Miranda

Miranda is the innermost and smallest of the five major moons of Uranus and, topographically, the most varied of the group. It was discovered in telescopic photographs of the Uranian system in 1948 by the Dutch American astronomer Gerard P. Kuiper, who named it after a character in William Shakespeare's play The Tempest. Miranda revolves around Uranus once every 1.413 days in a nearly circular orbit at a mean distance of 129,800 km (80,654 miles) from the centre of the planet. Slightly nonspherical in shape, it has a mean diameter of about 470 km (290 miles).

Because of the trajectory that the U.S. Voyager 2 spacecraft followed in its flyby of Uranus in 1986 (in order for it to be redirected to Neptune), the probe had the opportunity to study Miranda more closely than any other Uranian moon. Photographs from Voyager revealed that Miranda's surface is a bizarre patchwork of winding valleys, parallel grooves, fault scarps, and cratered highlands. The basic surface is heavily cratered, but it is interrupted by three lightly cratered regions that astronomers have named coronae

Miranda in a mosaic of images obtained by Voyager 2 on Jan. 24,1986. In this south polar view, old, heavily cratered terrain is interspersed with large sharp-edged patches of young, lightly cratered regions characterized by parallel bright and dark bands, scarps, and ridges. The patches, called coronae, appear to be unique to Miranda among all the bodies of the solar system. U.S. Geological Survey/NASA/JPL

Miranda in a mosaic of images obtained by Voyager 2 on Jan. 24,1986. In this south polar view, old, heavily cratered terrain is interspersed with large sharp-edged patches of young, lightly cratered regions characterized by parallel bright and dark bands, scarps, and ridges. The patches, called coronae, appear to be unique to Miranda among all the bodies of the solar system. U.S. Geological Survey/NASA/JPL

(but which are not geologically related 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.

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 was 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's topographic variation was a surprise because the moon was thought too small—being only a third the diameter of its much less topographically diverse siblings Titania and Oberon—to have experienced the extensive tectonic activity needed to fashion this varied terrain. It remains to be determined whether this activity resulted from extrinsic forces, such as one or more shattering collisions in the moon's early history, or intrinsic ones, such as eruptions from its interior caused by past tidal heating (as now occurs on Jupiter's volcanically active moon Io).

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