Uranus

Tilted and beautiful, Uranus is the seventh planet out from the sun. The photograph below is colored to show cloud regions. The planet itself appears a very lovely blue color.

It’s not visible to the naked eye, and became the first planet discovered with the use of a telescope. Uranus is tipped over on its side with an axial tilt of 98 degrees. It is often described as “rolling around the Sun on its side.”

Its average distance from the sun is 19 AU. It takes 84 Earth years to travel around the sun once -- for one Uranus year. It takes 17 hours to turn once on its axis, but is that a day like we know it? Not at all. Look at the illustration above. When one of the poles is facing the sun, its entire side will stay light for the full 17 hours and the other pole will stay dark. It is not until it orbits a quarter way around the sun that its "equators" get direct sunlight and there is a day and night sequence for a little while.

Before it was known that Uranus itself was tilted to such a high degree, the tilt was suspected. The moons were seen going around the planet at right angles to the planet's orbit around the sun. The main moons of all the other planets travel around the approximate equator of their planet, and that keeps them on the same plane as the planet's orbit around the sun. But here were Uranus' moons going around the planet "up and down." If the moons were traveling around the planet's equator, then that planet must be really highly tilted -- and that is just what they found. The tilt, and the crazy seasonal changes, produce immense storms. They are quite visible in infra-red, as can be seen in the top photograph here.

Uranus' diameter is 32,000 miles -- four times the size of Earth's. Like all four of the gas giants, all we can see without the help of radar imaging and similar helps is a cloudy cover. The temperature at the tops of the clouds ranges from -243F to -370F -- giving Uranus the coldest atmosphere in the solar system. The clouds are composed of hydrocarbons (methane and ethane) and ammonium hydrosulphide. Like Jupiter, the colored stripes are caused by the sulfur and phosphorus compounds of hydrogen. However, Uranus has so much more methane than either Jupiter or Saturn that its color appears to be primarily blue. This is because methane absorbs the red wavelengths of light and reflects back the blues and greens. Ultraviolet light from the sun turns some of the methane into a hydrocarbon haze, so we can't see the lower layers of the atmosphere.

However, as Voyager flew by, we were able to learn a little more about Uranus. Look at the picture below:

We could not see below the cloud cover, but there is something Voyager could see. Some sunlight which went through the cloud layer did not hit the planet, but was high enough or low enough to simply skim through the atmosphere, in one side and out the other. When this happened, we found spectral signatures of the light which told us the atmosphere below the top of the clouds contains hydrogen (82.5%), helium (15.2%), and methane (2.3%). Ices made up of water and ammonia also are in the atmosphere, but in very small amounts.

Uranus's core is very small for its size, as can be seen above. The orbital characteristics of the moons going around a planet tell us a good deal about the planet's mass, and therefore about its core.

Like all the gas giants, Uranus has rings and moons. In the top photographs the rings are clear and the four little white dots are four of its 27 moons we have discovered so far. The rings are dark and narrow. The particles in Uranus' rings appear as dark as lumps of coal, ranging in size from 0.1 to 10 meters (12yards) in diameter. Like Saturn and Jupiter, the ring system is within 2 of the planet's radii out from the center of the planet (with Saturn, they start there and extend out much farther).

In the same way the F ring of Saturn has shepherd moons, so does Uranus. Because Uranus is so far away, we are still finding out how many of its moons behave in this way. We are pretty sure about 15 moons do.

The illustration above shows the shepherd moons in the rings. Of these, Miranda is the largest. It is also the closest of the five largest moons: Miranda, Ariel, Umbriel,Titania, and Oberon -- all of which orbit around the planet's equator the same way the shepherd moons pictured above do. Those five are a mixture of water ice, methane ice and rock. We assume the other moons above are the same.

The smallest of the five, Miranda, has some very interesting features:

Miranda is only 390 miles across, so what are these square features? We have no idea. There is a suggestion that there has been tidal heating because it is so close to Uranus, and this has caused heating of its interior. This would cause melting in areas, subsequent sinking and then resurfacing. But why squares? The only other suggestion is that this moon was hit hard enough at one time for it to split along fault lines, but not hard enough to shatter it. It has cliff which can be seen at the top left that is thirteen miles high (twice as high as Mt Everest and here it is on the smallest spherical object in the solar system). It is the largest known cliff in the solar system. What geologists have recognized on this little moon are craters, geological regions, fault scarps and parallel grooves. The square features are cetainly "geological regions."

Uranus also has a number of captured 'rocks' which orbit it at odd angles and therefore are also called "moons." Here is an illustration of their orbits:

Because Uranus is tilted so severely, we were not sure where its north and south pole would be. Here is what was found:

Because of this, the plasma sphere is a little different at different times of Uranus' year: