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Moon of Saturn
Iapetus
Saturn's two-tone moon, half as dark as coal and half as bright as snow
Key facts
- Diameter
- About 913 miles / 1,470 km (mean radius 734 km)
- Distance from Saturn
- About 2,210,000 miles / 3,561,700 km
- One orbit
- 79.33 Earth days
- One day (rotation)
- 79.33 Earth days, the same as its orbit (tidally locked)
- Orbit tilt
- About 15 degrees to Saturn's equator
- Density
- About 1.09 g/cm³, mostly water ice
- Discovered
- 25 October 1671 by Giovanni Domenico Cassini
- Named after
- Iapetus, a Titan in Greek mythology, father of Atlas and Prometheus
- Surface
- One hemisphere very dark (albedo 0.03 to 0.05), the other bright ice (albedo 0.5 to 0.6), with an equatorial ridge 6.2 to 8.1 miles / 10 to 13 km high
- Temperature
- About -144 °C on the dark side, about -160 °C on the bright side
- Visited by
- Voyager 1 (1980), Voyager 2 (1981) and Cassini (closest flyby 10 September 2007 at 762 miles / 1,227 km)
- Saturn's known moons
- 293 as of August 2026
About Iapetus
Iapetus is a moon of Saturn. It is a very strange moon. One side is dark like coal. The other side is bright like snow. Nobody has seen a moon like it. A tall ridge runs round its middle. The ridge looks like a belt. Its mountains are taller than Everest. Iapetus takes a long time to go round Saturn. One trip takes 79 days. It is made mostly of ice. A man called Cassini found it long ago. Would you like to visit it?
Iapetus is the third largest moon of Saturn, and one of the strangest. It is a sphere about 1,470 km across, made mostly of ice. Its density is only about 1.1 times that of water. Iapetus is a two-tone moon, with one half dark and one half bright.
The dark side reflects only about 4 percent of the sunlight that hits it. The bright side reflects more than half of it, like fresh snow. Scientists think dark dust from another moon, Phoebe, lands on the leading side. That dark surface warms up in sunlight, so its ice slowly escapes.
The ice then settles again on the colder, brighter side. A huge ridge of mountains runs along the equator, 10 to 13 km high. Iapetus orbits Saturn once every 79 days, about 3.5 million km away. Its orbit is tilted, so it gives a rare view of Saturn's rings.
Like our Moon, it takes the same time to rotate as to orbit. Giovanni Cassini discovered Iapetus in 1671, and he noticed its odd brightness. The Cassini spacecraft flew past it in 2007, just 1,227 km above the surface.
Iapetus is Saturn's third largest moon, with a mean radius of about 734 km (NASA gives 736 km), so a diameter of roughly 1,470 km. It orbits at a semi-major axis of about 3,561,700 km with a period of 79.33 days, and rotates synchronously, keeping one face towards Saturn. Its orbit is unusually inclined, about 15 degrees to Saturn's equator, which is why Iapetus is the only large Saturnian moon from which the rings would appear open rather than edge-on. With a mean density of about 1.09 g/cm³, it is composed of roughly three quarters water ice and one quarter rock.
Giovanni Domenico Cassini discovered Iapetus on 25 October 1671. He noticed that he could see it only on one side of Saturn, and correctly deduced that one hemisphere is far darker than the other and that the moon rotates synchronously. This is the famous brightness dichotomy: the leading hemisphere (Cassini Regio) has an albedo of about 0.03 to 0.05, while the trailing hemisphere reflects about 0.5 to 0.6 of incoming sunlight. The name, after the Titan Iapetus, was proposed by John Herschel in 1847.
The current explanation combines two processes. Dark reddish dust, probably from the retrograde outer moon Phoebe, is swept up by the leading face. Because Iapetus rotates so slowly, that slightly darkened surface warms enough (about 129 K, or -144 °C, in Cassini Regio) for water ice to sublimate and migrate to colder, brighter regions (about 113 K, or -160 °C). This thermal segregation is a runaway feedback, which is why the boundary between dark and bright is so sharp. It is often misstated that the dark side is simply Phoebe dust; the dust only triggers the process, and the ice migration does most of the work.
Iapetus also has an equatorial ridge, discovered by Cassini in 2004: a chain of mountains about 1,300 km long, up to 20 km wide and 10 to 13 km high, giving the moon a walnut-like profile. Its origin is still debated; ideas include a collapsed ring, or a relic of rapid early spin.
Voyager 1 (1980) and Voyager 2 (1981) returned the first detailed images, Voyager 2 passing at about 909,000 km. Cassini made the only close flyby on 10 September 2007, at 1,227 km. Saturn has 293 confirmed moons as of August 2026.
Did you know?
- Cassini worked out in the 1670s that Iapetus had a dark side, because he could only see it on one side of Saturn.
- The ridge round Iapetus's equator is about 1,300 km long and 10 to 13 km high, so the moon looks a bit like a walnut.
- The bright side of Iapetus reflects more than ten times as much sunlight as the dark side, a bigger contrast than any other moon.
Neighbours around Saturn
Questions children ask
Why is one side of Iapetus dark and the other bright?
Dark dust, probably from the moon Phoebe, lands on the front of Iapetus as it orbits. The darker patches warm up in sunlight, so their ice turns to vapour and settles on the colder, brighter side. Over time this makes the dark side darker and the bright side brighter.
What is the ridge round the middle of Iapetus?
It is a line of mountains along the equator, about 1,300 km long and 10 to 13 km high, which is taller than Mount Everest. Nobody is sure how it formed. Some scientists think a ring fell onto the moon; others think Iapetus once spun much faster.
Has a spacecraft ever visited Iapetus?
Yes. Voyager 1 and Voyager 2 took the first close pictures in 1980 and 1981. The Cassini spacecraft, named after the man who discovered the moon, flew past just 1,227 km above the surface on 10 September 2007.
How long is a day on Iapetus?
A day on Iapetus lasts about 79 Earth days, the same time it takes to orbit Saturn. Like our Moon, it always keeps the same face towards its planet. That slow rotation lets the dark side warm up enough for ice to escape.
Quick quiz
Three questions for space cadets. Nothing is recorded.
- What is unusual about the surface of Iapetus?
- How long does Iapetus take to orbit Saturn?
- Which spacecraft made the closest flyby of Iapetus, in 2007?
Titan
Rhea
The Moon