Iapetus

What is Iapetus?

Moons· Diameter 1,469 km. Saturn's third-largest moon and one of the strangest in the solar system: its leading face is coal-dark, its trailing face snow-white, and a 20-km-high ridge girdles the equator like the seam of a walnut. See its distant, tilted orbit in the orbit view.

The orbit of Iapetus at this moment: https://nowsky.org/solar/orbit?fw=moon:saturn:iapetus&sp=0.06 — replayed live, no login.

Orbit and size

Figures are representative approximations. Source: compiled from NASA and IAU public data.

Parent planet and sibling moons

Frequently asked

How big is Iapetus?

1,469 km.

How far is Iapetus from Earth?

3,561,000 km.

What is notable about Iapetus?

one hemisphere coal-dark, the other snow-bright.

Which planet does Iapetus orbit?

Saturn.

The equatorial ridge — the seam of a fallen ring?

Along Iapetus's equator runs a ridge 20 km high and 20 km wide for more than 1,300 km — 1.4 percent of the moon's diameter, the equivalent of a range twice the height of Everest tracking Earth's equator exactly. When Cassini found it in 2004 the problem was that no known geological process makes so narrow and straight a band confined to the equator.

The leading explanation is that a ring or small moon once orbiting Iapetus broke up and rained down along the equator. Only there because rings lie in the equatorial plane; and the ridge sits on older cratered terrain while carrying craters of its own, placing it early but after the surface formed. An alternative holds that it is the remnant of an equatorial bulge from a time when Iapetus spun far faster, before tides slowed it to its present 79-day rotation.

The moon in depth

Cassini discovered Iapetus in 1671 and soon noticed that he could see it only when it lay west of Saturn — it vanished on the eastern side. Three centuries before spacecraft, he had correctly deduced that one hemisphere is about ten times darker than the other. The moon is 1,469 km across, orbits 3.56 million km from Saturn every 79 days, and its path is tilted a striking 15 degrees from Saturn's equator.

The dark leading face, Cassini Regio, is explained by a feedback loop: black dust drifting in from the outer moon Phoebe warms the ice, which sublimates and refreezes on the bright trailing side. In 2004 the Cassini spacecraft found a ridge 1,300 km long and up to 20 km high running along the equator — one of the tallest mountain ranges in the solar system. Whether this walnut seam is a collapsed ring or a relic of a faster spin remains unsettled.

Track Saturn in the orbit view and zoom out to watch Iapetus crawl around far beyond Titan. It is the only large moon whose orbit is tilted enough to show Saturn's rings edge-on and face-on in turn, so a visitor standing there would enjoy the finest ring views in the system.

Related

Figures are representative approximations. Simulated positions use J2000 mean elements.