Uranus' Major Moons Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 16, 2011


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.

Major Moons of the Outer Planets Ariel

Figure 10-6. The four largest satellites of Uranus, along with Earth, as they would appear if placed in close proximity. The relative size of Uranus is represented by the gray curve.

Practice problems of this concept can be found at: Major Moons of Outer Planets Practice Problems

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