Iapetus

Key facts

Diameter

1,469 km42% of Earth's Moon

Mass

1.81 × 10²¹ kg

Surface gravity

0.223 m/s²0.023 g

Density

1,088 kg/m³

Distance from Saturn

3,560,820 km≈ 59.1 Saturn radii

Orbital period

79.3 daysSame as its rotation (tidally locked)

Discovered

1671Giovanni Cassini

Surface

Dark leading side, bright icy trailing side

Surface temperature

About −173 °C to −143 °C

Rotation

Tidally locked, about 79 days

Notable feature

Equatorial ridge up to about 20 km high

Named after

Iapetus, a Titan of Greek mythology

Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.

Surface and geology

The dark region, Cassini Regio, covers most of the leading hemisphere and reflects only around 3 to 5 per cent of sunlight. The bright trailing hemisphere and the poles reflect ten times as much or more. The boundary between them is surprisingly patchy, with dark material concentrated on sun-facing slopes.

Along the equator runs a ridge more than 1,300 kilometres long, with peaks rising up to around 20 kilometres high, more than twice the height of Everest. Its origin is still debated: it may have formed when a rapidly spinning young Iapetus slowed down, or from material that fell from an ancient ring or collapsed sub-moon.

Iapetus is also heavily cratered, with several basins wider than 300 kilometres. Some, such as Engelier and Turgis, show enormous landslides that slid tens of kilometres across their floors.

Interior and composition

With a low density, Iapetus is made mostly of water ice with a smaller amount of rock. Its shape is noticeably flattened at the poles, more than its present slow rotation would cause, suggesting that it froze into shape early on when it spun in perhaps around 16 hours. A thick, rigid outer shell must have formed quickly to lock in that ancient bulge.

Orbit and rotation

Iapetus orbits far from Saturn, about 3.6 million kilometres out, taking about 79 days per circuit. Its orbit is tilted by roughly 15 degrees to Saturn's equator, so from Iapetus the rings would appear at an angle, unlike from the inner moons.

Because it is tidally locked, its leading hemisphere always sweeps forward along its path. This turns out to be key to explaining its two-tone appearance.

Origin

Iapetus likely formed in the outer part of the disc around young Saturn. Its colour contrast is thought to arise in two steps: dark dust from Saturn's outer moons, especially from the vast Phoebe ring discovered in 2009, coats the leading side, then the darkened surface warms enough for ice to evaporate and refreeze on the colder, brighter areas, exaggerating the difference over time.

Exploration

Iapetus was the second Saturnian moon to be discovered, found by Cassini in October 1671. Its distance from Saturn has made close spacecraft encounters rare.

The moons of Saturn

Surprising facts about Iapetus

• Cassini guessed in the 1670s that Iapetus had one bright and one dark side, nearly three centuries before spacecraft confirmed it.
• Iapetus' equatorial ridge gives it the profile of a walnut, and some peaks rise roughly twice as high as Mount Everest.
• The dark coating may be surprisingly thin in many places, perhaps only tens of centimetres deep, like a layer of soot on snow.
• In Arthur C. Clarke's novel 2001: A Space Odyssey, the mysterious monolith and its star gate were placed on Iapetus.
• From Iapetus, Saturn's rings would be seen at a tilt, thanks to the moon's inclined orbit.

Keep exploring

• Rhea, moon of Saturn: saturn's ancient frozen giant
• Titan, moon of Saturn: lakes of methane under orange skies
• Callisto, moon of Jupiter: a battered relic of the early solar system
• Saturn: the planet guide
• Watch Iapetus orbit Saturn in the interactive 3D solar system
• All the major moons of the Solar System

Sources

• NASA Science — Iapetus
• JPL Solar System Dynamics — Planetary satellite physical parameters
• Wikipedia — Iapetus