Mimas

Mimas is the innermost and smallest of Saturn’s major moons. NASA gives its mean diameter as about 394 kilometres and describes its shape as slightly elongated rather than perfectly spherical. It orbits roughly 186,000 kilometres from Saturn, completing one orbit in about 22 hours and 36 minutes. Like Earth’s Moon, Mimas is tidally locked, so the same hemisphere generally faces its planet. Its low density and the detection of water ice on its surface point to an ice-rich body.

The moon’s most recognizable landmark is Herschel, a large impact crater named for Mimas’s discoverer, William Herschel. At about 130 kilometres across, it spans roughly one third of the moon’s diameter. The crater has high walls and a central peak; its scale gives Mimas its familiar resemblance to the fictional Death Star, though that comparison says nothing about the moon’s actual structure. The crater preserves evidence of a major impact, but NASA notes that the impact’s effects on the far side are discussed as a possibility rather than a certainty.

Mimas’s cratered surface long made it appear geologically inactive. In 2024, researchers analyzing Cassini measurements of its orbital motion, particularly the gradual drift of its periapsis, argued that the observations are best explained by a global ocean beneath the ice. Their model placed the ocean 20–30 kilometres below the surface and suggested it formed relatively recently, within the past 25 million years. This is an inference from dynamics, not a direct detection of liquid water. The apparently unmodified surface and the ocean interpretation pose open questions; a 2025 review reported that evidence was strengthening while important contradictions remained under discussion.

William Herschel discovered Mimas on 17 September 1789 using a reflecting telescope. It remained an unresolved point of light to ground-based observers until Voyager 1 and Voyager 2 imaged it in 1980. NASA’s Cassini spacecraft later made close approaches and returned detailed images and measurements that enabled the study of its surface and orbit. Mimas has not been visited by a lander, and current knowledge of its interior relies on spacecraft observations and scientific models.