Miranda is the smallest and innermost of Uranus's five round satellites. It was discovered by Gerard Kuiper on 16 February 1948 at McDonald Observatory in Texas, and named after Miranda from William Shakespeare's play The Tempest. Like the other large moons of Uranus, Miranda orbits close to its planet's equatorial plane. Because Uranus orbits the Sun on its side, Miranda's orbit is nearly perpendicular to the ecliptic and shares Uranus's extreme seasonal cycle.
At just in diameter, Miranda is one of the smallest closely observed objects in the Solar System that might be in hydrostatic equilibrium (spherical under its own gravity), and its total surface area is roughly equal to that of the U.S. state of Texas. The only close-up images of Miranda are from the Voyager 2 probe, which made observations of Miranda during its Uranus flyby in January 1986. During the flyby, Miranda's southern hemisphere pointed towards the Sun, so only that part was studied.
Miranda probably formed from an accretion disc that surrounded the planet shortly after its formation and, like other large moons, it is likely differentiated, with an inner core of rock surrounded by a mantle of ice. Miranda has one of the most extreme and varied topographies of any object in the Solar System, including Verona Rupes, a roughly scarp that may be the highest cliff in the Solar System, and chevron-shaped tectonic features called coronae. The origin and evolution of this varied geology, the most of any Uranian satellite, are still not fully understood, and multiple hypotheses exist regarding Miranda's evolution.
Discovery and name
Miranda was discovered on 16 February 1948 by planetary astronomer Gerard Kuiper using the McDonald Observatory's Otto Struve Telescope. Its motion around Uranus was confirmed on 1 March 1948. It was the first satellite of Uranus discovered in nearly 100 years. Kuiper elected to name the object "Miranda" after the character in Shakespeare's The Tempest, because the four previously discovered moons of Uranus, Ariel, Umbriel, Titania, and Oberon, had all been named after characters of Shakespeare or Alexander Pope. However, the previous moons had been named specifically after fairies, whereas Miranda was a human. Subsequently discovered satellites of Uranus were named after characters from Shakespeare and Pope, whether fairies or not. The moon is also designated Uranus V.
Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed an M (the initial of Miranda) combined with the low globe of Jérôme Lalande's Uranus symbol as the symbol of Miranda (). This symbol is not widely used.
Orbit Of Uranus's five round satellites, Miranda orbits closest to it, at roughly 130 000 km from the surface; about a quarter again as far as its most distant ring. It is the round moon that has the smallest orbit around a major planet. Its orbital period is 34 hours and, like that of the Moon, is synchronous with its rotation period, which means it always shows the same face to Uranus, a condition known as tidal locking. Miranda's orbital inclination (4.42°) is unusually high for a body so close to its planet – roughly ten times that of the other major Uranian satellites, and 73 times that of Oberon. The reason for this is still uncertain; there are no mean-motion resonances between the moons that could explain it, leading to the hypothesis that the moons occasionally pass through secondary resonances, which at some point in the past led to Miranda being locked for a time into a 3:1 resonance with Umbriel, before chaotic behaviour induced by the secondary resonances moved it out again. In the Uranian system, due to the planet's lesser degree of oblateness and the larger relative size of its satellites, escape from a mean-motion resonance is much easier than for satellites of Jupiter or Saturn.
Observation and exploration
Miranda's apparent magnitude at opposition peaks at +15.8, making it invisible to many amateur telescopes. Virtually all known information regarding its geology and geography was obtained during the flyby of Uranus made by Voyager 2 on 24 January 1986, The closest approach of Voyager 2 to Miranda was —significantly less than the distances to all other Uranian moons. Of all the Uranian satellites, Miranda had the most visible surface. The discovery team had expected Miranda to resemble Mimas, and found themselves at a loss to explain the moon's unique geography in the 24-hour window before releasing the images to the press.Miner, 1990, pp. 309-319 In 2017, as part of its Planetary Science Decadal Survey, NASA evaluated the possibility of an orbiter to return to Uranus some time in the 2020s.Vision and Voyages for Planetary Science in the Decade 2013–2022 Uranus was the preferred destination over Neptune due to favourable planetary alignments meaning shorter flight times.Revisiting the ice giants: NASA study considers Uranus and Neptune missions. Jason Davis. The Planetary Society. 21 June 2017.
Composition and internal structure
At 1.12 g/cm3, Miranda is the least dense of Uranus's round satellites. That density suggests a composition of more than 60% water ice. Miranda's surface may be mostly water ice, though it is far rockier than its corresponding satellites in the Saturn system, indicating that heat from radioactive decay may have led to internal differentiation, allowing silicate rock and organic compounds to settle in its interior. Miranda is too small for any internal heat to have been retained over the age of the Solar System. Miranda is the least spherical of Uranus's satellites, with an equatorial diameter 3% wider than its polar diameter. Only water has been detected so far on Miranda's surface, though it has been speculated that methane, ammonia, carbon monoxide or nitrogen may also exist at 3% concentrations. These bulk properties are similar to Saturn's moon Mimas, though Mimas is smaller, less dense, and more oblate. In 2024, Strom et al. conducted geological mapping of Miranda's southern hemisphere based on spatially resolved images from the Voyager 2 fly-by, followed by comparison of the observed geological features to modelling of tidal stress in natural satellites. This study suggested that Miranda might have had a liquid ocean of about 100 km thickness beneath the surface within the last 100–500 million years.
Precisely how a body as small as Miranda could have enough internal energy to produce the myriad of geological features seen on its surface has not been established with certainty, though the currently favoured hypothesis is that it was driven by tidal heating during a past time when it was in 3:1 orbital resonance with Umbriel. The resonance would have increased Miranda's orbital eccentricity to 0.1, and generated tidal friction due to the varying tidal forces from Uranus. As Miranda approached Uranus, tidal force increased; as it retreated, tidal force decreased, causing flexing that would have warmed Miranda's interior by 20 K, enough to trigger melting. The period of tidal flexing could have lasted for up to 100 million years. Also, if clathrate existed within Miranda, as has been hypothesised for the satellites of Uranus, it may have acted as an insulator, since it has a lower conductivity than water, increasing Miranda's temperature still further. Miranda may have also once been in a 5:3 orbital resonance with Ariel, which would have also contributed to its internal heating. However, the maximum heating attributable to the resonance with Umbriel was likely about three times greater.
Geography Miranda has a unique surface. Among the geological structures that cover it are fractures, faults, valleys, craters, ridges, gorges, depressions, cliffs, and terraces. This moon is a mosaic of highly varied zones. Some areas are older and darker. As such, they bear numerous impact craters, as is expected of a small inert body. Other regions are made of rectangular or ovoid strips. They feature complex sets of parallel ridges and rupes (fault scarps) as well as numerous outcrops of bright and dark materials, suggesting an exotic composition. This moon is most likely composed only of water ice on the surface, as well as silicate rocks and other more or less buried organic compounds.
+ Main geological structures visible on the known part of Miranda Direct link to all official names of features on Miranda unavailable due to target site structure. Select "Miranda" from "Target" dropdown to view.(all named in reference to works by William Shakespeare) Name Type Length(diameter)(km) Latitude(°) Longitude(°) Origin of the name Mantua RegioRegiones 399 −39.6 180.2 Italian region of part of the plot of The Two Gentlemen of Verona Ephesus Regio 225 −15 250 The twins' house in Turkey in The Comedy of Errors Sicilia Regio 174 −30 317.2 Italian region of the plot of The Winter's Tale Dunsinane Regio 244 −31.5 11.9 Hill in Scotland at which Macbeth is defeated Arden Corona Coronae 318 −29.1 73.7 Forest in England where the plot of As You Like It takes place Elsinore Corona 323 −24.8 257.1 Castle in Denmark that is the setting for Hamlet Inverness Corona 234 −66.9 325.7 Macbeth's castle in Scotland Argier RupesRupes 141 −43.2 322.8 Region of France where the beginning of the plot of The Tempest takes place Verona Rupes 116 −18.3 347.8 Italian city where the plot of Romeo and Juliet takes place Alonso Impact crater 25 −44 352.6 King of Naples in The Tempest Ferdinand 17 −34.8 202.1 Son of the King of Naples in The Tempest Francisco 14 −73.2 236 A lord of Naples in The Tempest Gonzalo 11 −11.4 77 An honest old councilor from Naples in The Tempest Prospero 21 −32.9 329.9 Legitimate Duke of Milan in The Tempest Stephano 16 −41.1 234.1 A drunken butler in The Tempest Trinculo 11 −63.7 163.4 A jester in The Tempest
Regiones The regiones identified on the images taken by the Voyager 2 probe are named "Mantua Regio", "Ephesus Regio", "Sicilia Regio", and "Dunsinane Regio". They designate major regions of Miranda where hilly terrain and plains follow one another, more or less dominated by ancient impact craters. Normal faults also mark these ancient regions. Some escarpments are as old as the formation of the regions while others are much more recent and appear to have formed after the coronae. These faults are accompanied by grabens characteristic of ancient tectonic activity. The surface of these regions is fairly uniformly dark. However, the cliffs bordering certain impact craters reveal, at depth, the presence of much more luminous material.
Coronae
Miranda is one of very few objects in the Solar system to have crowns (also called coronae). The three known coronae observed on Miranda are named Inverness Corona near the south pole, Arden Corona at the apex of the moon's orbital motion, and Elsinore Corona at the antapex. The highest albedo contrasts on Miranda's surface occur within the Inverness and Arden coronae.
Inverness Corona