All of Uranus’ larger moons, including Ariel, are thought to consist mostly of roughly equal amounts of water ice and silicate rock. Carbon dioxide has also been detected on Ariel.
Ariel’s surface appears to be the youngest of all the moons of Uranus. It has few large craters and many small ones, indicating that fairly recent low-impact collisions wiped out the large craters that would have been left by much earlier, bigger strikes. Ariel is also thought to have had the most recent geologic activity of Uranus’ larger moons. It is transected by grabens, which are fault-bounded valleys.
Ariel has the brightest surface of the five largest Uranian moons, but none of them reflect more than about a third of the sunlight that strikes them. This suggests that their surfaces have been darkened by a carbonaceous material. Ariel’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.
Measurements have shown that Ariel’s surface temperature rises and falls quickly with the coming and going of sunlight, without a “thermal inertia” lag. That supports the picture of a porous surface, which would tend to insulate the moon and keep the subsurface from heating up. This texture could be the result of eons of micrometeorite strikes tilling the soil.
In an earlier time, Ariel appears to have undergone enough heating to allow differentiation — a separation in which heavier material sinks toward the core and lighter material “floats” at or near the surface. Models indicate that tidal interactions with Uranus may provide an important source of heat.
Like the other large Uranian moons, Ariel keeps the same face toward Uranus as it orbits the planet (just as our Moon keeps the same face always toward Earth). Ariels’ orbit is prograde, with a low-inclination and eccentricity in respect to Uranus. This is known as a regular orbit.
Ariel was discovered 24 October 1851 by William Lassell, one of 19th century England’s Grand Amateur astronomers, who used the fortune he made in the brewery business to finance his telescopes.
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“The surface features revealed by Voyager for Ariel are much more complex than those seen on the outer satellites. The older, cratered terrain on Ariel has been extensively fractured faults and grabens. However, very old, large craters appear to be missing, perhaps having been obliterated by an early period of resurfacing. Smaller (less than 60 kilometers in diameter) craters are found in some area; other areas consist of smooth plains, some of which may have been emplaced as flows of ice or water which buried the older terrain. Some of the grabens on Ariel show sinuous valleys on their floors that may represent fluid, or glacier-like flows. Alternatively, these features could be sinuous faults.”
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“The discovery marks the first detection of dry ice on a Uranian moon, the surfaces of which are mostly composed of water ice. Carbon dioxide ice has recently turned up in other places, including Neptune’s largest moon (Triton), the Martian polar caps, and two of Jupiter’s moons. It is also found in comets and in some interstellar dust particles.”
“We’re still working to figure out what combination of processes controls the distribution of the dry ice,” Grundy said, “but it is exciting to have directly detected carbon on Ariel’s surface, in addition to the oxygen and hydrogen of its water ice. The four main elements of life are carbon, oxygen, hydrogen, and nitrogen. Being based on at least three of these four key building blocks, Ariel’s surface chemistry could be remarkably complex.”