Two-tone moon of SaturnThe moon with two faces
Iapetus
Iapetus is split between bright and dark hemispheres, with a mountain ridge running along its equator.
- Host
- Saturn
- Type
- Two-tone icy moon
- Mean diameter
- 1,469 km
- Mean orbital radius
- 3,560,800 km
- Orbital period
- 79.33 days
- Surface gravity
- 0.223 m/s²
- Key feature
- Equatorial ridge
- Discovered
- 1671 · Giovanni Cassini
Iapetus up close
NASA imagery
A moon divided
Iapetus's leading hemisphere is coated in dark material, while much of the trailing hemisphere is bright water ice. The dark layer is associated with complex organic compounds and likely results from a feedback process involving sunlight, ice loss and redistribution.
The colour contrast is visible through a small telescope. Giovanni Cassini noticed that the moon disappeared when viewed on one side of Saturn and reappeared on the other.
The ridge
A system of mountains rises along Iapetus's equator, reaching about 13 kilometres above the surrounding terrain in places. The ridge stretches across much of the moon and remains unexplained.
Possible origins include uplift from crustal compression, emplacement of icy material from a former ring, or the remains of an ancient collision. Better gravity and topography data would distinguish among these scenarios.
Worth knowing
Iapetus's two hemispheres differ in brightness by a factor of about ten.
Its equatorial ridge is taller than Mount Everest in places.
The moon's orbit has helped constrain the structure of Saturn's extended outer ring.
Watch and explore
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Saturn's Strangest Moon: Iapetus SpaceAtlas
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Research & further reading
Mission imagery and measurements are interpreted alongside orbital data. Follow the source links for the current NASA overview and precise satellite parameters.