Iapetus is the Solar System’s two-tone moon: the hemisphere that faces forward along its orbit is as dark as coal, while the rear-facing hemisphere is as bright as snow. Adding to the strangeness, a ridge of mountains up to 13 km high runs almost exactly along its equator, giving the moon the profile of a walnut.
Its dark face reflects ~4% of sunlight; the bright face reflects over ten times more.
The equatorial ridge rises up to 13 km - taller than any mountain on Earth.
Giovanni Cassini realised in the 1600s that he could only see it on one side of Saturn - the first hint of its two-tone surface.
A runaway of light and dark
Dark dust shed by distant Phoebe drifts inward and dusts Iapetus’ leading face. Dark ice absorbs more sunlight, and over the moon’s slow 79-day rotation it warms to around -145 °C - enough for the ice to sublimate away and recondense on cold bright terrain. Dark areas therefore get darker and bright areas brighter: a thermal feedback loop that painted a moon in black and white.
Physical characteristics
Diameter
1,469 km
Mass
1.810e+21 kg
Surface gravity
0.22 m/s²
Density
1.09 g/cm³
Rotation period
1,904 h
Tidally locked
Yes
Mean temperature
-180 °C
Atmosphere and composition
Atmosphere
None.
Composition
Mostly water ice with a little rock; a dark carbon-rich veneer coats the leading face.
Discovery
Discovered by
Giovanni Cassini
Year
1671
Missions
Voyager 2NASA · Flyby probe · 1977 · active
Launched 16 days before its twin, Voyager 2 is the only spacecraft ever to visit all four giant planets - Jupiter, Saturn, Uranus and Neptune. It entered interstellar space in 2018 and is still returning data.
Cassini orbited Saturn for 13 years, delivering ESA’s Huygens probe to the surface of Titan in January 2005 - the first landing in the outer Solar System. It ended with a deliberate plunge into Saturn in 2017 to protect the potentially habitable moons Enceladus and Titan.