Uranus' Major Moons Help

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

Introduction to Uranus' Major Moons

Uranus has numerous moons. Four of them can be considered major satellites, in the sense that they have diameters greater than 1,000 km (620 mi). These four moons are Titania, Oberon, Umbriel , and Ariel . As we have already learned, Uranus is tilted on its axis so much that its equator lies almost perpendicular to the plane of its orbit around the Sun. The moons of Uranus orbit in or near the plane of the planet’s equator, rather than the plane of the planet’s orbit around the Sun. This is to say that the entire planet-moon system is tipped almost perfectly on its side.

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.

Major Moons of the Outer Planets Oberon

Figure 10-5. The relative size of Uranus’ largest moon Titania, the planet Earth, and the planet Uranus.

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.

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