Currently, we know that Uranus has at least twenty-one moons. The above image is a "family portrait" of the five largest moons of the planet. From left to right, they are Miranda, Ariel, Umbriel, Titania and Oberon. The Voyager mission discovered ten more moons when it visited the planet in 1986 and since then, astronomers have discovered six more.

Miranda
Although it is the smallest of the five larger moons of Uranus, Miranda nonetheless has presented us with a number of puzzles. If you look closely at the picture at the right, you will notice that about half of the moon looks like a lot of the other moons in our solar system. It has lots of craters that tell us that it has been hit by many objects during its long life.
The other half of the moon is covered by what appear to be giant cliffs, grooves, gullies and other features a lot like you might find in the American West, although much, much larger. What has caused all of these unusual features? In all honesty, nobody really knows, and the only we will find out is to send another spacecraft to the distant planet for an extended visit.

The half of Miranda that is covered with canyons, valleys and giant ridges is the most interesting part of this tiny world. Among the fascinating features of this region is the giant cliff shown in the picture at right.
Although the moon is a little under three hundred miles in diameter, the ridge shown in the picture is taller than Mount Everest here on Earth. How did such a small world become the home of such a large cliff? It is one of the many questions that remain unanswered.

Ariel
The second of the large Moons of Uranus is Ariel. Ariel, with a diameter of almost 720 miles, is much larger than Miranda and is made up mostly of rocky material and ice. The pictue at right shows an interesting world that one side has lots of craters much like our own Moon, but the other side is criss-crossed by what appear to be large grooves of some kind.
What causes such unusual features? One explanation may be that at some time in the distant past Ariel might have been much warmer than it is now. If this is the case, some of the ice in the moon may have melted and smoothed out the surface of the moon and then more recent geological activity may have formed the long, wide grooves we see in the picture.
Once again, it's a question that only extended research will provide the answer to.

Umbriel
The third in the family of the five largest moons of Uranus is Umbriel. Although Umbriel is about the same size as Ariel, the family resemblance ends there.
Where Ariel shows us a face that has lots of interesting and mysterious features, Umbriel is more like what most moons look like. It does have lots of impact craters, but other than that, it's a pretty cold, plain place.
The only really interesting feature of this moon is the bright crater you can see at the top of the picture at the right. This crater is very large, almost seventy miles across, and is also very bright, compared to the surface of the rest of the moon.
Does the interior of this moon contain material that is actually much lighter in color or was the crater just created recently? Either of these would provide an explanation as to why the crater is so much brighter than the rest of the surface.

Titania
At a little over 980 miles in diameter, Titania is the largest moon of Uranus, as befits its name. Like the other moons of the planet that we have seen, Titania is made up of rocks and ice.
Another thing that Titania has in common with its family members is the fact that part of its surface is heavily cratered, and part of it has lots of ridges, gullies and other features that look like erosion features that we see here on Earth and on Mars. What special set of circumstances have occurred during the planet's history could have caused this?
Hopefully, some future mission to the planet will provide answers to the many questions we have.

Oberon
The last of the largest moons of Uranus is Oberon. At a little over 945 miles in diameter, this moon is just a little smaller than Titania.
What Oberon doesn't have that its other family members have is a region that is covered with ridges and gullies. This moon is almost entirely covered by impact craters, much like most of the other moons in the solar system.
If you look closely, though, at the bottom left hand side of the image you can just barely make out what appears to be a "mountain" rising from the surface of the moon. This mountain is over three miles high and is the most interesting feature on the surface of this moon.