Visions of the Cosmos

Planetary Science

The Solar System

The Moons of Uranus


Illustration Credit National Aeronautics and Space Administration NASA

Uranus is inclined on its axis 97.86 degrees.  It takes 84.02 years to orbit the Sun and has a day of 17.24 hours. but has a mass 14.37 that of the Earth and a density of a little over half that of Earth. 

Credit: Lawrence Sromovsky, University of Wisconsin-Madison/ W. M. Keck Observatory. The images were obtained on July 11 and 12, 2004.

Notice how Uranus is inclined on its axis at 97.86 degrees

Once considered one of the less interesting planets Uranus been revealed as a dynamic world with some of the brightest clouds in the outer solar system and 11 rings. The first planet found with the aid of a telescope, Uranus was discovered in 1781 by astronomer William Herschel. The seventh planet from the Sun is so distant that it takes 84 years to complete one orbit. Uranus, with no solid surface, is one of the gas giant  planets (the others are Jupiter, Saturn, and Neptune).  The term gas giant will be explained in more detail under Jupiter.  Strictly speaking Uranus and Neptune may be better described as ice giants

The atmosphere of Uranus is composed primarily of hydrogen and helium, with a small amount of methane and traces of water and ammonia. Uranus gets its blue-green color from methane gas. Sunlight is reflected from Uranus' cloud tops, which lie beneath a layer of methane gas. As the reflected sunlight passes back through this layer, the methane gas absorbs the red portion of the light, allowing the blue portion to pass through, resulting in the blue-green color that we see. The planet's atmospheric details are very difficult to see in visible light. The bulk (80 per-cent or more) of the mass of Uranus is contained in an extended liquid core consisting primarily of 'icy' materials (water, methane, and ammonia), with higher-density material at depth.

It is about four times the diameter of the Earth but has a much lower bulk density.

Moons of Uranus

While most of the satellites orbiting other planets take their names from Greek mythology, the moons of Uranus are unique in being named for Shakespearean characters, along with a couple from the works of Alexander Pope.  Altogether 27 moons are listed on the web by NASA on 3 January 2007 update. 

Oberon and Titania (king and queen of the fairies) are the largest Uranian moons, and were first to be discovered - by William Herschel in 1787. William Lassell, who had been first to see a moon orbiting Neptune, discovered the next two, Ariel and Umbriel. Nearly a century passed before Gerard Kuiper found Miranda in 1948. And that was it until a NASA's robot made it to distant Uranus.

It was not until the coming of the space craft that more were discovered.  The Voyager 2 spacecraft visited the Uranian system in 1986.  It discovered 10 more between 26-154 kilometres in diameter namely  Juliet, Puck, Cordelia, Ophelia, Bianca, Desdemona, Portia, Rosalind, Cressida and Belinda.  Since then astronomers using the Hubble Space Telescope and improved ground-based telescopes have raised the total number to 27 known moons.  They have found 12 more moons which have been named Mab, Perdita, Cupid, Francisco, Caliban, Stephano, Trinculo, Sycorax, Margaret, Prospero, Setebos and Ferdinand.  These later moons are only around 12-16 kilometres across.

In addition to the moons Uranus is also surrounded by a ring system - nothing like as grand as those of Saturn

All of Uranus's larger moons appear to be roughly half water ice and half rock. The composition of the moons outside the orbit of Oberon remains unknown, but they may be captured asteroids.

Miranda, the innermost and smallest of the five major satellites, has a surface unlike any other moon that's been seen. It has giant fault canyons as much as 12 times as deep as the Grand Canyon, terraced layers, surfaces that appear very old, and others that look much younger.

Ariel has the brightest and possibly the youngest surface among all the moons of Uranus. It has few large craters and many small ones, indicating that fairly recent low-impact collisions wiped out the large craters that would have been left by much earlier, bigger strikes. Intersecting valleys pitted with craters scars its surface.

Umbriel is ancient, and is the darkest of the five large moons. It has many old, large craters and sports a mysterious bright ring on one side.

Oberon, outermost of the five major moons, is old, heavily cratered, and shows little signs of internal activity. Unidentified dark material appears on the fl

Titania, the largest moon of Uranus,  is marked by a few large impact basins, but is generally covered with small craters and very rough rocks. There are many faults on Titania indicating there has been internal forces molding its surface.

The Moons of Uranus Statistical Data

Name of Moon Approximate radius/diameter in kilometers Mean Distance from Uranus
Miranda 235  /   470 129,872
Ariel 580  /  1,160 190,945
Umbriel 585  /  1,190 265,998
Titania 789  /  1,578 436,298
Oberon 761  /  1,522 583,519


Miranda Ariel Umbriel Titania Oberon

Photographs taken during the Voyager Mission to the Outer Planets

More about Miranda

The bizarre surface of Miranda is revealed in this mosaic of Voyager 2 images. The bright chevron-shaped marking lies within a feature called Inverness Corona; the bullseye pattern along the bottom edge is called Arden Corona. Although only a very small moon, Miranda's extensive geologic activity has baffled scientists.

Because the Voyager 2 spacecraft was able to approach Miranda far more closely than the other larger moons (to within 13,160 kilometres) it was possible to obtain much higher resolution photographs.  It was not the satellite scientists would have chosen to get close to if they had had a choice, but they had no choice. Voyager 2 had to fly close to the planet in order to get the boost it needed to go to Neptune.   The resolution at which the larger satellites were photographed was around 2 to 3 kilometers.  On the other hand, details on the order of a few hundred meters can be seen on Miranda.   Fortunately, Miranda turned out to be the most remarkable of all the satellites.

Miranda is a small satellite with a mean diameter of 470 kilometers. . Its surface is unlike anything in the solar system with features that are jumbled together in a haphazard fashion. Miranda consists of huge fault canyons as deep as 20 kilometers, terraced layers and a mixture of old and young surfaces.

Given Miranda's small size and low temperature (-187 C )  the degree and diversity of the tectonic activity on this moon has surprised scientists. It is believed that an additional heat source such as tidal heating caused by the gravitational tug of Uranus must have been involved.

Miranda was named after the daughter of the magician Prospero in Shakespeare's play The Tempest.

The Solar System