Uranus' Major Moons Help (page 2)
Introduction to Uranus' Major Moons
The major moons of Uranus are believed to resemble “dirty snowballs,” a mixture of water ice and rock. The minor moons, with the exception of Miranda , are much smaller than the major ones and in some sense can be considered captive comets, containing a higher proportion of ice and less rocky material. None of the moons of Uranus has any appreciable atmosphere. Like most of the moons of major planets, they each keep the same side facing their parent at all times, and their orbits are nearly perfect circles.
Titania, the largest moon of Uranus, is only 1,580 km (980 mi) in diameter. It orbits its parent planet at a distance of 436,000 km (271,000 mi). Titania is much smaller than Uranus and between one-eighth and one-ninth the diameter of the Earth (Fig. 10-5). Observations of this moon and analysis of the light reflected from its surface indicate that it is made up of approximately half water ice and half rocky material.
In addition to the usual craters, the surface of Titania has long cracks or valleys. The reason for the existence of these fracture zones is unknown, but one popular theory holds that Titania was liquid at one time and then it froze from the outside in. As the water beneath the surface froze, the ice above cracked because water expands when it freezes. Another theory suggests that heat from the interior produces occasional eruptions of hot liquid or gas that penetrates the surface.
Oberon is just a little bit smaller than Titania, with a diameter of about 1,520 km (950 mi). It orbits Uranus at a distance of 583,000 km (362,000 mi). This moon has a composition similar to that of Titania, but there is some indication that the surface features are older. Fracture zones exist, and their origins suggest that Oberon was geologically active for a while after it formed, but it appears as if Oberon has been a “dead world” for much of its existence.
One of the most interesting features of Oberon is dark material inside many of its craters. The surface consists largely of water ice. At Uranus’ distance from the Sun, ice is as hard as granite unless heating occurs as a result of some other action such as tidal forces or internal activity. Neither of these factors seems to play a role on Oberon, and this makes the origin of the dark material somewhat mysterious. Some astronomers think that the dark material is volcanic lava, but there is little evidence to support this kind of activity on Oberon. Another theory holds that the floors of these craters are relatively smoother than the surrounding terrain and that this is why they appear darker. When a large meteorite strikes an icy body such as Oberon, the heat of impact melts the ice in and around the point of impact. The liquid water pools inside the crater and then refreezes, producing a smooth landscape that reflects relatively little light. You have seen this effect if you have ever looked at a smooth, well-kept outdoor skating rink surrounded by snow.
This satellite has a diameter barely large enough to satisfy our arbitrary minimum size limit to qualify it as a major moon: 1,170 km (730 mi). It orbits 266,000 km (165,000 mi) above its parent planet and takes a little more than 4 Earth days to revolve once around.
Umbriel is notable for its low albedo. The surface is almost entirely charcoal black. The only reason we can see it at all is that it isn’t a perfect light absorber; it reflects approximately one-fifth of the light it receives from the Sun. The orb resembles a gigantic dirty ice ball. Most of the solid material on the surface is water ice mixed with rocky material, but there also appears to be some frozen methane and trace amounts of other frozen elements and compounds that are gases in the familiar environs of our planet Earth.
The entire surface of Umbriel is pitted with craters. One feature, a bright ringlike mark, is thought to be the outline of a crater in which the rim mountains have more exposed ice than either the interior or exterior lowlands. Umbriel shows no signs of geologic activity in the recent past (meaning within the last several million years). Some scientists believe that it has not undergone much change since it was formed as part of the Uranian system.
Ariel is, in terms of size, practically a twin of Umbriel. It measures 1,160 km (720 mi) in diameter. It is much closer to Uranus, orbiting at a mean altitude of 190,000 km (120,000 mi). It takes only 2½ Earth days to orbit once around the planet. Like all the other moons of Uranus, Ariel orbits in a nearly perfect circle and keeps the same face toward Uranus all the time.
Ariel reflects about twice as much light as Umbriel, leading astronomers to surmise that its surface consists of relatively more icy material and less rock. The whole surface is cratered, but there are huge rift valleys and canyons too. There is evidence that a mixture of liquid ammonia and methane once flowed across the surface of this moon.
The long cracks in the surface of Ariel suggest that the moon has expanded or contracted since it was formed, resulting in fault lines. Some of the canyons have ridges inside them, as if liquids once flowed out from the interior and then froze solid when exposed to the chill of space in the outer Solar System.
Figure 10-6 is a size comparison of Titania, Oberon, Umbriel, and Ariel, along with the Earth and the curvature of Uranus.
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