Moons · Space Codex
Iapetus
One hemisphere is as dark as coal and the other as bright as snow, and a ridge up to 10 km high wraps its equator.
The two-faced moon
At a glance
Key points
From its tilted, distant orbit it has the best view of Saturn's rings of any large moon.
- One side as dark as coal, the other as bright as snow
- A 10 km-high ridge circles its equator
- Its tilted orbit gives a clear view of Saturn's rings
Two faces
Dark reddish dust, probably swept up from the direction of the outer moon Phoebe, coats Iapetus's leading side. The dark patches absorb more sunlight and warm up, so ice sublimates from them and refreezes on the brighter, colder areas. The process runs away until the boundary is sharp. Cassini noticed in 1671 that he could see the moon on one side of Saturn but not the other, and correctly guessed why.
The ridge
A mountain ridge up to 10 km high runs almost exactly along the equator, breaking into isolated peaks on the side facing away from Saturn. Combined with the moon's squashed shape, it gives Iapetus the look of a walnut.
What we still don't know
How the equatorial ridge formed: suggestions include a collapsed ring of debris, a once-faster spin frozen into the crust, and upwelling from the interior.
Missions
- 1977–
The only spacecraft to visit Uranus (1986) and Neptune (1989); interstellar since 5 Nov 2018.
- 1997–2017
Cassini–Huygens
NASA / ESA / ASICompletedOrbited Saturn 2004–2017 and ended by diving into the planet.
First-hand · our 3D model
How we render it
Iapetus is a sphere wrapped in a global map built from USGS and NASA spacecraft mosaics.
It is drawn at about 5.2% of Saturn's radius, enlarged from its true 1.3% so it stays visible. It orbits in Saturn's equatorial frame, inclined 7.6°, with a 79.33-day period kept in true ratio to its neighbours, and keeps one face towards Saturn.
Moon sizes and orbits are compressed so a whole system fits on screen, but never reordered: each moon is drawn at 0.72 × (true size ratio)^0.6 of its planet, capped so no large moon is drawn bigger than it really is, and orbit distances are log-compressed into a band that clears the planet's rings. Orbital periods keep their true ratios, every moon is tidally locked, and on lighter devices only the two largest moons of each planet are drawn.
Related entries
- SaturnRinged gas giant
Sources
- NASA NSSDC Saturnian Satellite Fact Sheet (opens in a new tab)
- JPL Solar System Dynamics: satellite physical parameters (opens in a new tab)
- JPL Solar System Dynamics: satellite mean orbital elements (opens in a new tab)
- NASA Science: Iapetus (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.