major moon · moon of Saturn
Iapetus
Saturn's two-toned moon, with one hemisphere dark as coal and the other bright as snow, and a strange ridge along its equator.
- Distance from Earth today
- 1.28 billion km8.564 AU · light: 1 h 11 min
- Distance from the Sun today
- 1.41 billion km9.425 AU
- As of
- 3 November 2026position from NASA/JPL orbital elements
Where it is today
Top view of the plane of Earth’s orbit, compressed distance scale. Open on the big map with the time slider →
Facts
- Diameter
- 1,469 km
- Mass
- 1,806 × 10¹⁸ kg
- Density
- 1,089 kg/m³
- Mean distance from the planet
- 3,561,700 km
- Orbital period
- 79.3 days
- Orbital eccentricity
- 0.028
- Orbital inclination
- 7.6°
Source of the figures: JPL SSD – Planetary Satellites (public domain).
Discovery: 1671 – Giovanni Domenico Cassini
Description
Iapetus is Saturn's third-largest moon and the most distant of its major moons. Giovanni Domenico Cassini discovered it in 1671 and noticed that he could see it on only one side of Saturn – the first hint of its striking two-tone surface. Its leading hemisphere is very dark, while the trailing side is bright ice; the dark material is thought to come partly from dust swept up from Saturn's outer moons, enhanced by ice migrating away from the warmer dark regions. Cassini images also revealed a long mountain ridge running along much of its equator, giving it a walnut-like shape.