Mimas: The Moon That Looks Like the Death Star

That's no moon — the Death Star resemblance

The comparison is almost too on-the-nose. The Death Star in Star Wars (released May 1977) has a hemispherical dish — the superlaser focusing lens — set into one face of a grey sphere. Mimas, imaged in detail for the first time by Voyager 1 on 12–13 November 1980, has exactly that: a single dominant crater, Herschel, set into an otherwise smooth grey globe. The timing means the resemblance is pure coincidence; George Lucas's designers had never seen a high-resolution picture of Mimas, because none existed.

What makes the illusion so convincing is scale mismatch with symmetry. Herschel is about 139 km across on a body of only ~396 km mean diameter — roughly one-third of the moon's face. Its rim walls rise about 5 km, parts of its floor sink up to 10 km below the surrounding terrain, and a 6 km central peak sits in the middle like the dark focusing point of the fictional weapon. Because Mimas is tidally locked to Saturn (one face always turned inward, like our Moon toward Earth), and Herschel sits near the leading hemisphere, spacecraft repeatedly caught it in profile — cementing the Death Star nickname in the public imagination.

The resemblance is a reminder that impact craters are the default sculptor of small airless worlds. Without an atmosphere to burn up incoming debris or erosion to soften old scars, every hit is preserved. Mimas simply wears the single most dramatic hit in the Saturn system as its face.

The impact that nearly destroyed a world

Herschel is not just picturesque — it is a record of a near-catastrophe. Planetary scientists calculate that the impactor that dug Herschel released energy comparable to a substantial fraction of the moon's gravitational binding energy. A slightly faster or larger projectile would have delivered enough shock to fragment Mimas entirely, scattering it into a ring of debris around Saturn.

Evidence of the near-miss shows up on the far side of the moon. Seismic waves from the Herschel impact appear to have propagated through Mimas and produced chevron-shaped grooves and disrupted terrain on the antipodal point — the location diametrically opposite the crater. This 'antipodal focusing' is inferred on other battered bodies too, most clearly Mercury (the Caloris basin) and the Moon (Imbrium), where a single giant impact appears to leave a signature clear across the world.

Some numbers put the violence in perspective:

• Crater diameter (~139 km) versus moon diameter (~396 km): the wound spans roughly 35% of the face.
• Rim-to-floor relief reaches about 10 km — deeper than Earth's Mariana Trench, on a world you could drive across in a long day.
• The central peak (~6 km) formed from rock and ice that rebounded upward in the seconds after impact, frozen in place because Mimas is too cold and rigid to relax the topography.

On a larger, warmer moon, such a crater would slowly flatten as the crust flowed. On frigid Mimas — surface temperature about 64 K (−209 °C) — the ice behaves like brittle rock, so Herschel has stood essentially unchanged for what is likely billions of years.

A ball of ice, barely denser than water

Mimas is the smallest and innermost of Saturn's major, gravitationally-rounded moons. Its mean radius is 198.2 km and its mass is only 3.75×10¹⁹ kg — about 6 millionths of Earth's Moon. Divide mass by volume and you get a density of just 1.15 g/cm³, barely above liquid water's 1.00 g/cm³.

That single number tells you what Mimas is made of. Rock has a density around 3 g/cm³; water ice is close to 0.92 g/cm³. A body measuring 1.15 g/cm³ must be overwhelmingly water ice with only a small rocky fraction. In effect, Mimas is a dirty snowball large enough for its own gravity to have pulled it into a sphere — indeed, it is often cited as the smallest known body that self-gravity has rounded, though its shape is measurably ellipsoidal (about 416 × 393 × 381 km across) because Saturn's tides stretch it.

Because it is small and icy, Mimas has weak surface gravity — roughly 0.064 m/s², less than 1% of Earth's. A person who could weigh 70 kg on Earth would tip a scale at under half a kilogram there. Its escape velocity is only about 0.16 km/s (160 m/s), so a well-thrown baseball could, in principle, leave the moon forever. These are the physics of a world where mountains of ice can stand tall precisely because gravity is too feeble to crush them.

Racing around Saturn, locked in resonance

Mimas hugs Saturn at a semi-major axis of 185,539 km — closer than our Moon is to Earth (about 384,400 km), yet orbiting a planet 95 times Earth's mass. The result is blistering speed: Mimas completes one orbit in just 0.942 days — about 22 hours and 36 minutes. Its orbital eccentricity is a modest 0.0196, and because it is tidally locked, its rotation period matches this exactly, so it spins once per revolution.

Mimas does not orbit alone. It is caught in a 2:1 mean-motion resonance with the larger moon Tethys — Mimas completes two orbits for every one of Tethys. Regular gravitational nudges at the same points keep the pair's orbits synchronized, a textbook example of the resonances that pattern the entire Saturn system.

Mimas also sculpts Saturn's rings. Its gravity is responsible for clearing the Cassini Division, the prominent dark gap between the A and B rings visible even in small telescopes. Ring particles orbiting at the inner edge of the division are in a 2:1 resonance with Mimas: they feel a repeated tug every second orbit, which pumps up their orbital motion and drives them out of the gap. A moon barely 400 km wide thereby carves a lane roughly 4,800 km across in a ring system 280,000 km in diameter — a striking demonstration of resonant amplification.

The 2024 surprise: a young ocean under the ice

For decades Mimas was dismissed as a dead, frozen relic — heavily cratered, geologically inert, the opposite of its restless neighbor Enceladus with its erupting geysers. That story changed on 8 February 2024, when Valéry Lainey and colleagues published in Nature a dynamical analysis of Mimas's motion using data from NASA's Cassini mission.

The clue was in the orbit. The team measured tiny wobbles — the way Mimas's orbit precesses (its long axis slowly rotates) and how the moon librates (rocks back and forth as it circles Saturn). A completely solid Mimas would move one way; a Mimas with a hidden liquid layer sloshing beneath a rigid shell would move slightly differently. The data matched the second case. Their best model places a global ocean of liquid water 20–30 km beneath the surface, making up a large fraction of the moon's interior.

The most startling result was the ocean's age: likely less than 25 million years old, perhaps only 5–15 million — a geological eyeblink. That youth neatly explains why the surface shows no cracks, ridges, or fresh terrain: the ocean simply hasn't existed long enough to deform the ancient, crater-saturated crust from within. Researchers suspect a past increase in Mimas's orbital eccentricity — driven by resonances with other moons — generated internal tidal heating that recently melted its interior. Some caution remains — the ocean is inferred from orbital dynamics, not directly detected — but Mimas has been promoted from 'dead ice ball' to candidate ocean world, a category once reserved for bodies like Europa, Enceladus, and Titan.

Discovery, naming, and how to see it

Mimas was found by William Herschel on 17 September 1789, using his newly built 40-foot (12 m) reflector — at the time the largest telescope in the world. Herschel had already discovered Uranus in 1781, and had used the same 40-foot reflector to find Enceladus about three weeks earlier, on 28 August 1789 — the telescope's first-light night. Mimas is faint (visual magnitude around 12.9) and buried in the glare of Saturn's rings, which is why it took the era's biggest instrument to reveal it. It carries the designation Saturn I as the innermost of the classical moons.

The name comes later. In 1847, William Herschel's son John Herschel proposed naming Saturn's moons after the Titans and giants of Greek mythology. Mimas was one of the Giants (Gigantes) slain in the war against the Olympian gods — a fitting name for a battered little world. The crater Herschel, of course, honors the discoverer.

For a backyard observer, Mimas is a genuine challenge. Points to keep in mind:

• You need a telescope of roughly 250 mm (10-inch) aperture or larger, dark skies, and steady seeing to glimpse it as a faint speck.
• Timing matters: Mimas is easiest to spot near its greatest elongation, when it swings farthest from Saturn's glare rather than being lost against the rings.
• Everything we truly know about its surface comes from spacecraft — Voyager 1 and 2 in 1980–81, and above all Cassini, which orbited Saturn from 2004 to 2017 and mapped Mimas in detail, measured its libration, and provided the data that would later reveal the hidden sea.

Two Saturnian ocean moons: the ancient, active Enceladus versus the young, quiet Mimas.

Property	Mimas	Enceladus
Mean diameter	≈396 km	≈504 km
Density	1.15 g/cm³	1.61 g/cm³
Surface	Ancient, saturated with craters	Young, fractured 'tiger stripes'
Ocean depth below surface	≈20–30 km	≈20–25 km (south pole)
Ocean age	Likely < 25 million years	Possibly ~1 billion years or more
Active plumes?	None observed	Yes — geysers feed Saturn's E ring