The first is what it looks like. Mimas has an enormous crater on one side called Herschel, which is so wide and so well-placed that the whole moon looks uncannily like the Death Star from Star Wars.
The resemblance is real, not designed — and yes, the timing is funny (more on that below). The second big thing is what scientists discovered hiding inside Mimas in February 2024: a global ocean of liquid water beneath its frozen surface.
Nobody expected it. For decades, Mimas was the example scientists used of a small moon that absolutely did not have a hidden ocean.
Then, in 2024, careful analysis of how Mimas wobbles as it orbits Saturn proved that it actually does. The whole picture of where liquid water can exist in the solar system shifted in a single paper.
Mimas is small. Mimas looks dramatic. And Mimas turns out to be one of the most surprising worlds in our solar system.
Mimas is one of Saturn's smaller moons — about 396 km across, made mostly of water ice. It was discovered in 1789 by the British astronomer William Herschel, using the largest telescope in the world at that time.
Mimas is famous for its huge Herschel crater (named after its discoverer) which is so big it covers nearly a third of the moon's face. The crater makes Mimas look uncannily like the Death Star from Star Wars — the resemblance was noticed after the Voyager 1 spacecraft photographed Mimas in 1980, three years after Star Wars first came out.
In February 2024, scientists announced an even bigger surprise: they found strong evidence that Mimas has a hidden global ocean of liquid water beneath its surface, probably only a few million years old. That would join Mimas with Enceladus, Europa and Titan as 'ocean worlds' in our solar system.
Quick Mimas facts
A pocket-sized list of the most asked-about Mimas numbers.
| Type | An icy moon of Saturn — Saturn's 7th-largest moon. |
| Diameter | About 396 km (250 miles) — about a tenth of Earth's Moon. |
| Made of | Mostly water ice (about 97%) with a small amount of rock. |
| Density | About 1.15 g/cm³ — only slightly denser than water. |
| Orbital distance | About 186,000 km from Saturn (closer than our Moon is to Earth). |
| Orbital period | About 22.6 hours (less than one Earth day). |
| Rotation | Tidally locked — the same face always points toward Saturn. |
| Surface temperature | About −209°C — even colder than most of Saturn's moons. |
| Discovered | 17 September 1789, by William Herschel. |
| Most famous feature | Herschel crater — about 130-140 km wide, with walls 5 km high and a central peak as tall as Mount Everest. |
| Hidden secret | Strong evidence for a global subsurface ocean of liquid water, announced in February 2024 — probably only a few million years old. |
| Named after | Mimas, a Giant from Greek mythology (a sibling of the Titans). |
Why Mimas looks like the Death Star
Mimas is famous, around the world, for one reason: it looks unmistakably like the Death Star space station from the original Star Wars films. A small grey-white sphere, scarred with craters — and on one side, a single enormous crater that creates exactly the look of the Death Star's giant superlaser dish. The resemblance is uncanny.
Here is the funny part. Star Wars came out in 1977.
Mimas had been discovered in 1789, but until the late 1970s, no one had ever seen it as anything more than a tiny dot of light — even the biggest telescopes on Earth couldn't make out any detail on its surface. The first close-up photographs of Mimas were taken by NASA's Voyager 1 spacecraft in November 1980, three years after Star Wars came out.
So the resemblance came to light only after Star Wars made the Death Star a famous shape — the designers of Star Wars couldn't have copied Mimas because nobody had ever properly seen it. It's a strange coincidence of cosmic timing.
The resemblance is so famous that NASA itself jokes about it. The official NASA page for Mimas has more than once linked the moon to the Star Wars comparison.
But Mimas is just a real little world, doing what it has been doing for billions of years — and the giant crater that makes it look like a battle station is actually a scar from one of the most violent impacts anywhere in Saturn's moon system. The Death Star is fictional.
Mimas is real, and arguably more interesting.
Herschel crater — the impact that almost destroyed Mimas
Herschel is the giant crater that gives Mimas its Death Star look. It is enormous — about 130 to 140 km wide (different sources give slightly different measurements), which covers nearly a third of the entire moon's face.
The walls of Herschel rise about 5 km above the surrounding surface (taller than most mountains on Earth), the floor of the crater plunges about 10 km below it, and in the very middle there is a central peak about 6 km tall — roughly the height of Mount Everest. Imagine a single hole in the ground big enough to hold a mountain the size of Everest, with walls higher than the Alps, on a tiny moon you could drive across in four hours.
That is Herschel.
The crater was made by an impact — something crashed into Mimas long ago, with enough energy that scientists believe it nearly broke the moon apart. The object that hit was probably the size of a small city — several kilometres across.
If it had been only a little bit bigger, or hit a little bit harder, Mimas would have shattered into millions of pieces, and there would be no moon there at all today. Instead, Mimas just barely survived.
The crater is named after William Herschel, the astronomer who discovered Mimas in 1789 — long before anyone could see this crater for themselves.
Picture a baseball-sized world. Now hit it with a marble.
Hard. The marble carves a crater that covers a third of the baseball, with a walls higher than your hand, a floor deeper than your finger can reach into, and a tiny mountain in the middle.
The baseball just barely doesn't crack. That's roughly the proportion of what happened to Mimas.
The 'baseball' kept spinning around Saturn afterwards — but it has worn that scar for hundreds of millions of years.
Scientists used to assume that the Herschel impact was so violent that it would have completely shattered any hidden ocean Mimas might have had. Whatever was beneath Mimas's icy crust at the time of the impact would have been broken open.
That, until very recently, was one big reason astronomers were sure Mimas didn't have any ocean inside it. Which brings us to the surprise of 2024.
The first close-up photographs of Mimas came from the Voyager 1 spacecraft in November 1980.
The 2024 surprise — Mimas has a hidden ocean
On 7 February 2024, an international team of scientists led by Dr Valéry Lainey at the Paris Observatory published a paper in the scientific journal Nature that overturned decades of thinking about Mimas. They had found, hidden beneath that battered icy surface, a global ocean of liquid water. It was an enormous surprise — because Mimas was the moon that was supposed to definitely not have one.
How do you find an ocean you can't see?
From Earth (and even from spacecraft flying past), the surface of Mimas just looks like a heavily-cratered ball of ice. There are no cracks, no plumes of water shooting into space, no signs of recent geological activity — none of the things that gave away the hidden ocean of Saturn's other ocean moon, Enceladus. So how did the 2024 team find Mimas's ocean?
The answer is in how Mimas wobbles as it goes around Saturn. Every moon has a tiny rocking motion in its orbit, called libration.
Scientists can measure libration very precisely — and importantly, the size of the wobble depends on what's inside the moon. A moon with a completely solid interior wobbles one way.
A moon with a sloshing liquid ocean inside wobbles a different way. By carefully analysing 13 years of Cassini-mission data on Mimas's exact position over time, the 2024 team showed that Mimas's wobble could not be explained by a fully solid interior.
It needed a liquid layer. The simplest explanation was a global ocean of water — about 20 to 30 km below the surface, beneath the ice shell.
Explore the Cassini-Huygens mission whose data revealed Mimas's hidden ocean.
Why this was such a shock
Here is what made the 2024 discovery so dramatic. Mimas had been the test case for a rule scientists call the 'Mimas Test'.
The rule was: any theory that explained why Saturn's moon Enceladus has a hidden ocean needed to also explain why Mimas, very similar in size and made of similar stuff, definitely does not. Mimas looked geologically dead.
It had no plumes, no fresh ice, no fractures, no surface activity. The Herschel impact would have broken open any ocean that was there at the time.
So if Mimas could keep a tiny ocean, why was Enceladus so much more dynamic? The Mimas Test was used for years to constrain theories about ocean moons.
And then the 2024 paper showed that Mimas did, after all, have an ocean. The Mimas Test wasn't broken because the rule was wrong — it was broken because we discovered the rule was based on a false fact. Mimas was not the simple solid moon scientists thought it was. It just looks that way.
Mimas's ocean is almost unimaginably young, in solar system terms. Most ocean worlds — Europa, Ganymede, Enceladus, Titan — have had liquid water inside them for billions of years.
Mimas's ocean appears to be only a few million years old. That sounds like a long time, but it's a tiny sliver of the age of the solar system.
It is so young that life would not have had nearly enough time to form there. But the discovery is still big news, because it means small moons can grow oceans relatively quickly under the right conditions — opening up many more places to look for hidden water elsewhere.
Where the heat comes from
So how does a small icy moon like Mimas have a liquid ocean inside it, when its surface temperature is colder than −200°C? The answer is the same as for Enceladus and many other ocean moons: tidal heating from Saturn.
Mimas orbits in an elliptical (not quite circular) path around Saturn, so the gravitational pull from Saturn changes a tiny bit during each orbit. That stretching-and-relaxing flexes Mimas's interior — and friction from that flexing generates heat.
It's the same effect that warms up a paperclip if you bend it back and forth many times. Over millions of years, that tiny constant flexing produced enough heat to melt some of the interior ice into liquid water.
The result is the global ocean we now know is there.
Tidal heating: the warming of a moon's interior caused by the constantly changing gravitational pull from its parent planet. As the moon orbits in an elliptical path, the planet's gravity stretches and squeezes it slightly.
That flexing creates friction inside the moon, which produces heat. Tidal heating is the reason small worlds far from the Sun, where sunlight is too weak to do anything, can still have liquid water inside them.
It's powered by gravity, not sunlight.
Mimas vs Enceladus — two very different ocean moons
Saturn now has two confirmed ocean moons: Enceladus and Mimas. They are similar in size — both small icy worlds, both made mostly of water ice, both kept warm inside by tidal heating from Saturn. But they could hardly be more different in personality.
Enceladus is geologically alive on the outside. It has cracks across its south pole called 'tiger stripes', from which fountains of salty water and ice shoot into space — plumes that Cassini flew through several times.
The Enceladus ocean is ancient (billions of years old), is in contact with a rocky seafloor, and has all the chemistry scientists think is needed for life. Enceladus is one of the top targets in the whole solar system for searching for alien microbes.
Mimas, by contrast, looks completely dead on the outside. There are no plumes, no cracks, no recent activity at all.
Its ocean appears to be very young (probably only a few million years old), is hidden under a thick ice shell, and has not yet had time to develop the chemical conditions for life. If life is going to form in Mimas's ocean, it will need a lot more time.
But the very fact that Mimas appears to have an ocean at all is exciting, because it suggests that almost any icy moon, anywhere in the universe, might have liquid water inside if the right tidal heating conditions exist.
Picture two snowballs. Both have water inside them.
The first snowball has cracks on its outer surface, and water keeps leaking out. The second snowball looks completely sealed — but if you cut it open, you'd find water inside there too.
That's the difference between Enceladus and Mimas. Enceladus is leaky.
Mimas is sealed. Both contain water.
Just over four years ago, we didn't even know the sealed one did.
Meet Enceladus, the other ocean moon — leaky and full of plumes.
Why Mimas is named after a Giant
Like all of Saturn's biggest moons, Mimas is named after a figure from Greek mythology — specifically, after Mimas, one of the Giants (called the Gigantes in Greek) who fought in the great war between the Giants and the Olympian gods. In the story, Mimas was a fearsome warrior Giant who was eventually defeated by the god Hephaestus during the war.
The name was given by the British astronomer John Herschel — son of William Herschel, who discovered the moon — when he proposed in 1847 that all of Saturn's moons should be named after Titans and Giants from Greek mythology. The pattern fit the planet itself, which is named after Saturn, the Roman god equivalent to the Greek Cronus (leader of the Titans). Mimas the Giant fits the Saturn family of names just as well as Mimas the moon fits its place in Saturn's collection.
There's also a little extra fit to the naming. In some versions of the myth, the Giant Mimas was struck down with extreme force — leaving a wound that mirrors, in a strange poetic way, the giant Herschel crater that scars the moon Mimas.
The Greeks didn't know any of that when they told the story. The moon just happens to wear its mythology on its face.
Read more about Saturn in mythology — the god, the Giants, and how all the moons got their names.
Why Mimas matters
Mimas is a small moon. By every measure of size, it sits well below Titan, Enceladus, and even some of Saturn's other moons. For most of the time astronomers have known about it, Mimas was mostly remembered for two things: a giant crater that survived almost destroying it, and a coincidence with Star Wars.
Then 2024 changed everything. The discovery of a hidden, young, global ocean inside Mimas means our solar system has yet another world where liquid water exists.
It also means that scientists have to rethink what 'looks like a dead, boring moon' really tells us. Mimas looks dead.
Mimas looks heavily cratered, frozen, ancient. And inside that dead-looking shell, a brand-new ocean is sloshing around right now, only millions of years old.
Most of the solar system's hidden surprises are probably still waiting to be found.
So the next time you see a photo of Mimas — that tiny grey moon with the giant Herschel crater that looks so much like the Death Star — remember: it's not just a battle-station look-alike. Inside, it probably has a hidden ocean, only a few million years old, born long after the dinosaurs went extinct on Earth, still warming up. Mimas turned out to be much more than its surface ever showed.
- Mimas is a small icy moon of Saturn — about 396 km across, the planet's 7th-largest moon.
- It was discovered by William Herschel in 1789, using the world's largest telescope at the time.
- Its most famous feature is the Herschel crater — about 130-140 km wide, covering nearly a third of the moon's face.
- Herschel crater has walls 5 km high, a floor 10 km deep, and a central peak as tall as Mount Everest.
- The impact that made Herschel crater nearly broke Mimas apart.
- Mimas looks uncannily like the Death Star from Star Wars — a coincidence, since the first close-up Mimas photos came in 1980, three years after Star Wars.
- In February 2024, scientists discovered Mimas has a hidden global ocean of liquid water beneath its surface.
- The ocean appears to be only a few million years old — extremely young in solar-system terms.
- The discovery overturned the 'Mimas Test' — the rule scientists had used for years to constrain theories of ocean moons.
- Mimas is named after a Giant from Greek mythology, fitting Saturn's family-of-Titans naming pattern.
Mimas — frequently asked questions
Quick answers to the most common questions.
What is Mimas?
Mimas is one of Saturn's smaller moons — about 396 km wide, the 7th-largest of Saturn's moons. It is made almost entirely of water ice. It is best known for the giant Herschel crater on one side of it, which gives Mimas a striking resemblance to the Death Star from Star Wars.
Why does Mimas look like the Death Star?
Because of the Herschel crater — a single giant crater about 130 to 140 km across, which covers nearly a third of the moon's face. The size, shape, and placement of the crater create the same look as the Death Star's superlaser dish. The resemblance was noticed in 1980 when Voyager 1 took the first close-up photographs of Mimas. Star Wars had come out three years earlier in 1977, so it's a coincidence of timing — the Death Star wasn't based on Mimas, and Mimas obviously wasn't based on the Death Star.
How big is the Herschel crater?
Enormous in proportion to Mimas. The crater is about 130 to 140 km wide (sources vary slightly), with walls 5 km high (taller than most mountains), a floor 10 km deep, and a central peak about 6 km tall — roughly the height of Mount Everest. The crater covers nearly a third of the entire moon's face. The impact that made it almost shattered Mimas completely.
Does Mimas have an ocean?
Almost certainly — and this is one of the most recent surprises in planetary science. On 7 February 2024, scientists led by Dr Valéry Lainey at the Paris Observatory published a paper in the journal Nature presenting strong evidence that Mimas hides a global ocean of liquid water beneath its icy surface, about 20-30 km below the surface. The ocean appears to be only a few million years old, which is very young in solar-system terms. The discovery would add Mimas to other ocean worlds like Enceladus, Europa, and Titan in our solar system.
How did scientists discover Mimas's hidden ocean?
By measuring exactly how Mimas wobbles as it orbits Saturn. Every moon has a tiny rocking motion called libration, and the size of that wobble depends on what's inside the moon. The 2024 team used 13 years of Cassini-mission data to measure Mimas's wobble very precisely. The wobble was inconsistent with a fully solid interior — but matched perfectly if there was a liquid layer beneath the ice. That liquid layer is the ocean.
Why is Mimas's ocean so young?
Scientists think Mimas's interior took a long time to warm up enough to melt some of the ice into a liquid ocean. The heat source is tidal flexing from Saturn — gravity stretching and squeezing Mimas as it orbits — which is slow but steady. Over millions of years, the friction from that flexing produced enough heat. The ocean has only existed for the last few million years; for most of Mimas's history, it was completely frozen all the way through.
How big is Mimas compared to Earth's Moon?
Much smaller. Earth's Moon is about 3,475 km across — Mimas is only about 396 km. So Earth's Moon is about 9 times wider than Mimas. By volume, Earth's Moon is roughly 700 times bigger. Mimas would fit inside Earth's Moon hundreds of times over.
Who discovered Mimas?
The British astronomer William Herschel, on 17 September 1789. He used his huge 12-metre telescope (the largest telescope in the world at the time) to spot Mimas as a tiny dot near Saturn. The Herschel crater is named after him. His son John Herschel later named the moon Mimas in 1847, after one of the Giants in Greek mythology.
Is Mimas tidally locked to Saturn?
Yes. Mimas always shows the same face toward Saturn, just as Earth's Moon always shows the same face toward Earth. That's why the Herschel crater is always visible from one particular angle as Mimas orbits. The Herschel-side face is sometimes called the 'leading hemisphere'.
What is the 'Mimas Test'?
Before 2024, scientists used Mimas as a benchmark: any theory that explained why Saturn's moon Enceladus has a hidden ocean had to also explain why Mimas — very similar in size and composition — did not. This rule was called the 'Mimas Test', and it helped constrain ideas about ocean moons for decades. The 2024 discovery overturned the Mimas Test entirely — it turns out Mimas does have an ocean, just a very young one.
Could there be life on Mimas?
Probably not — at least not yet. Mimas's ocean appears to be far too young (probably only a few million years old) for life to have had time to start there. Compare that to Enceladus, whose ocean has existed for billions of years and is now considered one of the best places in the solar system to search for alien microbes. Mimas is a fascinating new candidate for the ocean-worlds family, but Enceladus is still the better target for the search for life around Saturn.
Mini quiz — test your Mimas knowledge
Try these without scrolling back up. (Answers below.)
- What is the name of the giant crater on Mimas?
- Roughly how wide is Mimas across its diameter?
- Which fictional space station does Mimas famously resemble?
- In what year did scientists confirm Mimas has a hidden ocean?
- Where is Mimas's ocean — on the surface, in the atmosphere, or beneath the ice?
- How was Mimas's hidden ocean discovered?
- True or false: Mimas's hidden ocean is one of the oldest in the solar system.
- Imagine if Mimas had been hit by something just a little bit bigger than the object that made the Herschel crater. What might have happened?
- Challenge: explain in your own words why the 2024 Mimas-ocean discovery was such a big surprise to scientists.
1) Herschel crater, named after William Herschel, who discovered Mimas in 1789.
2) About 396 km (about 250 miles) across.
3) The Death Star from Star Wars.
4) February 2024 — published on 7 February in the journal Nature.
5) Beneath the ice — about 20 to 30 km below the surface.
6) By measuring exactly how Mimas wobbles as it orbits Saturn (its libration). The size of the wobble matched a moon with a liquid ocean inside, not a moon with a solid interior.
7) False. Mimas's ocean appears to be unusually young — probably only a few million years old. Other ocean moons like Enceladus, Europa, and Ganymede have had liquid water inside them for billions of years.
8) Several good answers. Mimas would probably have shattered completely — the Herschel impact almost broke it apart as it was, and a slightly bigger impact would have finished the job. Saturn would have one fewer moon today, and there might be another small ring around Saturn made of the pieces.
9) Several good answers. Scientists had used Mimas for years as the example of a small moon that definitely did not have a hidden ocean — they called it the "Mimas Test." The 2024 discovery showed they were wrong, which means many other moons that look "dead" on the outside might secretly have oceans too. It widens where in the solar system we should look for hidden water and possibly life.
Teacher and parent notes
Hooks for using this page in lessons or homework time.
Curriculum links:
- UK KS2 Year 5 — Earth and Space: moons of other planets; impact craters; the search for water in the solar system.
- UK KS3 Physics — gravity, tidal forces; how spacecraft measure distant objects; how scientific findings can be overturned by new data.
- US NGSS MS-ESS1 — Earth's place in the solar system; MS-LS4 / MS-PS4 — how scientific evidence is gathered and revised over time.
- Cross-curricular: science history (how a 2024 paper changed long-held beliefs); language (the Star Wars cultural connection); maths (the crater is about 130-140 km wide on a 396 km moon — what fraction does that cover? use 130 km for the maths); geography (how impact craters here on Earth compare to the Herschel crater on Mimas).
Talking-point prompts:
- "For years, scientists used Mimas as the example of a moon that definitely did not have an ocean. Then a 2024 paper showed they were wrong. Is that a failure of science — or is that how science is supposed to work?"
- "The Herschel crater covers a third of Mimas. What does that look like in proportion? What would an equivalent crater look like on Earth?"
- "Mimas's ocean appears to be only a few million years old. Earth has had oceans for over 4 billion years. Why might Mimas's ocean be too young for life to have started there yet?"
Activity suggestions:
- Proportion drawing: have students draw a circle for Mimas (396 km wide) and then add the Herschel crater (130 km wide) in proportion. Discuss how big a third of the face really looks.
- Libration demonstration: use a ball on a string, swung in a circle, with a marble taped inside that can shift around. Show how a ball with a sloshing weight inside wobbles differently from a ball with a fixed weight. Discuss how scientists used this same idea to find Mimas's ocean.
- Timeline of discovery: build a Mimas timeline showing 1789 (discovery), 1847 (named after the Giant), 1980 (first photographs), 2010 (Cassini's closest flyby), 2024 (hidden ocean discovered). Each date is a step in our growing understanding.
- Star Wars vs Mimas comparison: have students look at both images side by side and list what's the same and what's different. (Mimas isn't a perfect sphere, the crater is rougher, etc.) Discuss why coincidences happen in the real world.
More Saturn adventures
Pick what you want to explore next.
The other ocean moons
- Enceladus, the geyser moon — Saturn's other ocean moon, with cracks and plumes of water shooting into space.
- Titan, Saturn's biggest moon — with methane lakes on the surface and a hidden water ocean deeper inside.
The missions that found Mimas's ocean
- The Cassini-Huygens mission — whose 13 years of data revealed Mimas's hidden ocean.
- The Pioneer and Voyager flybys — Voyager 1 took the first close-up photos of Mimas in 1980.
The naming story
- Saturn in mythology — Mimas was a Giant from Greek mythology, and all of Saturn's moons follow the same naming pattern.
Where Mimas fits in the family
- Saturn the planet itself — Mimas's home planet, with 285+ confirmed moons.
- How to see Saturn tonight — Mimas is too small to see from a back-garden telescope, but you can spot Saturn itself easily.
Sources and last updated
This page is fact-checked against current sources from NASA and peer-reviewed planetary science.
- Lainey, V. et al. (2024). 'A recently formed ocean inside Saturn's moon Mimas.' Nature, 7 February 2024 — the discovery paper for Mimas's hidden ocean.
- NASA — Mimas overview page (science.nasa.gov) for basic data: 396 km diameter, 22.6-hour orbit, tidally locked.
- NASA / JPL Cassini mission archive — Mimas Cassini imagery and ocean-discovery context.
- Sci.News, National Geographic, CNN, Reuters — multiple independent science-press summaries of the 2024 Nature paper.
- Space.com — 2025 follow-up coverage of the 'Mimas Test' and crater-formation simulations.
- Standard reference data on Herschel crater dimensions (130 km diameter, 5 km walls, 6 km central peak) — NASA / JPL imaging archives.
Last updated: May 2026. Mimas physical data (diameter 396 km, density, orbital period, tidal locking) verified against NASA mission pages.
Herschel crater dimensions verified against NASA / JPL imaging archives; the diameter is given as 130 km in most NASA references but cited as up to 139 km in some others, so the page uses an 'about 130-140 km' range where appropriate. Hidden-ocean evidence (Lainey et al., Nature, 7 February 2024) verified against the original paper and multiple independent science-press summaries.
Ocean age framed as 'probably only a few million years' to reflect the Nature paper's actual finding (the ocean interface reached its present depth less than 2-3 million years ago).
Written and fact-checked by the Planets for Kids editorial team.
Explore the Rest of Saturn
- Saturn Facts for Kids — The Complete Guide
- Saturn's Rings
- Saturn's Hexagon Storm
- Saturn's Seasons & Ring Cycle
- Saturn's Atmosphere
- How Saturn Formed
- Titan
- Enceladus
- Mimas
- Rhea
- Iapetus
- Pan, Daphnis & the Shepherd Moons
- Why Saturn Has So Many Moons
- Pioneer 11 & the Voyager Flybys
- The Cassini-Huygens Mission
- The Huygens Probe on Titan
- Dragonfly — Titan Quadcopter
- How to Observe Saturn
- Could Humans Live on Saturn?
- Saturn in Mythology
- Saturn vs Jupiter
- Saturn vs Earth
- For Teachers & Parents