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Uranus - Introduction

Uranus was discovered by accident by Sir William Herschel in 1781. While systematically surveying the sky, he noticed a "star" that seemed to shift in one direction in the sky. After plotting its course and applying Newtonian math, he was able to determine this star was in fact a planet, twice the distance from the Sun as Saturn.

We know very little about Uranus and much of our knowledge comes from only one probe and Earth based observation. What we do know is there are differences when compared to Jupiter and Saturn. In addition, the axis of rotation is tilted by 98 so that the north and south poles face the Sun at alternate times.

Uranus - A Quick Summary: (More information at Uranus Fact Sheet and Uranian Rings Fact Sheet)

Average Distance from Sun: 2.871 x 109 km
Eccentricity of Orbit: 0.0429
Average Orbital Speed: 6.83 km/s
Orbital Period: 84.099 years
Rotational Period: 17.24 hours
Inclination of Equator to Orbit 97.86
Diameter: 51,118 km
Mass: 8.682 x 1025 kg
Average Density: 1318 kg/m3
Escape Speed: 21.3 km/s
Albedo: 0.56
Average Cloud-Top Temperature: -218 C
Atmospheric Composition 82.5% hydrogen
15.2% helium
2.3% methane

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The Atmosphere and Interior:

The atmosphere of Uranus is gaseous like Jupiter and Saturn, but contains a higher ratio of heavier elements (to Astronomer's, a heavy element is any element heavier than helium). There is also a greater percentage of methane which absorbs longer wavelengths resulting in the bluish color. Like Jupiter and Saturn, Uranus rotates differentially; however, there are no clearly defined bands of clouds on Uranus. The wind speeds on Uranus are at high speed - near 700 km/h - but are in reverse at the equator (rotating to the west instead of east). It is presumed this is the result of the extreme tilt of Uranus resulting in more sunlight at the poles verses the equator.The third image above demonstrates the heating at the poles as a result of the tilt. The diagram below demonstrates the atmospheres of the Gas Giants, including Uranus:

The image above provides much information like gross composition, temperature and altitude.

The image below demonstrates the interiors of the Gas Giants:

Major differences between Uranus and both Jupiter and Saturn is the lack of liquid metallic hydrogen.

The differences between Uranus/Neptune and Jupiter/Saturn is an area of active study. There are two competing theories to explain the structural differences and atmospheric differences:

  • Uranus and Neptune formed closer to the Sun, at a distance between 4 and 10 A.U. then moved outward

  • Uranus and Neptune formed independently from the rest of the Solar System, but used material from the Solar Nebula

In other words, Uranus and Neptune should have less heavier elements than Jupiter and Saturn, not more. To add to confusion, the magnetic field of Uranus is tilted by 59 versus the average tilt of about 12.

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The Rings:

The rings of Uranus are very thin and are possibly made of methane-ice. The very narrow orbits of these rings are possibly the result of the numerous shepherd moons surrounding the ring. Collectively, the rings are called the U-Rings - U meaning Uranus.

The dark color of the rings may be the result of converting methane-ice to carbon ice, a process called radiation darkening. This is one explanation for the low reflectivity of the rings.

The image below demonstrates a color enhancement of a Voyager 2 image. This shows all 9 rings.

The beautiful colors are the result of the computer enhancements and are not a natural feature of the ring.

Hubble recently imaged the ring system whichs shows just how narrow they can be.

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