Iapetus

Moon· orbits Saturn

Iapetus is Saturn's two-faced moon: one hemisphere is as dark as coal, the other as bright as snow, and a strange ridge of equatorial mountains circles much of the body.

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

Key facts

Mass

1.81 × 10²¹ kg

0.0246 Moon masses

Radius

734 km

42% of our Moon's radius

Surface gravity

0.224 m/s²

2% of Earth's gravity

Day length

79.3 days

Orbit around Saturn

79.33 Earth days

Science

Physical

Mass: 1.81 × 10²¹ kg · 0.0246 Moon masses; JPL / IAU mean radii; spacecraft-tracked GM where measured
Radius: 734 km · 42% of our Moon's radius; JPL SSD Planetary Satellite Physical Parameters (mean radius)
Density: 1,089 kg/m³ · 1.1× water; Computed from mass and radius
Surface gravity: 0.224 m/s² · 2% of Earth's gravity; Computed from mass and radius
Escape velocity: 573 m/s · 5% of Earth's escape velocity; Computed from mass and radius

Orbit around Saturn

Distance from Saturn: 3,561,700 km; From JPL orbital elements
Orbital period: 79.3 days; From JPL orbital elements
Eccentricity: 0.028; From JPL orbital elements
Inclination to Saturn's Laplace plane: 7.60 °; 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: 79.3 days
Tidal locking: Tidally locked to its parent

Temperature

Mean: 115 K (-158 °C)

Composition

Bulk composition: Mostly water ice with rock; one hemisphere is coated in a thin, very dark, organic-rich dust.

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: 0.20 · reflects 20% of incoming sunlight; Strongly two-faced — the leading side is very dark (~0.04) and the trailing side bright (~0.6); this is an approximate global average.
Equilibrium temperature: 86 K (-188 °C); Computed from albedo + distance from the Sun (airless black body)
Measured mean temperature: 115 K (-158 °C)
Greenhouse warming: +29 K; Measured temperature minus the airless equilibrium — the greenhouse effect

Discovery & history

Discovered: October 25, 1671
By: Giovanni Domenico Cassini
Method: Visual discovery (telescopic)
Name: Iapetus, a Titan of Greek myth, father of Prometheus and Atlas.

Iapetus, found by Cassini in 1671, is the Solar System's great two-faced world: one hemisphere is as dark as coal, the other as bright as snow. Cassini correctly reasoned it must keep one face toward Saturn, and a strange equatorial ridge gives it the outline of a walnut.