Mimas

Mimas is the smallest and innermost of Saturn's major moons, a ball of nearly pure water ice just under 400 km across — small enough that its own gravity has barely managed to pull it into a sphere. It orbits deep inside Saturn's realm, skimming just beyond the visible rings, and completes a full circuit of the planet in a little under a day — about 22.6 hours. For most of the two centuries since its discovery it was treated as an unremarkable frozen relic, a cratered snowball keeping the same face turned toward its giant parent.

Two things have since made Mimas one of the more talked-about small moons in the solar system: a colossal impact scar that gives it an uncanny resemblance to a piece of science-fiction hardware, and a 2024 finding that this dead-looking iceball may be hiding a global ocean of liquid water beneath its surface.

The Death Star moon

Mimas is defined by a single feature. Herschel, an impact crater about 130 km wide, spans roughly a third of the moon's own 396 km diameter — as if a wall-to-wall bite had been taken out of one hemisphere. Its ramparts rise about 5 km above the crater floor, and a central peak stands some 6 km tall, taller than most mountains on Earth. Seen from the right angle, the crater and its shadowed pit give Mimas an almost exact likeness to the Death Star from Star Wars — a coincidence not lost on anyone who saw Cassini's sharp portraits of it.

The impact that carved Herschel came remarkably close to shattering the moon outright. On the far side of Mimas, directly opposite the crater, the surface is broken by long grooves and fractures — chasmata that many researchers think were cut by shock waves that raced around and through the little world and converged on the antipode. A slightly harder blow, and Mimas might not exist as a single body today.

A world of ice and craters

With a mean radius of about 198 km and a density only slightly greater than water, Mimas is almost entirely frozen H2O with just a small fraction of rock. That low density, combined with its heavily battered surface, long marked it as a geologically dead object — a place that has recorded impacts for billions of years without ever resurfacing itself. Craters crowd nearly every part of the terrain, the largest reaching tens of kilometres across, and the moon's slightly ovoid shape betrays how weak its self-gravity is at this size.

Mimas is tidally locked, keeping one hemisphere permanently facing Saturn as it goes around, much as our own Moon does with Earth. Surface temperatures hover near −200 °C, and in 2010 Cassini's infrared instrument mapped an odd, sharply bounded warm-and-cold pattern across the surface — a 'Pac-Man' thermal anomaly whose exact cause is still debated but likely reflects differences in how the icy ground conducts heat.

A shepherd at the edge of the rings

Mimas orbits at an average distance of about 185,500 km from Saturn, close in among the ring system, and its gravity leaves a visible mark on the rings themselves. The moon is largely responsible for clearing the Cassini Division, the broad dark gap between Saturn's A and B rings: particles orbiting there fall into a 2:1 resonance with Mimas — circling Saturn twice for every one Mimas orbit — and the repeated tugging sweeps them out over time. It also helps sculpt sharp edges elsewhere in the rings, making this small moon a genuine architect of Saturn's most famous feature.

The hidden ocean

In February 2024, a team led by researchers at the Paris Observatory published a result in Nature that upended the picture of Mimas as inert. By tracking tiny irregularities in the moon's orbital motion in Cassini data — the slow drift of its closest approach to Saturn and the way it rocks, or librates, as it circles — they concluded that Mimas cannot be solid all the way through. The measurements fit only if a global ocean of liquid water lies beneath the ice, at a depth of roughly 20 to 30 km.

The strangest part is the ocean's apparent youth. The same orbital analysis suggests it is geologically recent — likely less than about 25 million years old, and perhaps still growing. That would explain the paradox of a heavily cratered, undisturbed surface sitting atop liquid water: the ocean simply has not existed long enough for cracks, ridges, or geysers to break through the crust. If confirmed, Mimas joins Enceladus, Europa, and a handful of other icy moons as an 'ocean world' — and hints that hidden seas may be far more common than the frozen faces of these moons suggest.

Discovery and exploration

Mimas was found on 17 September 1789 by William Herschel, using the enormous 40-foot reflecting telescope he had recently completed in England — the same instrument with which he spotted the moon Enceladus a few weeks earlier. It was later named, following a suggestion by his son John Herschel, after one of the Giants of Greek mythology; fittingly, the crater that dominates it carries the family name.

For nearly two centuries Mimas remained a mere point of light. Pioneer 11 and the two Voyager probes returned the first real images as they swept past Saturn between 1979 and 1981, with Voyager 1 revealing Herschel in 1980. The definitive views came from NASA and ESA's Cassini orbiter, which studied Saturn from 2004 to 2017 and made its closest Mimas approach on 13 February 2010, passing within about 9,500 km. Cassini's data — gathered years before anyone suspected an ocean — is what later made the 2024 discovery possible.

How to spot Mimas

Mimas is a difficult target for backyard observers. At around 12th–13th magnitude it is faint, and it sits so close to Saturn that the planet's glare and the brilliance of the rings tend to swamp it. Seeing it generally calls for a telescope of at least 250 mm (10 inches) of aperture, steady skies, and timing that catches Mimas at greatest elongation, when it swings out to the side of Saturn rather than in front of or behind the glare. Even then it is a fleeting speck — but knowing an entire hidden ocean may lie beneath that speck makes the hunt worthwhile.