Formation

The formation of Uranus has long been a mystery. If the planet formed in the manner that Jupiter and Saturn did, it would need to be older than the solar system. An alternative scenario considers the fact that bodies as far from the Sun as Uranus likely contained large amounts of ice. Collisions between bodies would have been powerful enough to vaporize the ices. While most would simply condense due to the extremely cold temperatures, some of the vapors would mix with the gaseous nebula, increasing its density. As the density of gasses increases, the ease with which a smaller planet core could trap them increases. As a result, cores as small as Mars could have acquired very dense gaseous envelopes at the distance of Neptune. This result allows for Uranus to have formed within the solar system.

Composition

The average density of Uranus is much higher than the density of the gas giants and therefore indicates that their internal structures are very different. If they were of comparable composition, Uranus’s smaller mass would produce less gravitational pull and therefore less compression. However, with an average density of 1320 kg/m3 Uranus’s density is comparable to that of Jupiter (1330 kg/m3)and greater than that of Saturn (690 kg/m3). It can be concluded that Uranus contains much greater proportions of heavy elements than those of the gas giants.

Although Uranus contains a greater percentage of heavy elements than its proportions of hydrogen and helium, it does not support the idea that with increasing distance from the Sun, less hydrogen and helium are vaporized. There should be a greater abundance of hydrogen and helium based on their distance from the Sun.

Further confusion arises when addressing the masses of the two outer most planets. With a solar nebula so sparse at the location of Uranus and Neptune, it should have taken tens of hundreds of millions of years for them to grow to their current size around a core of icy planetesimals. Observations of other protoplanetary disks do not appear to survive that long without dissipation. This evidence leads not only to questioning the composition and mass of Uranus and Neptune but their existence at all.

One hypothesis for such occurrences is that these planets formed in the denser regions of the solar nebula between 4 and 10 AU. At this location they could have grown to their present size more rapidly, and gravitational interactions with Jupiter and Saturn forced them into their current locations. A much sparser solar nebula stopped the growth of the planets leading to their present sizes.

An alternative hypothesis is that Uranus and Neptune formed in their current locations from the gasses of the solar nebula, rather than around a core of icy planetesimals. With a ball of gas formed, icy particles would have migrated to the center forming a solid core. Through this method, the formation of the planets would have taken only a few hundred years. However, their sizes and percentages of hydrogen and helium would have been much greater. If the presence of a hot star existed near the solar nebula, large amounts of ultraviolet radiation would have stripped the planets of their hydrogen and helium leaving smaller planets with higher levels of heavy elements. The thicker areas of the solar nebula in which Jupiter and Saturn are present would have protected them from such ultraviolet radiation leaving them in their presents states.

A current model for the internal composition of Uranus suggests a rocky core roughly the size of Earth. This core is surrounded by a mantle of liquid water and ammonia. There is also a layer of liquid helium and hydrogen with a small percent of methane which surrounds the mantle. This helium and hydrogen layer is much shallower than that of the gas giants. Furthermore, with smaller masses, there is not enough pressure to turn the liquid hydrogen into liquid metallic hydrogen as seen in Jupiter and Saturn.

Atmosphere

Uranus’s atmosphere is very hydrogen rich with helium some methane and traces of water and ammonia. Uranus gets its blue green color from the methane present in its atmosphere. Sunlight is reflected from Uranus's 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 seen from Earth.

Uranus has a very high proportion of heavy elements compared when compared with Jupiter and Saturn. This is probably because Uranus retained less of its original hydrogen atmosphere than the larger planets have.

There appears to be little structure to the atmosphere besides some hazy layers. The atmosphere is cold and clear to great depths and there appear to be no clouds or haze in the lower atmosphere. There is a polar haze which is most likely composed of methane and acetylene.

The temperature at Uranus’s cloud tops is 57 K and its atmosphere is cold to great depths. The most unusual aspect of Uranus’s atmospheric temperature is that the equator is cooler than each of the poles and the pole facing the sun is cooler than the shadowed pole.

As the spring equinox began on Uranus in 2004, storm clouds formed in the northern hemisphere where winds gusted to 112 m/s. Scientists predict that the cloud and banded structures in the northern hemisphere may become more pronounced as Uranus continues around the Sun.

Magnetic Field

Information retrieved from the Voyager 2’s magnetometer and radio emissions from charged particles at it’s magnetosphere showed that the magnetic field of Uranus was titled at a steep angle to it’s axis of rotation. The magnetic axis of Uranus is tilted 59o from its axis of rotation. The magnetic field is also off set from the center of the planet's mass. It is believed that this misalignment may be due to a reversal of the magnetic field, similar to that which the Earth has experienced several times. Another source may have been catastrophic collisions with large planet size bodies, as suggested by its rotation axis and system of moons.

Uranus’s magnetic field is not generated in the same fashions as that of the gas giants. Under high pressures found in the liquid mantles of the planet, dissolved molecules such as ammonia become ionized as they loose electrons and become electrically charged. As water is a conductor in the presence of charged particles, electric currents in this fluid are believed to be the source of Uranus’s magnetic field.

Unlike the Earth's magnetic field, and those of the other planets (except Neptune), Uranus' magnetic field is not axisymmetric nor is it extremely strong in its dipole component when compared to its quadropole and octopole components. The reasons for this strange behavior are not entirely known, but a theory has been proposed recently that attributes the oddity to the composition of Uranus' dynamo.

The Earth's dynamo consists of a solid inner core and a convecting, liquid outer core. In the proposed theory, the dynamo of Uranus would consist of three layers: a solid inner core, a stably stratified fluid region, and a convecting liquid outer region. In this model, the convecting liquid region acts as a thin shell to the stratified liquid region and the solid inner core is very small. Simulations of planets with such a dynamo do in fact produce non-axisymmetric, non-dipolar magnetic fields.

Alternatively, the solid inner core of Uranus might be a much, much weaker conductor than its fluid outer core and the stratified liquid region need not exist.

Unique Features

One of the most unique things about Uranus is its orientation in orbiting the sun. Unlike the other seven planets, Uranus orbits the Sun on its side, with axial tilt of 97 degrees. Due to this unique tilt of the planet a night at one of its poles lasts up to 21 Earth years, and during this time there would be no light or heat received from the sun.

The Numbers

Discovered By	William Herschel
Discovery Date	1781
Average Distance from the Sun	2,870,972,200 km (19.191 AU)
Perihelion	2,735,560,000 km (18.286 AU)
Aphelion	3,006,390,000 km (20.096 AU)
Equitorial Radius	25,559 km
Equitorial Circumference	160,592 km
Volume	5.9142 * 1013 km³
Mass	8.6849 * 1025 kg
Density	1.30 g/cm³
Surface Area	8.1156 * 109 km²
Equitorial Surface Gravity	8.43 m/s²
Escape Velocity	76,640 km/h
Sidereal Day	-0.7196 Earth days (retrograde)
Sidereal Year	84.02 Earth years
Mean Orbital Velocity	24,607 km/h
Orbital Eccentricity	.047168
Orbital Inclination to Ecliptic	0.770 degrees
Equitorial Inclination to Orbit	97.86 degrees
Orbital Circumference	1.762 * 1010 km
Effective Temperature	-216 °C
Namesake	Roman god of the sky

Source: http://sse.jpl.nasa.gov/planets