Oberon is the outermost of Uranus’ major moons and the most heavily cratered - an ancient icy surface that has barely changed in four billion years. Dark material pooled on some crater floors hints that muddy ice once seeped up from inside, but on the whole this is a world that switched off early.
A mountain roughly 6 km high pokes above its edge in Voyager 2’s images - taller than any peak in the Alps.
Herschel discovered Oberon and Titania on the same January night in 1787.
At around -200 °C, its water-ice crust is as rigid as granite.
Distance decides destiny
Line up Uranus’ moons by distance and a pattern appears: inner Miranda and Ariel show dramatic resurfacing, while outer Umbriel and Oberon keep their oldest faces. The closer a moon orbits, the stronger the tides and the more likely it is to be caught in resonances that flex and heat it. Oberon, orbiting farthest out, escaped that heating almost entirely - so its craters are a preserved record of the early Solar System’s bombardment.
Physical characteristics
Diameter
1,523 km
Mass
3.000e+21 kg
Surface gravity
0.35 m/s²
Density
1.63 g/cm³
Tidally locked
Yes
Mean temperature
-200 °C
Geometric albedo
0.23
Atmosphere and composition
Atmosphere
None detected.
Composition
Water ice and rock; dark material coats the floors of some large craters, possibly erupted from the interior.
Discovery
Discovered by
William Herschel
Year
1787
How
his own 18.7-inch reflecting telescope
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.