It resembles the Death Star and hides a secret that could reshape how we see icy moons. Saturn’s smallest round moon, Mimas, long thought geologically dead, may conceal a liquid ocean beneath its frozen surface.
From a crater that nearly destroyed it to a thermal image shaped like Pac-Man, Mimas has surprised scientists again and again.
Discovered by a Giant Telescope
William Herschel discovered Mimas on 17 September 1789 using his massive 40-foot reflecting telescope with a 48-inch mirror. He noted the satellite at its maximum elongation. His son, John Herschel, later named it after a figure from Greek mythology in 1847, along with Saturn’s other moons, drawing from Titans and Giants.
The Impact That Nearly Shattered It
Mimas is 396.4 kilometers (246.3 miles) in diameter and composed mostly of water ice. It is marked by the giant Herschel crater—139 kilometers (86 miles) wide. The impact left shock damage on the opposite side of the moon. In photos, Mimas’s crater gives it a resemblance to the Death Star from Star Wars.
Orbital Influence and a Pac-Man Signature
Mimas’s gravity creates the Cassini Division, a major gap in Saturn’s rings, through a 2:1 orbital resonance. It’s also linked to other rings and moons, like Tethys and Pandora. In 2010, Cassini’s infrared scans revealed a strange temperature distribution. When overlaid on a map, the shape looked like the classic arcade character Pac-Man.
A Hidden Ocean Beneath the Ice
Unusual libration in Mimas’s orbit led researchers in 2014 to propose an internal ocean. In 2022, new models supported this, showing that an ocean could exist beneath 20–30 km of ice without causing surface cracks. A 2024 study found Mimas’s orbit precesses more slowly than expected for a solid body, reinforcing the ocean hypothesis. The ocean likely formed within the last 25 million years.
Exploration Over the Decades
Pioneer 11 passed by in 1979, followed by Voyager 1 in 1980 and Voyager 2 in 1981. Cassini, arriving in 2004, provided the clearest images and closest flyby in February 2010 at 9,500 km (5,900 mi). Cassini confirmed Mimas’s oblong shape, cratered terrain, and thermal quirks, making it one of Saturn’s most studied and unexpectedly intriguing moons.
It resembles the Death Star and hides a secret beneath its icy surface.
Saturn’s moon Mimas, once thought geologically inactive, may harbor a liquid ocean.
A massive crater almost shattered it, and a thermal scan once revealed the shape of Pac-Man on its face…🧵👇 pic.twitter.com/ojrYzgd8cx
— Detective Tiger's Stories (@TigerDetective) May 30, 2025