, what was the motivation behind this recent study regarding finding an internal ocean on Mimas?

Dr. Gabriel Tobie, who is a planetary scientist at Nantes Université in France and a co-author on the study, tells Universe Today, “One of the initial motivations to study Mimas was to understand why it is so different from the neighboring moon, Enceladus, which is characterized by a very active surface with direct communication with a global surface ocean. On Enceladus, we know that all the observed activity is controlled by tidal forces generated by Saturn. Mimas is closer to Saturn and should normally experience even more intense tidal forces. So why Mimas’ lack sign of activity?”

Discovered by William Herschel on September 17, 1789, Mimas is best known for its Death Star appearance due to Herschel Crater, which spans 139 kilometers (86 miles) in diameter, or just over one-third the diameter of Mimas at 396 kilometers (246 miles). Unlike other ocean worlds like Europa and Enceladus, whose surfaces are largely devoid of craters due to the frequent resurfacing from their respective internal oceans, the surface of Mimas possesses countless craters with no indications of resurfacing. Therefore, the debate for Mimas possessing an internal ocean has raged for years, including a 2014 study published in Science and a 2017 study published in JGR: Planets.

2022 video discussing the possibility of an ocean on Mimas based on research at the time.

Dr. Tobie continues by telling Universe Today, “It was initially thought that Mimas remained frozen since its formation and that the conditions to initiate ice melting in its interior were never met. This new finding we report in this study shows that Mimas in fact is not that different than Enceladus. It also has a global ocean, but in contrast to Enceladus, such an ocean was formed very recently, explaining the lack of surface activity.”

After analyzing data from NASA’s Cassini, the researchers concluded that an internal ocean exists on the heavily cratered Mimas approximately 20-30 kilometers (12-18 miles) beneath its surface, forming less than 25 million years ago, which is young in geologic terms. Additionally, the team concluded the juncture where the internal ocean and ice interact reached less than 30 kilometers (18 miles) from the surface only 2-3 million years ago, indicating the ocean is potentially still developing and growing. Therefore, what implications does finding an ocean on Mimas have for other potential ocean worlds in our solar system?


Saturn’s moon, Mimas, captured by NASA’s Cassini spacecraft in 2010. (Credit: NASA/JPL-Caltech/Space Science Institute)

Dr. Alyssa Rhoden, who is a Principal Scientist at the Southwest Research Institute (SwRI) in Boulder, Colorado and co-authored an article in Nature discussing the groundbreaking discovery, tells Universe Today, “As far as stealth ocean moons go, Mimas pretty much takes the cake. Its surface betrays nothing of the ocean underneath. Icy moons around Uranus, for example, do show some geologic activity on their surfaces that have (in absence of other options) have been attributed to oceans enabling tidal stresses and/or enhanced heating to drive processes like convection. More importantly, Mimas is showing us that it’s not too late for moons to undergo massive changes. Maybe triggering ocean formation later in a moon’s lifetime is more common in this size range of moons than we initially considered.”

As noted, an internal ocean on Mimas indicates it could possess habitable conditions for life as we know it like Europa and Enceladus. This is because scientists have hypothesized that internal