Statistics: 

• Planet Diameter:  51,000 km, 4.1 Earth Diameters
• Planetary Mass:  8.68x10²⁵ kg, 14.6 Earth masses
• Average Distance from Sun:  2.87 billion km, 19.191 AU
• Length of Day:  17 hours 14 minutes; retrograde
• Length of Year:  84.07 Earth Years
• Atmosphere:  Gas Giant; mostly hydrogen and helium
• Day/Night Surface Temperature:  58 degrees Kelvin

Exploration of Uranus
The Atmosphere
Ring & Moon System of Uranus
Planetary Collisions

Uranus has the distinction of being the first planet in our solar system that was not known to the Ancients.  This is due to the fact that Uranus is so distant from the Earth & Sun that it is not easily seen with the naked eye.  The English astronomer and musician William Herschel discovered Uranus.  In 1781, Herschel reported the discovery to the world.  Interestingly enough, Uranus actually appeared on star charts a hundred years prior to Herschel's work, but it was labeled as a star not a planet.  It was not until 1986, when the Voyager 2 probe encountered Uranus, that we began to learn more about this distant world.

Uranus' Atmosphere

Like Jupiter and Saturn, Uranus is another gas giant planet.  Because the overall density of Uranus has been calculated to be about 1.2 grams per cubic centimeter, scientists long speculated that it was made up primarily of lighter elements such as hydrogen and helium; these speculations were confirmed with the coming of Voyager 2 in January of 1986.  Even when the Voyager 2 probe came to within 80,000 kilometers of the surface of Uranus, it did not see any appreciable detail in the cloudy structure of the gas giant.  Indeed, it has been reported by some to be "somewhat bland."  This fact confirms scientists expectations of the planet; Uranus was known to be very cold in comparison to Jupiter and Saturn, so astronomers guessed that Uranus had no internal heat source, and this would explain the lack of complex activity in the Uranian atmosphere.  Uranus has a blue-green appearance because of an abundance of methane (CH₄) in the lower layers of the atmosphere, and methane molecules absorb primarily red light in the visible wavelengths.

Rings and Moons of Uranus

Rings -- In 1977, Uranus (from the vantage point of the Earth) passed in front of a distant star; this process is known as occultation.  Observers from the university of Cornell boarded NASA's Kuiper Airborne Observatory, an airplane specially fitted for astronomical observations, and flew to the Indian Ocean to observe this occultation.  The team observed that before the occultation, the star seemed to flicker in brightness; likewise, after the occultation the star flickered in brightness again.
 

These dips and flickers in the star brightness were interpreted by the scientists to have been the result of the distant starlight passing through a system of rings around the planet Uranus.  This would explain the flickering both before and after the occultation.  Further observations of more occultations with Uranus revealed more detail about it's ring system.

When Voyager 2 arrived at Uranus in 1986, it confirmed the ring hypothesis.  The probe found a total of 11 faint rings surrounding the planet.  It was also discovered that the rings are not quite following circular orbits around Uranus, and they also have different colors & chemical compositions.  In addition, these rings were found to vary in width and darkness as one followed them around the gas giant; this confused astronomer and planetary scientists as they could not explain the strange structure of these rings.  However, Voyager 2 helped to shed some light on this dilemma when it discovered what are known as shepherd moons.  This is because some scientists believe that these small moonlets serve to shepherd the rings into their current structure through gravitational effects.
 

These newly discovered moons within the actual structure of the rings themselves has allowed scientists to form a better picture of the structure and stability of the Uranian ring system.  As far as the source of the rings and how they might have formed, check out the explanation of the Saturnian ring system.

Moons -- Uranus has a total of 15 known satellites, but for our purposes we will focus on the five largest moons:  Miranda, Ariel, Umbriel, Titania, and Oberon.  The names for these moons came from a colleague of William Herschel named William Lassell; Lassell named these moons for various mythological figures from the fairy kingdom.
 

  

 

All of these moons have similar features; for instance, it turns out that the calculated densities of all five moons is about 1.5 grams per cubic centimeter.  Each moon is seemingly a mixture of rock, water ice, ammonia ice, and methane ice.  These moons can be thought somewhat as large slush balls.  In addition, all of these satellites bear the mark of violence in the form of numerous craters upon their surfaces.

Miranda is perhaps the most interesting moon of Uranus.  Though it is small (500 km in diameter), it shows an incredible variety of geologic features on its surface such as canyons, grooves, rolling hills, crater remains.  Some astronomers believe that perhaps in the distant past, Miranda collided with a body of similar size; however, the impact wasn't strong enough to send all of the pieces flying away from each other.  Eventually, scientists claim, the mutual gravitational attraction between the pieces brought them all back together in a jumbled lump of a moon.

Ariel is about 1200 km across and has many valleys and scarps covering its surface.  Many linear grooves and smooth patches also mark the landscape; these features are presumably the result of tectonic type activity beneath the surface.

Umbriel is comparable in size to Ariel, and it is also the darkest and least geologically active of Uranus' moons.  If one looks closely at the top of the image of Umbriel above, you'll see a bright ring; this is assumed to be a deposit of frost ringing an impact crater.

Titania is among the largest of the moons.  It is assumed that the surface of Titania is extremely old because of the high density of impact craters there.  In addition, like Umbriel, bright frost deposits may make some geologic features show up better.

Oberon is about the same size as Titania, and it contains many impact craters.  Thus, the surface is thought to be very old as well.  In addition, darker material filling in the lowlands of Oberon is thought to have originated inside the moon; therefore, Oberon might be volcanically active.

Planetary Collisions

One last interesting feature of Uranus, is that its rotational axis is tilted roughly 98^o with respect to the rotation of the Sun; most planets rotate on an axis that is roughly parallel with that of the Sun.  The result is that one of Uranus' poles will sometimes point towards the Sun for very long periods of time.  Scientists have speculated that this could be the cause of some very interesting weather & seasonal patterns on Uranus.
 

How could this happen?  It is thought that one explanation for the strange rotation of Uranus is that maybe in its past it collided with a very massive body.  Though the collision was not strong enough to destroy Uranus, it was strong enough to effectively knock the planet over onto its side.  Another explanation is that maybe some large body came close enough to Uranus so that the two interacted strong enough gravitationally that Uranus was pulled over onto its side.  This and many other features of the Uranus system are yet again testaments to the early violence & interactions between the primordial bodies of our solar system.

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