By Sabine Stanley, Ph.D., Johns Hopkins University
Uranus, like Jupiter, has multiple moons, but even the largest moons of Uranus are relatively small. Uranus also has multiple rings around it, just like Saturn. So, how were the moons of Uranus discovered, and what’s the structure of the moons like? And, what do we know about the rings of Uranus?
Discovery of the Moons of Uranus
William Herschel, the person who discovered Uranus, was also the first to discover moons orbiting the planet. Six years after discovering Uranus, on a single night, he found the two largest Uranian moons: Titania and Oberon. Both are less than half the diameter of Earth’s Moon.
It took another 60 years for astronomers to find the next two biggest moons, Ariel and Umbriel, which are only the diameter of Earth’s Moon. Then, it took almost 100 years more to find the next one, Miranda, which is only one-seventh the diameter of Earth’s Moon. That was in 1948, and the discoverer was Gerard Kuiper of ‘Kuiper Belt’ fame.
These five moons are the only moons of Uranus large enough to be spherical. The number is similar to what is there at Jupiter, which has four, and Saturn, which has seven. But Uranus has more than these 5 moons. It has at least 27. The others are all smaller and non-spherical.
The Voyager 2 mission found 10 moons during its 1986 flyby, while Earth-based telescopes after the Voyager flyby have found the rest. All the moons are named for magical spirits and characters from the works of Shakespeare and Alexander Pope.
The Regular and Irregular Moons of Uranus
Uranus’s moons can be classified as either regular or irregular, but unlike Jupiter or Saturn, most of the moons known for Uranus are so far regular. The 18 regular moons are on very circular orbits and are moving in the same direction that Uranus is rotating.
This suggests that they formed from an accretion disk surrounding Uranus. But because Uranus is rotating on its side, it means that these regular moons typically have extreme seasonal cycles, just like Uranus does.
One hemisphere of the moons constantly experiences day, while the other experiences night, in the respective summers and winters. Since Voyager 2 flew by Uranus during southern summer, that means we’ve only been able to see the close-up of half of each moon.
The nine irregular moons are on more elliptical, inclined, or retrograde orbits, suggesting they formed elsewhere, but were then captured by Uranus’s gravity field.
Learn more about the Earth-Moon system.
The Composition and Structure of the Moons of Uranus
All of Uranus’s moons are a mixture of ice and rock and tend to have a neutral gray color. The five big round moons are quite similar in terms of composition, being about half rock and half ice. They all have impact craters and tectonic features like canyons and cliffs.
They also tend to be darker than Saturn’s moons, with dark organics mixed in with the icy surfaces. Darker surfaces might suggest that they are older than Saturn’s moons, since being older would give them more time to pollute their surfaces.
The biggest moon, Titania, has a canyon called Messina Chasma, which is about three times longer than the Earth’s Grand Canyon. On Oberon, the highest mountain is similar in height to Mauna Kea on Earth.
At Umbriel, there is a crater called Wunda crater which has a strange bright ring of material in it. It may be carbon dioxide ice. The ring is about five kilometers in width. It’s not known why it’s there or how it could have formed. At Ariel, there are low-lying smooth plains that probably formed from cryovolcanism, where a water and ammonia solution erupted on the surface less than 100 million years ago.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, Wondrium.
Miranda: The Smallest Moon of Uranus
The smallest and innermost of Uranus’s round moons, Miranda, is different from the others. Its patchwork surface seems to have been stitched together from different parts. The topography on Miranda is highly varied.
For example, Miranda hosts the solar system’s largest cliff, called Verona Rupes, which extends 20 kilometers high. If a rock was dropped off the cliff, then because Miranda’s gravitational acceleration is over 100 times smaller than Earth’s, it would take almost 12 minutes for the rock to reach the bottom.
In addition to towering cliffs, Miranda also features grooved oval and chevron-shaped features called coronae. There are three of them in the hemisphere we’ve been able to image. It’s these coronae that give Miranda its patchwork appearance.
Perhaps long ago, Miranda was broken apart by a catastrophic impact, but then the pieces pulled back together gravitationally, but in a mixed up order. The grooved coronae would have then formed as the heavier, rocky parts of the moon descended to the interior and the more buoyant, ice-rich parts rose to the surface.
However, it’s unlikely that an impact so powerful would have left the pieces close enough to regroup. So, it’s more likely that these coronae are actually the result of tidal forces acting on Miranda, bit like what Jupiter is doing to Io.
Tidal forces from Uranus would have repeatedly flexed and squeezed Miranda, causing enough heat for the icy material of the moon to move easily. Rising intrusions of warm ice called diapirs would have pushed up on the surface of Miranda, causing the grooved terrain that is seen. It is possible that a similar mechanism is responsible for the coronae on Venus, except that Venus has rising diapirs of warm rock, not ice, that create the oval, crown- like surface features.
Learn more about Saturn and the rings.
The Rings of Uranus
In addition to moons, Uranus also has rings—13 of them. This sounds like a lot, but the rings are very thin and dark, with some wide separations. They are dark like Jupiter’s rings, but they aren’t dusty like them.
The particles in Uranus’s rings are actually quite big, typically ranging in size from basketballs to large houses. Saturn has some ring particles that are big, but the particles of Saturn’s rings are much brighter as they are more ice-rich. Though it is not known why Uranus’s ring particles are so dark, it may be because they are richer in organics than Saturn’s rings.
These rings were accidentally discovered in 1977. Scientists were interested in studying Uranus’s atmosphere, and they planned to do so by looking at distant stars through the atmosphere. This is called an occultation study.
In this style of study, one waits until the timing is just right, so that when looking from Earth, the planet to be studied passes in front of a faraway, fixed star. As the planet starts passing in front of the star, the planet’s atmosphere covers some of the light from the star.
This causes the star to appear fainter. But more importantly, the atmosphere absorbs certain frequencies of light from that star based on what the atmosphere is composed of.
The surprising thing that happened at Uranus was that even before it reached the star, the star disappeared and reappeared several times. Something was blocking the view of the star, and it wasn’t the planet. The same disappearing act happened in reverse order when the star emerged from the other side of Uranus.
This symmetry meant that what was being seen were not individual moons, but instead, rings. This initial discovery found five rings, and soon more followed. The rings were eventually imaged by Voyager 2.
Though it is not known what the rings are made of, they are likely a mixture of rock and ice. They may have formed from collisions of previous moons surrounding the planet.
Their narrowness suggests that either the rings are extremely young, like 1 million years, or they must have shepherd moons like seen at Saturn for some of its thin rings.
Some shepherd moons have been discovered for Uranus. For example, the small moons Cordelia and Ophelia shepherd one of the rings. In fact, the thinness of rings are inspiring searches for new shepherd moons surrounding the other rings.
The Voyager 2 flyby was the only mission ever sent to Uranus. And it only spent minutes close to the planet, collecting data to reveal much of what we know about the workings of this sideways planet and its equally tilted system of moons and rings.
Common Questions about the Moons and Rings of Uranus
The five largest and the only spherical moons of Uranus are Titania, Oberon, Ariel, Umbriel, and Miranda.
Uranus has a total of 27 moons. Of these, 5 are comparatively bigger and spherical, and the remaining are much smaller and non-spherical.
Miranda is the smallest and innermost of Uranus’s spherical moons. The topography on Miranda is highly varied, including the largest cliff in the solar system called Verona Rupes, which extends 20 kilometers high.
Uranus has 13 rings. These rings are very thin and dark, with some wide separations.