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Uranus’s five largest moons range from about 240 to 800 km (150 to 500 miles) in radius. All were discovered telescopically from Earth, four of them before the 20th century (see below Observations from Earth). Ten small inner moons were found by Voyager 2 in 1985–86. They are estimated to be between about 10 and 80 km (6 and 50 miles) in radius, and they orbit the planet at distances between 49,800 and 86,000 km (31,000 and 53,500 miles). The innermost moon, Cordelia, orbits just inside the outermost rings, Lambda and Epsilon. An 11th tiny inner moon, Perdita, photographed by Voyager near the orbit of Belinda, remained unnoticed in the images until 1999 and was not confirmed until 2003. Two additional inner moons—Cupid, near Belinda’s orbit, and the other, Mab, near Puck’s—were discovered in observations from Earth in 2003. All 18 of the above are regular, having prograde, low-inclination, and low-eccentricity orbits with respect to the planet.
Nine small outer moons in roughly the same size range as the Voyager finds were discovered from Earth beginning in 1997. These are irregular satellites, having highly elliptical orbits that are inclined at large angles to the planet’s equator; all but one also orbit in the retrograde direction. Their mean distances from the planet lie between 4 million and 21 million km (2.5 million and 13 million miles), which is 7–36 times the distance of the outermost known regular moon, Oberon. The irregular moons likely were captured into orbits around Uranus after the planet formed. The regular moons probably formed in their equatorial orbits at the same time that the planet formed. Properties of the known Uranian moons are summarized in the table. Names and orbital and physical characteristics are listed separately for the major moons and the 10 small inner moons originally discovered by Voyager.
The four largest moons—Titania, Oberon, Umbriel, and Ariel, in order of decreasing size—have densities of 1.4–1.7 grams per cubic cm. This range is only slightly greater than the density of a hypothetical object that would be obtained by cooling a mixture of solar composition and removing all the gaseous components. The object that remained would be 60 percent ice and 40 percent rock. In contrast to these four is Miranda, the fifth largest Uranian moon, but only half the size of Ariel or Umbriel. Like the smaller moons of Saturn, Miranda has a density (1.2 grams per cubic cm) that is slightly below the solar composition value, which indicates a higher ice-to-rock ratio.
Water ice shows up in the surface spectra of the five major moons. Because the reflectivities of the moons are lower than that of pure ice, the obvious implication is that their surfaces consist of dirty water ice. The composition of the dark component is unknown, but, at wavelengths other than those of water, the surface spectra seem evenly dark, indicating a neutral gray colour and thus ruling out material such as iron-bearing minerals, which would impart a reddish tinge. One possibility is carbon, originating from inside the moons in question or from Uranus’s rings, which could have released methane gas that later decomposed to produce solid carbon when bombarded by charged particles and solar ultraviolet light.
Two observations indicate that the surfaces of the major moons are porous and highly insulating. First, the reflectivity increases dramatically at opposition, when the observer is within 2° of the Sun as viewed from the planet. Such so-called opposition surges are characteristic of loosely stacked particles that shadow each other except in this special geometry, in which the observer is in line with the source of illumination and can see the light reflecting directly back out of the spaces between the particles. Second, changes in surface temperatures seem to follow the Sun during the day with no appreciable lag due to thermal inertia. Again, such behaviour is characteristic of porous surfaces that block the inward flow of heat.
Virtually all of what is known about the distinctive surface characters of Uranus’s major moons comes from Voyager 2, which sped past them in a few hours and imaged only their sunlit southern hemispheres. Oberon and particularly Umbriel display dense populations of large impact craters, similar to the highlands of Earth’s Moon and many of the oldest terrains in the solar system. In contrast, Titania and Ariel have far fewer large craters (in the range of 50–100 km [30–60 miles] in diameter) but have comparable numbers in the smaller size ranges. The large craters are thought to date back to the early history of the solar system more than four billion years ago, when large planetesimals still existed, whereas the smaller ones are thought to reflect more-recent events including, perhaps, the impacts of objects knocked loose from other moons in the Uranian system. Thus, the surfaces of Titania and Ariel must be younger than those of Oberon and Umbriel. These differences, which do not follow an obvious pattern with respect to either the moons’ distances from Uranus or their sizes, are largely unexplained.
Volcanic deposits observed on the major moons are generally flat, with lobed edges and surface ripples characteristic of fluid flow. Some of the deposits are bright, while some are dark. Because of the very low temperatures expected for the outer solar system, the erupting fluid was probably a water-ammonia mixture with a melting point well below that of pure water ice. Brightness differences could indicate differences in the composition of the erupting fluid or in the history of the surface.
Riftlike canyons seen on the major moons imply extension and fracturing of their surfaces. Miranda’s canyons are the most spectacular, some being as much as 80 km (50 miles) wide and 15 km (9 miles) deep. The rupturing of the crust was caused by an expansion in the volume of the moons, inferred to be in the range of 1–2 percent, except for Miranda, for which the expansion is thought to be 6 percent. Miranda’s expansion could be explained if all the water making up its interior were once liquid and then froze after the crust had formed. Freezing under low pressure, the water would have expanded and thereby stretched and shattered the surface. The presence of liquid water on the surface at any stage of the moon’s history seems unlikely.
Miranda has the jumbled appearance of an object formed from separate pieces that did not totally merge. The basic surface is heavily cratered, but it is interrupted by three lightly cratered regions that astronomers have named coronae (but which are not related geologically to surface features of Venus of the same name). These are fairly squarish, roughly the length of one Miranda radius on a side, and are surrounded by parallel bands that curve around the edges. The boundaries where the coronae meet the cratered terrain are sharp. The coronae are unlike any features found elsewhere in the solar system. Whether they reflect a heterogeneous origin for the moon, a giant impact that shattered it, or a unique pattern of eruptions from its interior is not known.