Mimas

Moon· orbits Saturn

Mimas is a small, heavily cratered ice moon of Saturn dominated by the 130-km crater Herschel, which gives it an uncanny resemblance to the Death Star.

Open in 3D viewSee Mimas in the live, interactive 3D map

Key facts

Mass

3.75 × 10¹⁹ kg

0.000511 Moon masses

Radius

198 km

11% of our Moon's radius

Surface gravity

0.0637 m/s²

1% of Earth's gravity

Day length

22.62 hours

Orbit around Saturn

22.6 hours

0.94 Earth days

Science

Physical

Mass: 3.75 × 10¹⁹ kg · 0.000511 Moon masses; JPL / IAU mean radii; spacecraft-tracked GM where measured
Radius: 198 km · 11% of our Moon's radius; JPL / IAU mean radii; spacecraft-tracked GM where measured
Density: 1,150 kg/m³ · 1.1× water; Computed from mass and radius
Surface gravity: 0.0637 m/s² · 1% of Earth's gravity; Computed from mass and radius
Escape velocity: 159 m/s · 1% of Earth's escape velocity; Computed from mass and radius

Orbit around Saturn

Distance from Saturn: 186,000 km; From JPL orbital elements
Orbital period: 22.62 hours; From JPL orbital elements
Eccentricity: 0.02; From JPL orbital elements
Inclination to Saturn's Laplace plane: 1.60 °; From JPL orbital elements · The Laplace plane is the one a moon's orbit precesses about: close in it is the planet's equatorial plane, the plane of the rings, and further out it tilts toward the planet's own orbit.

Rotation

Rotation period: 22.62 hours
Tidal locking: Tidally locked to its parent

Temperature

Mean: 64 K (-209 °C)

Composition

Bulk composition: Almost entirely water ice with a little rock; gravity data hint at a young internal ocean.

Radiation & temperature

Insolation: 0.0112 S⊕ · 1% of the sunlight Earth receives; Computed from the distance to the Sun (inverse-square; Earth's sunlight = 1), before any atmosphere.
Bond albedo: 0.55 · reflects 55% of incoming sunlight
Equilibrium temperature: 74 K (-199 °C); Computed from albedo + distance from the Sun (airless black body)
Measured mean temperature: 64 K (-209 °C)

Discovery & history

Discovered: September 17, 1789
By: William Herschel
Method: Visual discovery (telescopic)
Name: Mimas, one of the Giants of Greek myth.

Herschel discovered Mimas in 1789 with the great reflecting telescope he had just built. Its enormous Herschel crater — a third of the moon's width — gives it an uncanny resemblance to the Death Star, and wobbles in its motion hint at a hidden ocean beneath the ice.