The Death Star with a secret
Mimas
Moon of Saturn · discovered 1789 by William Herschel
Mimas is the smallest and innermost of Saturn's major moons, a cold ball of ice just under 400 kilometres across. One enormous impact crater, Herschel, dominates its face and gives it an uncanny resemblance to the Death Star from Star Wars. For decades it looked like the very picture of a dead, frozen world.
Then the numbers stopped adding up. Careful tracking of how Mimas wobbles and drifts in its orbit now suggests that beneath its scarred shell there may be a liquid ocean, and a remarkably young one. If that is right, even the most unlikely little moons can hide water.
Key facts
- Diameter
- 396 km11% of Earth's Moon
- Mass
- 3.79 × 10¹⁹ kg
- Surface gravity
- 0.064 m/s²0.0065 g
- Density
- 1,149 kg/m³
- Distance from Saturn
- 185,539 km≈ 3.1 Saturn radii
- Orbital period
- 22.6 hoursSame as its rotation (tidally locked)
- Discovered
- 1789William Herschel
- Surface
- Heavily cratered water ice
- Surface temperature
- About −209 °C (64 K)
- Rotation
- Tidally locked: one face always towards Saturn
- Notable feature
- Herschel crater, about 139 km wide
- Named after
- Mimas, a Giant of Greek mythology
Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.
Surface and geology
Herschel crater is roughly 139 kilometres wide, about a third of the moon's own diameter. Its walls rise around 5 kilometres above the floor, and its central peak rises roughly 6 kilometres above the crater floor. The impact that made it came close to shattering Mimas altogether, and some scientists link fractures on the opposite side of the moon to the shock waves it sent through the interior.
The rest of the surface is saturated with craters, many of them overlapping, which tells us that Mimas has seen little resurfacing since its early history. Unlike Enceladus next door, there are no obvious smooth plains, fresh cracks or plumes. Cassini's thermal instrument did, however, reveal an odd pattern: a warmer region shaped like the 1980s video game character Pac-Man, thought to arise because high-energy electrons from Saturn's magnetosphere alter the texture of the ice on one hemisphere.
Interior and composition
With a density only slightly above that of water ice, Mimas must be made mostly of ice with a modest amount of rock. In 2014 Cassini measurements showed that the moon rocks back and forth slightly more than expected as it orbits, a libration that could be explained either by an oddly shaped rocky core or by an ocean beneath the ice.
In 2024 a team led by Valéry Lainey reported that subtle changes in Mimas' orbit fit best with a global ocean lying roughly 20 to 30 kilometres below the surface. Their models suggest the ocean formed within the last 25 million years or so, recently enough that it has not yet left marks on the surface. The result is still being tested, but it has turned Mimas from a textbook dead moon into a candidate stealth ocean world.
Orbit and rotation
Mimas circles Saturn in less than a day, closer in than any other major moon. Like Earth's Moon, it is tidally locked, so the same hemisphere always faces the planet. Its orbit is slightly elongated, and this eccentricity is the engine that could flex and warm the interior.
Mimas also sculpts Saturn's rings. Ring particles at the inner edge of the Cassini Division would complete exactly two orbits for every one of Mimas, and those repeated tugs help keep that broad gap relatively clear of material.
Origin
Mimas probably formed from icy material orbiting the young Saturn, as did the planet's other mid-sized moons. Some researchers have proposed that the inner moons are much younger than the Solar System, perhaps assembled from material spread out from an ancient, more massive ring system, though this idea remains debated. A recently formed ocean would fit with a moon whose orbit has changed significantly over time.
Exploration
William Herschel discovered Mimas in 1789 with his 1.2-metre telescope, and the giant crater now bears his name. Everything we know about its surface comes from spacecraft flybys, culminating in more than a decade of observations by Cassini.
Pioneer 11 (1979)
The first spacecraft to visit Saturn returned only fuzzy views of Mimas but helped refine its size and orbit.
Voyager 1 and 2 (1980–1981)
The Voyagers revealed Herschel crater for the first time and showed a heavily cratered, icy surface.
Cassini (2004–2017)
Cassini mapped Mimas in detail, made its closest pass at about 9,500 km in February 2010, discovered the Pac-Man thermal pattern and supplied the orbital data behind the ocean hypothesis.
The moons of Saturn
| Moon | Diameter | Distance | Orbit |
|---|---|---|---|
| Mimas | 396 km | 185,539 km | 22.6 hours |
| Enceladus | 504 km | 237,948 km | 32.9 hours |
| Tethys | 1,062 km | 294,619 km | 45.3 hours |
| Dione | 1,123 km | 377,396 km | 2.74 days |
| Rhea | 1,528 km | 527,108 km | 4.52 days |
| Titan | 5,150 km | 1,221,870 km | 15.9 days |
| Iapetus | 1,469 km | 3,560,820 km | 79.3 days |
Surprising facts about Mimas
- The Death Star resemblance is pure coincidence: Star Wars was released in 1977, but Herschel crater was not seen until Voyager 1 flew past in 1980.
- The impact that created Herschel crater is thought to have come close to breaking Mimas apart entirely.
- If the 2024 analysis is correct, Mimas' ocean may be only a few million to 25 million years old, making it younger than the extinction of the dinosaurs.
- Cassini's heat maps showed a warm-and-cold pattern on Mimas that looks remarkably like the arcade character Pac-Man eating a dot.
- Mimas is often cited as the smallest known body in the Solar System massive enough to have pulled itself into a nearly round shape.