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.
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?
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 oceans are created from internal heat generated from the moons being tugged and pulled as they orbit their respective planets, also known as tidal heating. While such internal oceans are completely absent from receiving sunlight due to their outer surfaces, scientists hypothesize that the internal heat that creates the ocean could also harbor hydrothermal vents where life has been observed to exist here on Earth. Therefore, what implications does finding an ocean on Mimas have for finding life beyond Earth?
“Finding an ocean on Mimas demonstrates that habitable environments may be found even in small objects far from the Sun, which is already a great discovery,” Dr. Tobie tells Universe Today. “However, the chance to detect any sign of life on such objects is extremely low, as there is no direct communication between the subsurface ocean and the surface. Enceladus, with its very active jets, is a much better target to address the question of life beyond Earth. Mimas, however, provides an opportunity to study the first stage of ocean formation and potentially chemical complexification before life emerged, a fundamental stage which is still unknown on Earth.”
In terms of follow-up studies, Dr. Tobie tells Universe Today that the methods used for this recent study could also be applied for other moons in the solar system, specifically moons orbiting Uranus, along with providing an opportunity to use Cassini data to re-evaluate not only Mimas, but other mid-sized moons orbiting Saturn, including Enceladus.
With this groundbreaking discovery, Mimas joins several other planetary bodies within the solar system that can be called ocean worlds, which include the aforementioned Europa and Enceladus, but also the dwarf planets, Ceres and Pluto; Jupiter’s moons, Ganymede and Callisto; Saturn’s largest moon, Titan; and Neptune’s moon, Triton. It is through these fantastic and mysterious worlds that scientists from around the world are studying to better understand the conditions for life to exist, both here on Earth and beyond.2
“The main take-away from Mimas is that we ought to test ideas, even if they seem unlikely,” Dr. Rhoden tells Universe Today. “Mimas’ surface doesn’t show evidence of an ocean, so it was easy to dismiss the ocean hypothesis when it was first proposed. But to come to a scientific conclusion, we have to back up inferences with tests. Sometimes, we find that the tests confirm our expectations, and sometimes, we get to be surprised.”
What new discoveries will scientists make about Mimas and other ocean worlds in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!
Add more system bodies with subsurface water:
“A team co-led by Southwest Research Institute found evidence for hydrothermal or metamorphic activity within the icy dwarf planets Eris and Makemake, located in the Kuiper Belt. Methane detected on their surfaces has the tell-tale signs of warm or even hot geochemistry in their rocky cores, which is markedly different than the signature of methane from a comet.” [by Southwest Research Institute]
The Saturn system timing is curious. While a few moons like Mimas have been crater dated to over 4 billion years of age, the ice ring system have been dust dated to within 100 million years. Besides showing the system dynamics, it may connect a putative late onset of Mimas internal heating with a specific orbital upset.