A moon stitched together
Miranda
Moon of Uranus · discovered 1948 by Gerard Kuiper
Miranda is the smallest and innermost of Uranus' five major moons, a world only about 470 kilometres across. Yet it has one of the most bizarre landscapes in the Solar System: a jumble of ancient cratered plains, grooved racetrack-shaped regions and cliffs taller than any on Earth.
When Voyager 2 swept past in January 1986, scientists expected a small, dead, ice ball. Instead they found a moon that looks as if it had been broken apart and clumsily glued back together — and they are still debating how a body this small became so restless.
Key facts
- Diameter
- 472 km14% of Earth's Moon
- Mass
- 6.40 × 10¹⁹ kg
- Surface gravity
- 0.079 m/s²0.0081 g
- Density
- 1,200 kg/m³
- Distance from Uranus
- 129,390 km≈ 5.1 Uranus radii
- Orbital period
- 33.9 hoursSame as its rotation (tidally locked)
- Discovered
- 1948Gerard Kuiper
- Surface
- Water ice mixed with darker rocky material and carbon compounds
- Atmosphere
- None detected
- Surface temperature
- Roughly −190 °C to −210 °C
- Rotation
- Synchronous — always shows the same face to Uranus
- Named after
- Prospero's daughter in Shakespeare's The Tempest
Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.
Surface and geology
Miranda's surface is split between two very different kinds of terrain. Large parts are old, rolling and heavily cratered, but cutting across them are three enormous features called coronae — Arden, Elsinore and Inverness — each a few hundred kilometres wide and marked by parallel ridges and grooves that curve around their edges like the lanes of a racetrack. Inverness Corona carries a bright chevron-shaped marking that is visible even in low-resolution images.
The moon's most famous landmark is Verona Rupes, a fault scarp whose height is usually estimated at up to about 20 kilometres, though some studies suggest a somewhat lower figure. Even at the conservative end, it is one of the tallest known cliffs in the Solar System, and in Miranda's feeble gravity a dropped stone would take many minutes to reach the bottom.
Long fault canyons, scarps and tilted blocks run across much of the imaged hemisphere. Together they show that Miranda's crust was pulled apart and reshaped long after the moon first formed, even though it is far too small to have kept much internal heat on its own.
Interior and composition
Miranda has the lowest density of Uranus' major moons, so it is thought to be made mostly of water ice with a smaller share of rock. It may never have fully separated into a distinct core and mantle. Some researchers have argued that its coronae formed above rising plumes of warmer, slushy ice, and a 2024 modelling study of its surface features even suggested Miranda could have hosted a subsurface ocean in the geologically recent past — an intriguing but still debated idea.
Orbit and rotation
Miranda circles Uranus in a little under a day and a half, with its orbit tilted by about 4 degrees to the planet's equator — more than any of the other major Uranian moons. Like its siblings it is tidally locked, keeping one face turned towards the planet.
Because Uranus is tipped over by about 98 degrees and Miranda orbits close to its equatorial plane, the moon shares the planet's extreme seasons. Each pole spends roughly 42 years in continuous sunlight followed by about 42 years of darkness.
Origin
Miranda probably formed from a disc of gas, dust and ice that surrounded the young Uranus. Its scrambled appearance once led to the idea that it was smashed apart by a giant impact and reassembled, but most scientists now favour explanations involving internal heating.
One leading idea is that Miranda was once locked in an orbital resonance with another moon, possibly Umbriel. Tidal flexing during that period would have warmed its interior enough to drive upwelling ice and crustal faulting, before the resonance broke and the moon froze again.
Exploration
Only one spacecraft has ever visited Miranda. Because Voyager 2 arrived when Uranus' south pole faced the Sun, it could photograph only the southern hemisphere, so roughly half of the moon remains unseen at close range.
Discovery (1948)
Gerard Kuiper discovered Miranda in February 1948 using the McDonald Observatory in Texas, nearly a century after the previous Uranian moons were found. He named it after the heroine of The Tempest.
Voyager 2 (1986)
On 24 January 1986, Voyager 2 passed within about 30,000 kilometres of Miranda — closer than to any other Uranian moon, because its route to Neptune required a close pass of Uranus. Its images revealed the coronae and Verona Rupes in stunning detail.
Telescopes today
Large ground-based telescopes and the James Webb Space Telescope can measure Miranda's ice composition from afar, but cannot resolve its surface features.
Future missions
A Uranus Orbiter and Probe was ranked the top new flagship priority in the US planetary science decadal survey for 2023–2032. Such a mission could finally map Miranda's northern hemisphere.
The moons of Uranus
Surprising facts about Miranda
- Verona Rupes may be up to about 20 kilometres tall — roughly ten times the depth of the Grand Canyon — on a moon less than 500 kilometres wide.
- Miranda was the closest target of Voyager 2's entire Uranus encounter, partly because a tight pass of Uranus was needed to fling the spacecraft onwards to Neptune.
- The giant-impact 'reassembled moon' story is a popular one, but today most scientists think heating from tidal forces or rising ice better explains Miranda's patchwork look.
- Miranda's orbit is tilted more than four degrees from Uranus' equator, the steepest of the five major moons and a possible clue to a past orbital resonance.
- About half of Miranda has never been photographed in detail, because it was lost in the long polar night when Voyager 2 flew by.