1. Concepts 2. Solar System Origin 3. Planetary Processes 4. Earth Processes 5. Meteorites
6. Our Moon 7. Remote Sensing 8. Mercury 9. Mars 10. Venus, Our Twin
11. Jupiter & Jovian Moons 12. Saturn, Rings & Moons 13. Uranus 14. Neptune 15. Pluto, Charon & Comets

Uranus

by Kari Hetcher and Scott Hughes

Read: Chapter 4, Planetary Magnetospheres and the Interplanetary Medium
Re-read: Chapters 14 , Interiors of the Giant Planets, Chapter 15, Atmospheres of the Giant Planets, Chapter 16, Planetary Rings, Chapter 22, Midsize Icy Satellites, Chapter 23, Small Worlds: Patterns and Relationships

 

Obviously, the farther away from the Earth a planet is, the less we know about it. It takes years for a spacecraft to reach Uranus. This beautiful blue planet is twice as far from the Sun as Saturn. Uranus is so far from our own, that it wasn’t even discovered until 1781, after telescopes were invented!

Voyager 2 reached Uranus in 1986 and was able to take some revealing pictures of the planet and its 15 satellites.

 

Visit the site: Welcome to the Planets by California Institute of Technology

Features

Uranus is much smaller than both Jupiter and Saturn, yet has a density of 1.28g/cm3, higher than the other gas giants. Uranus' higher density suggests that Uranus must contain more ice and silicate rock than Jupiter and Saturn.

The most abundant ices in the solar system are water, ammonia, and methane.  Most workers believe that these ices form most of Uranus’ interior. The center of the planet probably consists of a small rocky core, about 40% the size of the Earth.

Uranus has a slight magnetic field, but it, too, surprised scientists. The axis of the magnetic field is tilted 60 degrees from the axis of the planet’s rotation and is offset from the center. How strange! ! ! Visit  Planetary Data System/NASA's  Voyager Uranus Science Summary and Jet Propulsion Laboratory's Voyager Uranus Science Summary to see how Voyager expanded our knowledge of Uranus.

Atmosphere

Even with the help of Voyager 2, Uranus remains a mystery. Thick blue clouds of methane gas obscure the planet, but studies suggest an active lower atmospheric layer of ammonia and water ice clouds circulating around the planet, similar to the Earth’s atmospheric system. The blue color is due to Rayleigh scattering of the blue light, as on the Earth, along with the preferential absorption of red light by the methane gas in the atmosphere. Enhancement of the photographs of Uranus show faint latitudinal bands in the methane clouds. The ammonia clouds below are believed to circulate around the planet parallel to the equator. This would not be surprising, as we see the same sort of circulation on Jupiter, Saturn, and the Earth, but Uranus is unique. The entire planet seems to be tipped onto its side! 

Go to NASA Space Science News to learn about storms on Uranus! 

Go to STScI- PRC99-11: Huge Spring Storms Rouse Uranus to see a terrific little movie about seasonal changes on Uranus (this is a very large file-check your computer to make sure you have enough memory before downloading).

On the Earth, we would attribute the cloud distribution and circulation to the distribution of incoming solar radiation. On Uranus the north pole faces the sun half the time, and the south pole the other half the time. Thus seasons last many years because of its 84 year orbital period. Think about this for a moment. Which pole is north and which one is south? Note that the planet has a retrograde rotation with an axial inclination a bit less than 90 degrees, unless you think of it as a direct rotation around an axis inclined GREATER than 90 degrees. The Nine Planets has an explanation of this process.

Rings

Ten dark, narrow rings surround Uranus. They are some of the darkest objects in our solar system and were not even discovered until 1977. Unlike the rings of Saturn and Jupiter, these rings are made up of huge chunks of rocks and ice, often house-sized boulders! Their dark grey color suggests they may be radiation- decomposed methane ice or an exotic rock type. Interestingly, the ring system lies in Uranus’ equatorial plane. Two shepherd satellites, Cordelia and Ophelia, are located on the inside and outside of the thickest ring.

 

 


Moons

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It is believed that Uranus was tipped on its side early in its accretionary history. The five major moons, along with 10 smaller ones discovered by Voyager 2, all lie along the planet’s equatorial plane. This suggests that the moons may have formed from a nebula of material surrounding Uranus’ equator after the putative Earth-sized impact

No atmospheres are found on any of the Uranian moons. The surfaces seem to consist primarily of water ice and rock. Their interiors probably contain silicate rock, water ice, and methane ice. These moons are denser than either Jupiter or Saturn’s moons. Does this make sense? Shouldn’t the bodies decrease in density farther from the sun because more ice and methane gas would be found there? Interestingly, at the distance from the sun where Uranus accreted, carbon monoxide may have been the most prominent gas. This volatile compound might have released the denser carbon required to make rocks and darken the moon and ring system.

We will discuss the five major moons from the closest to the farthest from Uranus. These are: Miranda, Ariel, Umbriel, Titania, and Oberon.

Miranda

Miranda is the smallest and innermost of the Uranian satellites. Because of its tiny size and location in the solar system, it was expected to be only a bland ball of ice. How surprised were scientists when Voyager 2 photographed Miranda and found it to be a moon unlike any other! Its surface consists primarily of two types of terrain.

The first is heavily cratered and very old, similar to the other satellites we have discussed.

The second terrain type is unique. It appears younger than the first type because it has fewer crater features. Instead, it is dominated by three large, oval structures, named coronae. The coronae consist of parallel bands of light and dark areas surrounding lighter blotches with distinct edges. The bands are possibly ridges and scarps that were created by global expansion in Miranda’s distant past. The ridges and bands seem to have an outer belt surrounding them, which resembles a racetrack wrapping around each corona. This racetrack is actually a large trough about 5 to 6 km deep.

Notice that any faults within the coronae are truncated by these troughs. It is suggested that the coronae formed when large blobs of volcanic ices rose to the surface of the satellite, and huge pieces of solid rock and ice sank into the center of the moon, creating the unique oval patterns. Perhaps some sort of convection started within Miranda, but stopped before the entire satellite could be resurfaced. NASA's Goddard Space Flight Center has a good website for more information on Uranian clouds, rings and moons.

Miranda's Coronae

Despite its small size (only 470 kilometers across), Uranus' small icy satellite Miranda has had a surprisingly diverse and complex geologic history, concentrated in three dark oval- to square-shaped regions called coronae. This image shows portions of two coronae, Arden Corona at lower left and Inverness Corona at right. Coronae are 100-300 kilometers across and consist of a central zone of chaotic ridges surrounded by a zone of concentric ridges and fractures. Ridges appear to be extensional fault blocks in some areas and volcanic extrusions in other areas. The extruded ridges are up to 2 kilometers high and may be composed of ammonia-water lavas.

The concentric pattern of volcanism and tectonism within coronae suggests that they formed over plumes of material rising from the core of Miranda. These plumes spread out as they neared the surface, fracturing the crust and triggering local volcanism. The geologic complexity of Miranda is puzzling because it should have been cold and inactive since shortly after its formation. The heat required to melt large parts of the interior may have been provided by tidal interactions with neighboring satellites and Uranus itself. Similar tidal heating powers the volcanism on Jupiter's moon Io. (Voyager 2 images 26846.11, 26846.14, 26846.26.)

Ariel

Like the moons of Saturn, Ariel shows evidence of two stages of bombardment. This moon has a very complex history for such a small satellite. Ariel has the youngest and brightest surface of all the Uranian moons. Its history can be divided into six main stages.

 

Umbriel

 

Though Umbriel is about the same size as Ariel, it is strikingly different in appearance. Umbriel is the darkest of the Uranian moons, and lacks the rayed craters and tectonic activity seen on Ariel. This may be the oldest surface of all the Uranian moons, having remained unchanged since the period of intense bombardment nearly 4 billion years ago. Its dark color may be caused by carbonaceous dust, though its origin is still uncertain.

 

Titania is the largest of Uranus’ moons, but resembles Ariel in many ways. Ariel’s stages 1-5 can be seen on Titania, though to a lesser extent. The smooth plains are less extensive, the fault systems are less dramatic, and there are smaller craters on Titania. Strangely, there are more craters on Titania.

Oberon is slightly smaller than Titania, but looks very different. Oberon is essentially covered with large impact craters, suggesting is has had little or no resurfacing since the period of intense bombardment. Bright-rayed craters can be seen. Some of the large craters are floored by dark, carbon-rich icy material, but no global resurfacing or smooth plains appear.

 

Assignment:

1.  What is the chemical composition of the atmosphere of Uranus?

2.  What is the composition of the interior of Uranus? What makes it different from the interiors of Jupiter and Saturn?

3.  Make a summary table for each of the five major moons of Uranus. Compare their densities, diameters in kilometers, surface features, ages, and locations in relation to Uranus.

4.  Ariel is too small to have enough accretionary or radioactive heat to produce all of these features. From where did the energy come?

5.  Can you think of any reasons the magnetic field would be tilted from Uranus’ axis? Has this ever happened on the Earth?


Web Sites about Uranus

NASA NASA's main home page, with an excellent search engine
Jet Propulsion Laboratory Another very good home page
Welcome to the Planets  California Institute of Technology
NASA's Spacekids
Ucar Tour of Uranus
Astronomy Today

Glossary
End Of The Module