At about 500 km in diameter, it’s only one-seventh as large as Earth’s moon, a size that seems unlikely to support much tectonic activity. Yet Miranda sports one of the strangest and most varied landscapes among extraterrestrial bodies, including three large features known as “coronae,” which are unique among known objects in our solar system. They are lightly cratered collections of ridges and valleys, separated from the more heavily cratered (and presumably older) terrain by sharp boundaries like mismatched patches on a moth-eaten coat. Miranda’s giant fault canyons are as much as 12 times as deep as the Grand Canyon. Due to Miranda’s low gravity and large cliffs, a rock dropped off the edge of the highest cliff would take a full 10 minutes to reach the foot of the cliff.
Scientists disagree about what processes are responsible for Miranda’s features. One possibility is that the moon may have been smashed apart in some colossal collision, and the pieces then haphazardly reassembled. Another, perhaps more likely, scenario is that the coronae are sites of large rocky or metallic meteorite strikes which partially melted the icy subsurface and resulted in episodic periods of slushy water rising to Miranda’s surface and refreezing.
Miranda’s surface is nearly as bright as that of Ariel, the brightest of the larger Uranian moons, but none of them reflect more than about a third of the sunlight that strikes them. This suggests that that their surfaces have been darkened by carbonaceous material. Miranda’s brightness increases dramatically when it is in opposition — that is, when the observer is directly between it and the Sun. This indicates that its surface is porous, casting reflectivity-decreasing shadows when illuminated at other angles. This texture could be the result of eons of micrometeorite strikes tilling the soil.
All of Uranus’ larger moons, including Miranda, are thought to consist mostly of roughly equal amounts of water ice and silicate rock. Unlike the other four main Uranian satellites, Miranda’s orbit is slightly inclined.
Of the five Uranian moons known before Voyager 2 visited the planet, Miranda is the smallest and closest to the planet.
Miranda was discovered in telescopic photos of the Uranian system by Gerard P. Kuiper on 16 February 1948 at the McDonald Observatory in western Texas. It was the last moon of Uranus to be discovered prior to Voyager 2’s visit in 1986.
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“Although Miranda is the smallest of the major Uranian satellites, it displays a complex geologic history. A wide variety of terrains can be identified on Miranda, including old, heavily cratered surfaces and geologically younger, complex terrains.” “Miranda also displays enormous fault scarps, some of which cut across the entire globe. The largest of these include grabens that are 10 to 15 kilometers deep and hundreds of kilometers long. Bright material is exposed in the walls of some fault scarps and craters. This bright material is apparently part of a layer perhaps 1 kilometer thick.”
“Miranda, the innermost of the five large moons, is in a category of its own and a strong candidate for the title of the strangest body yet observed in the Solar System. The impression given by the Voyager images, of a body which has been broken up and then reassembled, may in fact be the only good explanation for its bizarre features. Some areas of the moon exhibit cracking, including some huge fault canyons as deep as 20 km. Others seem to show layering and terraces, and a mixture of old, dark, pock-marked surfaces and younger, smooth bright regions. The latter are most likely the result of internal melting of ice and its subsequent release onto the surface, where it flowed for a while before refreezing. Given Miranda’s small size and low temperature (-187°C), it is hard to see how this could have happened. An additional heat source such as tidal heating caused by the gravitational tug of Uranus must have been involved, implying that Miranda was in a different orbit at some point in its relatively recent history. Even then, it is hard to make ice flow at such low temperatures and in the absence of a blanketing atmosphere, and again we have to think in terms of some mysterious cataclysm, possibly even the same one which titled Uranus sideways. Like much of the outer Solar System, Miranda calls for a more detailed investigation before we can begin to read its history.”
The Compact NASA Atlas of the Solar System by Ronald Greeley and Raymond Batson