Mimas, the "Death Star" moon
Mimas is the smallest and innermost of Saturn's major round moons, 396 km across, famous for its giant Herschel crater, which makes it look strikingly like the Death Star from Star Wars. Long assumed to be a frozen, inert ball of ice, Mimas became one of the big surprises of planetary science in 2024, when an analysis of its orbit pointed to a global ocean hidden beneath its crust, formed only within the last 25 million years or so.
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Discovery and name
William Herschel discovered Mimas in September 1789, about three weeks after Enceladus, using his 40-foot (12-meter) reflector, then the largest telescope in the world. Mimas orbits so close to Saturn that it is almost lost in the planet's glare, which is why it escaped earlier observers. John Herschel named it in 1847 after Mimas, one of the Giants of Greek mythology.
Mimas is one of the smallest bodies known to have been pulled into a roughly round shape by its own gravity. Saturn's tides stretch it slightly, so it is an ellipsoid elongated toward the planet, about 415 km along its longest axis and 381 km along its shortest.
Herschel crater
Voyager 1 imaged Mimas in November 1980 and revealed an enormous impact basin on its leading hemisphere, about 139 km wide, more than a third of the moon's diameter. Named Herschel, it has walls rising around 5 km, a floor up to about 10 km deep and a central peak roughly 6 km high, formed as the floor rebounded after the impact.
The impact that made it came close to shattering Mimas entirely. Fractures on the opposite side of the moon may be related to shock waves focused through its interior. The resemblance to the Death Star, the battle station of the 1977 film, is pure coincidence: the crater was discovered three years after the film's release.
The "Pac-Man" heat pattern
In 2010 Cassini mapped Mimas' daytime temperatures and found a sharp, V-shaped boundary between warmer and cooler regions, giving the leading hemisphere the shape of the 1980s video game character Pac-Man. The explanation is radiation: high-energy electrons from Saturn's magnetosphere preferentially strike one region of the moon and pack down its fluffy surface ice, which then conducts heat away faster and stays cooler by day. A similar pattern was later found on Tethys.
A hidden ocean?
Mimas has an unusually eccentric orbit for a moon so close to Saturn, and in 2014 a team led by Radwan Tajeddine found that it rocks back and forth (librates) more than a solid body should. Two explanations fitted: an elongated rocky core, or a liquid ocean decoupling the crust from the interior. Many researchers favored the core, because Mimas' heavily cratered surface shows no sign of the cracks and resurfacing seen on active moons.
In February 2024, a study led by Valéry Lainey (Paris Observatory) in Nature tipped the balance. By analyzing how the orientation of Mimas' orbit drifts over time, using Cassini observations, the team showed that a rigid core could not reproduce the data, while a global ocean beneath an ice shell about 20 to 30 km thick could. Crucially, the models also indicate that the ocean is young: it formed within roughly the last 25 million years and may still be spreading. That would explain the paradox of an ocean under a surface that looks ancient: the ice has not yet had time to crack and renew.
How Mimas came to warm up so recently remains open. One possibility is that its orbital eccentricity was higher in the recent past, perhaps because of changes in its resonances with other moons, boosting tidal heating. The finding added Mimas to the list of "stealth" ocean worlds and suggested that oceans might be more common, and more transient, than thought. See life on Titan and Enceladus for the broader context.
Sculptor of the rings
Mimas has an outsized influence on Saturn's rings. Ring particles orbiting at the distance where they would complete exactly two orbits for every one of Mimas receive a regular gravitational tug that clears them out. This 2:1 resonance falls at the outer edge of the B ring and the inner edge of the Cassini Division, the 4,700-km-wide gap visible in small telescopes (ring gaps and divisions). Mimas is also in a 4:2 resonance with Tethys, and tiny Methone, Anthe and Pallene, the Alkyonides, orbit in its gravitational neighborhood.
Exploration
Mimas has only been seen up close by the Voyagers and Cassini. Cassini's closest approach was in February 2010, at about 9,500 km, producing the best images of Herschel crater. No future mission currently plans a dedicated visit, though the ocean discovery has made Mimas an attractive target.
FAQ
Why does Mimas look like the Death Star?
Because of Herschel, a 139-km impact crater that covers about a third of the moon's diameter, with a central peak. The resemblance is a coincidence: Star Wars came out in 1977, three years before Voyager 1 revealed the crater.
Does Mimas have an ocean?
Probably. A 2024 analysis of Mimas' orbital motion in Nature concluded that a global ocean lies 20 to 30 km beneath its surface, and that it formed less than about 25 million years ago. It has not been confirmed by a direct measurement.
How big is Mimas?
Mimas is about 396 km in diameter on average, slightly elongated toward Saturn. It is the smallest of Saturn's seven major round moons.
Who discovered Mimas?
William Herschel discovered Mimas on 17 September 1789 with his 40-foot telescope in Slough, England. His son John Herschel named it in 1847.
Sources
- NASA Science: Mimas
- Lainey, V., et al., "A recently formed ocean inside Saturn's moon Mimas", Nature 626 (2024)
- Tajeddine, R., et al., "Constraints on Mimas' interior from Cassini ISS libration measurements", Science 346 (2014)
- Howett, C. J. A., et al., "A high-amplitude thermal inertia anomaly of probable magnetospheric origin on Saturn's moon Mimas", Icarus 216 (2011)