A new ocean in our solar system

Out of the roughly 290 moons in our solar system, the average person only really cares about the one in our night sky. However, every few years, we make a discovery on or about a moon millions of miles from our planet that is carried by virtually every headline in the world. I was one of the kids in 2012-2013 reading one such headline about Titan’s discoveries and nothing would interest pre-teen me more than the possibility of extra-terrestrial life on a moon in our solar system.

Similarly, in early 2024, Lainey et al ¹ published their findings in Nature about a sub-surface ocean on one of Saturn’s moons - Mimas.

This was surprising because Mimas’s surface looks old and inert, showing none of the usual signs (like cracks or geysers) that betray an internal ocean. Yet by carefully analyzing Mimas’s orbital and rotational behavior using data from NASA’s Cassini spacecraft, researchers found telltale clues pointing to a hidden sea under the ice.²

Irregularities that led to the discovery

Despite Mimas’s “Death Star” appearance – a 400 km-wide ice ball covered in craters – subtle irregularities in its orbit and rotation revealed something unusual inside. Cassini mission data showed that Mimas does not move exactly as a solid, rigid body would. Astronomers observed peculiarities in Mimas’s orbital motion, such as an unexpected drift in its periapsis (the point of closest approach in its orbit) and small wobbles in its rotation (librations).

These clues led scientists to suspect two possibilities: either Mimas contains an oddly elongated solid core or it has an internal ocean that decouples the crust from the core. By comparing thousands of Cassini images and precise tracking of Mimas’s movement against computer models, the team found that only the ocean hypothesis could explain the data. In other words, Mimas’s icy outer shell appears to be slipping over a liquid layer, causing the moon to wobble and orbit in a way that a solid Mimas could not. Crucially, Cassini had seen no fractures or geysers on Mimas’s surface, so these orbital anomalies were the sole evidence of the ocean – making Mimas a “stealth” ocean world.

What do we know?

The data so far implies that Mimas has a global ocean of liquid water sandwiched between its core and icy crust. The rigid outer ice shell is estimated to be only about 20-30cm thick, and under this layer there’s an ocean 80 km deep according to some estimates, which is an enormous volume given the moon’s small size. The ocean is believed to be global, extending under the entire surface rather than a localized pocket, and more importantly the water layer is in direct contact with the silicate rocky core at the center.

The ocean’s temperature is not uniform and may reach “tens of degrees Celsius” according to model calculations.³ This is above the freezing point, indicating active heating. The water is likely a briny liquid, like other icy-moon oceans, but remains liquid because of internal heat. I first thought antifreeze compounds like salt and ammonia might be at play but those were not reported and even pure water can stay liquid under the pressure and heat of Mimas. For context, the pressure at the base of ~70 km ocean under ice is a few hundred bar, when combined with elevated temperature, would keep the water from freezing solid. The total amount of liquid water in Minas is small compared to Earth - roughly estimated at only ~1.3% of Earth’s ocean volume, but its remarkable because the moon itself is much smaller than the size of the Earth.

The thickness of the ice shell is about 20-30 km and is comparable to that of Enceladus suggesting a similar structure. However, Mimas’ shell appears unfractured at the surface, while Enceladus’ surface distinctly is.

Young Ocean & Recent heating

The discovery of an oceans forces a reevaluation of Mimas’s geological history and internal structure. Preciously, Mimas was thought to be a cold, geologically dead

Previously, Mimas was thought to be a cold, geologically dead world – essentially a frozen ice-rock ball shaped by ancient impacts. Its heavily cratered surface (dominated by the giant Herschel crater) attests to an old, inactive exterior. However, the new findings point towards an active interior that has only become so in the recent past (in geological terms at least) The ocean is estimated to have formed less than 25 million years ago, and more likely only 5-15 million years ago. Considering Mimas has been around ~4.5 billion years⁴, it means only recently did the conditions change from frozen solid to liquid water in its interior.

Scientists believe the trigger was tidal heating – the same process warming other ocean moons, but unexpectedly intense for tiny Mimas. Mimas orbits Saturn on a slightly eccentric (oval) orbit, which means Saturn’s gravity flexes the moon rhythmically. If the orbit’s eccentricity were to increase significantly, the tidal flexing would become much stronger, heating the interior through friction (imagine squeezing and releasing a rubber ball repeatedly, which heats it up). The research team’s simulations suggest that gravitational interactions with other Saturnian moons pumped up Mimas’s orbital eccentricity to about three times its previous value at some point in the recent past. his could happen if Mimas fell into a temporary orbital resonance or alignment with other moons. The enhanced eccentricity led to intense tidal deformation of Mimas’s core, “warming it like a massaged squash ball” and melting the overlying ice.

In this scenario, Mimas’s once-solid interior began to melt from the bottom up, eventually creating a global ocean under the crust.⁵ Notably, the models indicate the ocean-ice boundary only reached as shallow as ~30 km below the surface in the last ~2-3 million years (sounds like a lot but is just 0.04% of its lifespan). This recency explains why Mimas’s surface hasn’t yet cracked or shifted: the ocean is so new that the icy crust has not had time to respond with tectonic activity. Basically, Mimas’s outward appearance is a snapshot of its old frozen state, while its interior is now undergoing a dramatic, still in-progress transformation. This is also very interesting because of its paradoxical nature of looking ancient but being fairly young internally.

Astrobiological Potential

The identification of a liquid water ocean inside Mimas has exciting implications for astrobiology and mission planning, though with some caveats. On one hand, liquid water in contact with a rocky core is one of the key ingredients for life as we know it. In Mimas’s ocean, water touching the silicate core could enable water–rock chemistry (for example, hydrothermal reactions) that produce nutrients or building blocks for life.

The Saturn system already has known habitable-zone candidates like Enceladus, where hydrothermal vents likely exist on the seafloor. Mimas now joins the exclusive club of ocean worlds (with Enceladus, Titan, Europa, Ganymede, etc.) and is notably the smallest such moon known to host a subsurface ocean. This suggests that even tiny, unassuming moons can maintain the conditions required for liquid water, expanding the range of potential habitats in the solar system.

However, there are significant challenges and uncertainties regarding life in Mimas. The ocean is very young, on the order of only ten million years or less. Life on earth took hundreds of millions years, if not longer to get started in the oceans, so a newly formed ocean might not have had time to develop even primitive life. There’s no known communication between its ocean and the surface. So even any possible life would be not be easy to detect from the surface. Considering both of these factors scientists believe that there are easier places to search for extraterrestrial life in the solar system like Enceladus’s plumes or Europa’s spots of thin ice.

That being said, Mimas offers a very unique chance to study a nascent ocean world in its early stages. Since it likely formed recently, it provides a natural laboratory for observing the initial conditions and processes that occur when an icy body first becomes an ocean world. This could yield insight into how habitability starts on icy moons. Future missions could target Mimas to answer key questions using ice-penetrating radar, gravity measurements and magnetometry to confirm the ocean’s presence and measure its properties.

Conclusion

the discovery of a subsurface ocean on Mimas fundamentally changes our picture of this little moon. It demonstrates that even the most unassuming celestial bodies can surprise us, and it broadens the criteria for where life-sustaining conditions might arise.

This revelation will undoubtedly influence future exploration of Saturn’s moons, as scientists seek to learn how common (or how fleeting) such hidden oceans are, and whether life could brew in these unlikely havens beneath the ice.

Sources

Footnotes

1. Lainey, V. et al. (2024). A recently formed ocean inside Saturn’s moon Mimas. Nature, 626: 280–282. Link to article ↩

2. Queen Mary Univ. of London / ScienceDaily (Feb 8, 2024). “Mimas’ surprise: Tiny moon of Saturn holds a young ocean beneath its icy shell.” Link to article ↩

3. Ian Sample, The Guardian (Feb 7, 2024). “Saturn’s ‘Death Star’ moon has hidden ocean under its crust, say scientists.” Link to article ↩

4. Will Dunham, Reuters (Feb 7, 2024). “Saturn’s 'Death Star' moon has a hidden secret – a subsurface ocean.” Link to article ↩

5. Observatoire de Paris / IMCCE Press Release (Feb 2024). “Presence of a ‘young’ ocean under the Mimas ice sheet.” Link to article ↩