Miranda (moon)
Miranda is the eleventh moon of Uranus, one of the largest gas giants in our solar system. Discovered in 1948 by astronomer Gerald Peter Kuiper, Miranda is notable for its unique geological features, including a heavily fractured surface that suggests it has been torn apart and reassembled multiple times. The moon’s surface is composed primarily of ice, with signs of significant meteor bombardment, resulting in a high density of craters. Notable formations on Miranda include three large coronae—Arden, Inverness, and Elsinore—which are characterized by alternating light and dark stripes, resembling racetracks.
Miranda exhibits synchronous rotation, always presenting the same face toward Uranus as it orbits the planet approximately every 1.41 Earth days. Its orbit is slightly inclined, raising questions about its formation; it may have been created in situ or captured through gravitational interactions. The surface temperature of Miranda is around 86 kelvins, which may allow for the melting of ammonia-water mixtures, contributing to its geological activity.
Studying Miranda helps enhance our understanding of satellite formation and geological processes, with implications for broader planetary science, including tectonics on Earth. Future exploration missions could provide deeper insights into this intriguing moon and its complex features.
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Miranda (moon)
Miranda is the one satellite of Uranus about which there is good information. The whole Uranus system is different because Uranus is tilted. Miranda is different from any of the other natural satellites studied.
Overview
Miranda is the eleventh of the twenty-seven known moons of Uranus, a gas giant planet that is the seventh planet from the Sun. Miranda was discovered by a Dutch-born American astronomer, Gerald Peter Kuiper, in 1948. Miranda is the daughter of the magician Prospero in William Shakespeare’s play The Tempest. Features and places on the moon Miranda are named for characters in Shakespeare’s plays. When Miranda was discovered, photographic emulsions and techniques had just developed to the stage to be very useful in astronomy. Kuiper was using the McDonald Observatory to photograph the four satellites of Uranus that were known at that time: Oberon, Titania, Umbriel, and Ariel. Upon developing the film, he found a bright spot. A few days later, it was proven that the spot could not be a star and determined that it was, rather, a satellite of Uranus.
Miranda is synchronous with Uranus—that is, Miranda presents the same face to Uranus all the time. As a synchronous satellite, Miranda revolves around Uranus in its orbit while it rotates about its own axis: 33.9 hours (1.41 Earth days). Miranda turns in the same direction as Uranus, which has a prograde rotation. Miranda’s orbit is about four degrees out of the plane of Uranus’s equator; its inclination is 4.3 degrees. The inclination is large enough to cause doubt about whether Miranda was formed by the same process by which Uranus was formed or was instead captured in a close encounter.
Because Uranus is tipped on its side, Miranda’s orbit is almost perpendicular to the orbit of Uranus. Miranda’s eccentricity is small, meaning that the orbit is close to circular. The larger the eccentricity, the more elliptical an orbit; zero eccentricity means a circular orbit. Miranda’s orbit is only slightly elliptical, with the maximum distance from Uranus being 130,069 kilometers and the minimum distance 129,731 kilometers. Miranda is a triaxial ellipsoid with a 480-kilometer diameter along the axis pointed at Uranus. The equatorial axis is 468 kilometers, and the pole-to-pole axis is 466 kilometers.
The surface of Miranda is fractured to such a degree that it seems as if Miranda was torn apart and the pieces put back together in a haphazard manner. Some scientists have stated that Miranda was torn apart and reformed at least five times. Other ideas are that tidal forces caused partial differentiation—a separation of materials with heavier rock sinking and forcing water ice to the surface. Near-infrared spectrometry has confirmed that the surface is ice. The surface shows it to be intensely cratered from meteor bombardment, although some craters have been reduced in height either by a change in the surface or by material ejected by other meteors covering the crater as the material fell back to the surface. One crater is about thirty kilometers across. There are more craters on Miranda than on other outer moons of Uranus. That is to be expected since the closer a satellite is to the planet and its gravitational pull, the greater the density of meteors it encounters.
There are also coronae—large areas of alternating light and dark stripes that are hundreds of kilometers wide. Miranda's three coronae—Arden, Inverness, and Elsinore—are unlike elsewhere in the solar system. They look like racetracks with several tracks side by side. Inverness looks like a chevron of bright material from space on a dark surface. The edges of the Arden and Inverness coronae are a trench with a cliff surrounding the coronae. These cliffs can be ten to twenty kilometers high. The coronae seem to be at a lower altitude than the surrounding surface. Two coronae, Arden and Inverness, have albedos (reflectivity) that differ from the surrounding material. Elsinore does not have the exterior trench and has much the same albedo as the surrounding material. The coronae were formed in order: Arden, Inverness, and Elsinore. Arden is in the leading hemisphere. Elsinore is in the trailing hemisphere. The sharp tip of the chevron edge of the Inverness Corona is very close to the south pole of Miranda.
Fracture lines and gorges are running across the older cratered surface. Few craters in the coronae suggest they are less old than the cratered surface area. Some scientists draw a correlation between the partial differentiation and the coronae. The coronae are a feature of partial differentiation caused by a Miranda-Umbriel-Ariel orbital resonance. The differentiation did not finish for some reason, but the rise of ice and sinking of heavier material formed the coronae.
Two other satellites of Uranus, Umbriel and Ariel, interact with Miranda. A gravitational pull occurs when the satellites are close and then dissipates when they are far apart. This pull, along with the gravitational pull of Uranus, causes the interior of Miranda to flex. This tidal flexing may cause heat, leading to coronae and surface fractures. The gravitational interaction also causes a change in the orbit of Miranda.
A surface temperature of eighty-six kelvins, determined by Voyager 2, may be enough to melt an ammonia-water mixture. Near-infrared spectrometry not only shows that the surface contains a large amount of water ice; there is also a feature in the spectra that may indicate the presence of ammonia hydrate. This mixture would lower the melting point enough that tidal flexing could account for the fracturing and coronae. The other surface material is carbonaceous and/or silicate matter. The density of Miranda (1.2 grams per cubic centimeter) is close to that of ice (1.0 grams per cubic centimeter), so there cannot be a large percentage of other material. The rock fraction can only be 33 percent.
The albedo of Miranda was measured by Voyager at 32 percent. Hubble Space Telescope (HST) measured it at a lower value. One question is, therefore, whether the change in value indicates a real difference, meaning that the northern hemisphere imaged by HST is darker than the southern hemisphere imaged by Voyager, or whether there is another explanation for the difference in values.
At least four theories exist about how Miranda and the other satellites were formed. One is the co-accretion model. Uranus’s gravitational force collected solid particles that were a part of the solar nebula to create a disk that later coalesced into satellites. The accretion disk model, by contrast, postulates that the gravitational force of Uranus collected material from the solar nebula. This material formed a disk and was later collected into a satellite. A third model is the spin-out model, which suggests that some material spun out from the planet as it contracted, forming Miranda and the other satellites. The blow-out model theorizes that an Earth-sized body struck Uranus, causing Uranus to tilt and eject material that formed a disk and later satellites. Some also subscribe to the theory that Miranda was a small body captured by Uranus’s gravitational force as it came close to the planet.
Knowledge Gained
Miranda can be seen with Earth-based instruments, but it is so close to Uranus that it is difficult to study its characteristics. Near-infrared spectra obtained using the United Kingdom Infrared Telescope (UKIFT) revealed much of what is known about the surface composition of Miranda. Earth-based instruments also serve well to study orbits, even for a faraway satellite like Miranda. Once Miranda was known, it was clear that it had been seen in other photographs. Putting all of the sightings together produced a knowledge of the satellite’s orbit.
The goals for the Voyager studies for Miranda were twofold: observational and magnetic. The magnetic goals were to see whether Miranda had a magnetic field (it was determined that it did not), how Miranda interacted with Uranus’s field, and whether there were charged particles in the atmosphere. The observational goals for Voyager were to note the satellite’s size, shape, mass, density, shape changes with time, types of surface structures, surface processes, and albedo change with phase angle. Voyager data concerning albedo are similar but slightly lower than that obtained from the Hubble Space Telescope. Voyager data and years of observations from Earth were used together to calculate better values for Miranda’s orbit, eccentricity, and angle of inclination. The measurement of the mass of Miranda was made from radio science data. Imaging data were used to determine size, which is more complicated than merely measuring distances in a single image. Several images were needed to cover Miranda. By reference to the same feature in at least two images, researchers then generated a mathematical model to calculate where each feature should be. From that model, the size could be calculated.
Comprehensive photometry has been done of Miranda using the HST. All possible phase angles from Earth were studied. A phase angle is the angle between the Sun-to-Miranda and Earth-to-Miranda lines. The geometric albedo and opposition surge were measured for Miranda. Opposition surge is the brighter reflection that occurs at a phase angle of zero.
The data from Voyager and the HST vastly increased our knowledge of Miranda and added to what is known of Uranus’s satellites in general. Before these space-based observations, the number of known satellites of the gas giant was five; it is now known that there are at least twenty-seven Uranian satellites.
Context
Data gained by a study of Miranda may be second in importance to the ideas generated by its unusual surface features. Scientists have to develop explanations for the generation of the features of Miranda. Some information is known, but the total explanation will require innovative ideas.
Ideas about tectonics and how the planet’s surfaces can move and change will help scientists understand Earth's creation and the continents' separation. Studies of tidal flexing, such as that in the Uranian system, may be critical to studying Earth’s plate tectonics. The effect of gravity on the tides is visible, but is that all the effect that gravitational pull has on Earth? Scientists are also learning more about melting points and sublimation values of mixtures as they try to explain Miranda.
The best way to study Miranda will be to mount another interplanetary probe, such as Voyager. Still, until such a mission to Uranus and its satellites can be planned, new techniques are being developed for using land-based instruments and the Hubble Space Telescope to study Miranda and other distant worlds. Humanity will have to improve the instrumentation used presently, and new devices and techniques will have to be designed and tested to expand the study of Miranda.
Bibliography
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Bond, Peter. Distant Worlds: Milestones in Planetary Exploration. Copernicus, 2007.
Corfield, Richard. "The Harmonies of the Ice Giants: Uranus and Neptune." Lives of the Planets. Basic, 2012.
Croswell, Ken. Ten Worlds: Everything That Orbits the Sun. Boyds Mills, 2007.
Elkins-Tanton, Linda T. Uranus, Neptune, Pluto, and the Outer Solar System. Chelsea, 2006.
Loewen, Nancy. The Sideways Planet: Uranus. Picture Window, 2008.
McFadden, Lucy-Ann Adams, Paul Robest Weissman, and T. V. Johnson, eds. Encyclopedia of the Solar System. Academic, 2007.
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"Miranda." NASA, 19 Dec. 2019, solarsystem.nasa.gov/moons/uranus-moons/miranda/in-depth. 15 Sept. 2023.