The ancient king of the outer moons
Oberon
Moon of Uranus · discovered 1787 by William Herschel
Oberon is the outermost and second-largest of Uranus' five major moons, an icy world about 1,520 kilometres across. Its ancient surface is saturated with craters, many of them with curiously dark floors, and it is cut by long fault valleys.
Seen only once up close, by Voyager 2 in 1986, Oberon still guards plenty of secrets — from a mountain that may rise around 11 kilometres high to the possibility, suggested by recent models, of a thin ocean buried beneath its crust.
Key facts
- Diameter
- 1,523 km44% of Earth's Moon
- Mass
- 3.08 × 10²¹ kg
- Surface gravity
- 0.346 m/s²0.035 g
- Density
- 1,559 kg/m³
- Distance from Uranus
- 583,520 km≈ 22.8 Uranus radii
- Orbital period
- 13.5 daysSame as its rotation (tidally locked)
- Discovered
- 1787William Herschel
- Surface
- Water ice with dark, carbon-rich material and some CO2 ice
- Atmosphere
- None detected
- Surface temperature
- Roughly −190 °C to −210 °C
- Rotation
- Synchronous — always shows the same face to Uranus
- Named after
- King of the fairies in A Midsummer Night's Dream
Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.
Surface and geology
Oberon is the most heavily cratered of Uranus' major moons, with a surface that appears to date back billions of years. Its largest well-imaged crater, Hamlet, is about 200 kilometres across, and like several others — including Othello and Macbeth — it has patches of very dark material on its floor. This dark stuff may have welled up from below after the impacts, or may be carbon-rich material darkened by radiation, but its origin is uncertain.
Many craters are surrounded by bright rays of ejecta, freshly excavated ice that contrasts with the reddish-grey plains. Oberon's craters are named after heroes of Shakespeare's tragedies, including Hamlet, Othello, Macbeth, Lear and Romeo.
Voyager 2 also spotted a mountain silhouetted on Oberon's limb, estimated to stand around 11 kilometres high — possibly the central peak of a large impact basin. A long canyon called Mommur Chasma suggests the crust was stretched early in the moon's history, perhaps as its interior expanded.
Interior and composition
Oberon is thought to be roughly half water ice and half rock, likely separated into a rocky core and icy mantle. NASA-led modelling in 2023 suggested that, like Titania, it might preserve a thin layer of salty liquid water between core and mantle, warmed by radioactive decay in the rock. This idea is plausible but unconfirmed, and a future orbiter would need to test it with magnetic and gravity measurements.
Orbit and rotation
Oberon takes about 13.5 days to orbit Uranus, on a nearly circular path close to the planet's equatorial plane. It is tidally locked, always showing the same face to the planet.
Because Uranus is tilted by about 98 degrees, Oberon's poles spend roughly 42 years in sunlight and then 42 years in darkness. Its distant orbit also carries it outside Uranus' magnetosphere for part of each circuit, so its surface is weathered differently from the inner moons.
Origin
Oberon probably formed in a disc of gas, ice and dust that surrounded the young Uranus, perhaps produced in the giant impact thought to have tilted the planet. Its crowded crater record suggests it saw relatively little geological activity after an early phase of internal heating and crustal stretching.
Exploration
Voyager 2 is the only spacecraft to have visited Oberon, passing at a distance of several hundred thousand kilometres. Because Uranus' south pole pointed towards the Sun at the time, only the southern hemisphere — roughly 40 per cent of the surface — was imaged.
Discovery (1787)
William Herschel discovered Oberon on 11 January 1787, the same night as Titania. His son John suggested naming the moons after characters from Shakespeare and Pope in 1852.
Voyager 2 (1986)
Voyager 2's January 1986 flyby revealed Oberon's dark-floored craters, bright rays, Mommur Chasma and the tall mountain on its limb.
Telescopes today
Ground-based infrared telescopes and the James Webb Space Telescope study Oberon's surface ices, including small amounts of carbon dioxide ice.
Future missions
A Uranus Orbiter and Probe, ranked the top flagship priority in the 2023–2032 planetary science decadal survey, could map Oberon's northern hemisphere for the first time.
The moons of Uranus
Surprising facts about Oberon
- A mountain on Oberon may rise around 11 kilometres high — taller than Mount Everest stands above sea level — and was spotted only because it poked above the moon's edge.
- Oberon is only slightly smaller than Titania and Saturn's moon Rhea, putting it among the ten largest moons in the Solar System.
- Unlike the inner Uranian moons, Oberon spends part of each orbit outside Uranus' magnetosphere, in the raw flow of the solar wind.
- Several of Oberon's craters have strikingly dark floors, a puzzle that Voyager 2's images could not solve.
- Oberon's craters honour Shakespeare's tragic heroes, so you can find Hamlet, Othello, Macbeth and King Lear all on one moon.