Miranda (moon)

Miranda is the smallest and innermost of Uranus's five round moons. It was discovered by Gerard Kuiper on 16 February 1948 at McDonald Observatory in Texas. It is named after Miranda, a character 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 470 km (290 mi) across, Miranda is one of the smallest closely observed objects in the Solar System that might be in hydrostatic equilibrium, which means it is shaped like a sphere by its own gravity. Its total surface area is roughly equal to that of the U.S. state of Texas. The only close-up images of Miranda come from the Voyager 2 probe, which studied Miranda during its flyby of Uranus in January 1986. During the flyby, Miranda's southern half faced the Sun, so only that part was studied.

Miranda probably formed from an accretion disc that surrounded the planet shortly after its formation. Like other large moons, it is likely differentiated, with a rocky inner core surrounded by a mantle of ice. Miranda has one of the most extreme and varied surfaces of any object in the Solar System. It includes Verona Rupes, a cliff about 20 kilometres (12 mi) high that may be the highest cliff in the Solar System, and chevron-shaped tectonic features called coronae. Scientists still do not fully understand how this varied geology formed, and several ideas exist about how Miranda changed over time.

Discovery and name

Miranda was discovered on 16 February 1948 by planetary astronomer Gerard Kuiper using the McDonald Observatory's 82-inch (2,080 mm) Otto Struve Telescope. Its movement around Uranus was confirmed on 1 March 1948. It was the first moon of Uranus found in nearly 100 years. Kuiper named it "Miranda" after a character in Shakespeare's The Tempest. The four moons found earlier, Ariel, Umbriel, Titania, and Oberon, had been named after characters from Shakespeare or Alexander Pope, but those characters were fairies, while Miranda was a human. Later moons of Uranus were named after characters from both writers, whether they were fairies or not. The moon is also called Uranus V.

Moons other than Earth's were never given symbols in astronomy books. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, suggested a symbol for Miranda that combined an M with the low globe of Jérôme Lalande's Uranus symbol. This symbol is not widely used.

Orbit

Miranda is the closest of Uranus's five round moons to the planet. It orbits about 130 000 km from the surface, roughly a quarter farther out than the outermost ring. It has the smallest orbit of any round moon around a major planet. Miranda takes 34 hours to orbit Uranus. Like the Moon, its orbital period matches its rotation period, so it always shows the same face to Uranus, a condition known as tidal locking. Its inclination (4.42°) is unusually high for a moon so close to its planet, about ten times that of the other major Uranian moons and 73 times that of Oberon. Scientists are still not sure why. There are no mean-motion resonances between the moons that could explain it, so one idea is that the moons sometimes pass through secondary resonances. This may once have locked Miranda into a 3:1 resonance with Umbriel for a time, before chaotic behaviour from the secondary resonances moved it out again. Because Uranus has a lesser degree of oblateness and its satellites are relatively larger, escaping a mean-motion resonance is much easier there than for moons of Jupiter or Saturn.

Observation and exploration

Miranda's brightness peaks at +15.8 when it is opposite the Sun in the sky, which makes it invisible to many amateur telescopes. Almost everything known about its geology and geography came from the flyby of Uranus by Voyager 2 on 24 January 1986. Voyager 2 came within 29,000 km (18,000 mi) of Miranda, much closer than it came to any other Uranian moon. Of all of Uranus's moons, Miranda had the most visible surface.

The discovery team had expected Miranda to look like Mimas. They were puzzled by the moon's unique landscape in the 24 hours before the images were released to the press. In 2017, as part of its Planetary Science Decadal Survey, NASA studied the idea of an orbiter returning to Uranus sometime in the 2020s. Uranus was preferred over Neptune because planetary alignments made the flight times shorter.

Composition and internal structure

Miranda is the least dense of Uranus's round moons, at 1.12 g/cm³. This suggests it is more than 60% water ice. Its surface may be mostly water ice, but it is much rockier than its matching moons in the Saturn system. Heat from radioactive decay may have caused internal differentiation, letting silicate rock and organic compounds sink into its interior. Miranda is the least round of Uranus's moons, and it is too small to have kept any internal heat since the Solar System formed. So far, only water has been found on its surface, though methane, ammonia, carbon monoxide or nitrogen may also be present in small amounts. Its features are similar to those of Saturn's moon Mimas, which is smaller, less dense, and more flattened. In 2024, scientists mapped the southern half of Miranda using images from the Voyager 2 fly-by and compared them with models of tidal stress in natural satellites. This suggested that Miranda may have had a liquid ocean about 100 km deep beneath its surface within the last 100 to 500 million years.

It is not yet certain how such a small body could have gained enough internal energy to create its many surface features. The favoured idea is that tidal heating occurred when Miranda was in a 3:1 orbital resonance with Umbriel. This raised its orbital eccentricity to 0.1 and caused tidal friction from the changing tidal forces of Uranus. The flexing warmed its interior by 20 K, enough to cause melting, and may have lasted up to 100 million years. If clathrate was present, it could have acted as an insulator and raised the temperature further. Miranda may also once have been in a 5:3 resonance with Ariel, which would have added some heating, but the heating from Umbriel was likely about three times greater.

Geography

Miranda has a unique surface, with faults, valleys, craters, ridges, gorges, depressions, cliffs, and terraces. It is a mosaic of very different zones. Some older, darker areas have many impact craters, as expected on a small inactive body. Other areas have strips with complex sets of parallel ridges and rupes (fault scarps), along with bright and dark patches that suggest an unusual makeup. The surface is most likely made of water ice, silicate rocks, and some partly buried organic compounds.

The regiones seen in images from the Voyager 2 probe are named Mantua Regio, Ephesus Regio, Sicilia Regio, and Dunsinane Regio. These areas have hills and plains, mostly covered with old craters. Escarpments and grabens mark past movement in the land. Miranda is one of very few objects in the Solar system with crowns (also called coronae). The three known coronae are Inverness Corona near the south pole, Arden Corona at the apex of the moon's orbit, and Elsinore Corona at the antapex. Inverness Corona is a shape about 200 km (120 mi) across, with a bright, chevron-shaped center and many gorges. It is the youngest of the three because it has few impact craters. Arden Corona is about 300 km (190 mi) wide, with dark, curved bands around a rectangular core. Elsinore is similar in size to Arden and is seen near the terminator. Its topography includes furrows called "sulcus," which are like those on Ganymede. Miranda also has huge escarpments, including a gigantic bright cliff called Verona Rupes that forms complex grabens. Simple and transitional craters are common, and no craters with a central peak were seen. Scientists count craters, as they do on Callisto, to estimate the age of the land.

Origin and formation

Scientists have proposed several ideas about how Miranda formed and changed over time. One idea is that it came from the accretion of a disk of gas and dust called a "subnebula". This subnebula may have existed around Uranus for a time after the planet formed, or may have been created after a cosmic impact that tilted Uranus's axis of rotation. Parts of this small moon look surprisingly young compared with the geological time scale, with its newest surfaces only a few hundred million years old. Models predict that a moon of Miranda's size would cool quickly, and neither the heat from its first formation nor the heat from fission of radioactive materials seems enough to explain this long period of activity.

Miranda has the youngest surface of all the moons of Uranus, which suggests its landscape has changed the most. Its history may involve processes such as tidal forces, orbital resonances, partial differentiation, and convection, though the exact mix is still unknown. The patchy surface may partly come from a collision with an impactor that broke the moon apart, after which the pieces joined back into the round shape that the Voyager 2 probe photographed. In 2011, this idea was set aside in favor of ideas involving Uranus's tidal forces. The oldest regions are cratered plains, such as Sicilia Regio and Ephesus Regio. The best explanation for the heat that powered the moon is the one that also explains the volcanism on Io: orbital resonance and tidal forces from Uranus. Later, Miranda cooled and formed grabens, and it may have been affected by resonances with Ariel and Umbriel. A third major period formed the Elsinore and Arden coronae, possibly from a diapir inside the moon, which may have changed its moment of inertia and caused a 90° reorientation. Miranda's history may have lasted more than 3 billion years, from about 3.5 billion years ago to a few hundred million years ago.