Thursday, July 21, 2011


Finally we come to Uranus, the last of the Jovian planets in the Solar System, and certainly the least spoken about. Uranus is the 7th planet from the sun and is an ice giant along with Neptune. Uranus has the third largest radius in the solar system, and is fourth largest by mass, essentially switching around the stats of Neptune. Uranus is unique in that it is the only planet in the solar system with a rotation that is not perpendicular, more or less, to the plane of the Solar System. Uranus is tilted 97.77 degrees, causing it to roll around the sun like a ball instead of a top, like all the other planets. Uranus's tilt makes it somewhat funny to view because its moons orbit like the hands of a clock, and its rings can make it look like a target.
Uranus seen with rings

The odd axial tilt of Uranus means that each hemisphere of the planet can be in darkness or light for 42 years. Oddly enough, the equator of the planet is still hotter than the pole of the planet facing the sun, even though the pole gets far more sunlight. We must consider, however, than Uranus is really far from the sun.

Uranus is the coldest planet in the Solar System, with its tropopause getting down to -224 degrees Celsius (49 Kelvin). The outer cloud layer on Uranus is composed largely of methane which gives it its cyan appearance. The inner most cloud layer is believed to be composed primarily of water. Like most gas giants, the atmosphere of Uranus is mostly hydrogen and helium, but because it is an ice giant, there are also large amounts of various ices, such as water and ammonia, mixed in as well. The atmospheres of Jupiter and Saturn account for a large portion of the planets mass, but the atmosphere of Uranus accounts for very little. Uranus has some pretty fast winds by earth standards, getting up to 900km/h, but they cannot compare to the winds on Neptune. These winds are fairly unremarkable by Jovian standards, they are fairly slow and they flow with the same rotation as the core.
Uranus has a fairly plane appearance

Like Neptune, the interior of Uranus is composed mostly of rock and ice. The very core of the planet is a combination of iron, nickel, and silicates, accounting for a small percentage of the planets mass. The next layer is the mantle. The mantle is composed of water, ammonia and other volatile fluids (fluids with a low boiling point). It is here that the water-ammonia ocean is formed. There may also be ionic and super ionic water present, but it's really hard to tell from the observations that we've made. The mantle accounts for most of the planets mass, about 13.4 earth masses. After the mantle there is a final layer of gas that envelopes the core. Temperatures in the core are probably around 5,000 Kelvin.

The magnetosphere of Uranus is tilted 57 degrees from the axis of rotation so that it is more like the magnetospheres of other planets. The magnetosphere is unique in that it does not originate at the planets core, but this isn't that odd for Jovian planets. It is often the case in planets like Neptune of Uranus that their magnetic fields are created by conductive materials at their cores, like the water-ammonia oceans or ionic water. In larger planets, like Saturn or Jupiter, it is the liquid hydrogen that makes these fields. Because of the orientation of Uranus, however, the magnetic field is asymmetric, about .1 gauss over the southern hemisphere to about 1.1 gauss over the northern hemisphere. The magnetic field on earth is pretty much symmetric.

Uranus has 27 moons, all of which are small. The five largest moons are called Miranda, Ariel, Umbriel, Titania, and Oberon with Titania being the largest. Most of the moons are named after characters from works of Shakespeare or Alexander Pope.
Five largest moons of Uranus

And finally, Uranus has a system of rings like all of the other Jovian planets, except, of course, for the fact that they are tilted 90 degrees. There are 13 rings in all made of ice and bits of dust. The particles in these rings are much smaller than the ones around Saturn. A curious feature of these rings is that some of them follow the orbits of some of the moons, suggesting that they got much of their materials from those orbiting bodies.
Orbital paths of moons and rings


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