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Neptune - Introduction

Neptune is my favorite planet - I have no idea why. Perhaps its the soothing cool-blue color, resulting from a higher concentration of methane gas. Uranus and Neptune are near twins with the exception of banding of atmospheric clouds on Neptune.

What is exciting about Neptune is not the planet itself, but the manner of discovery. Since Sir William Hershel discovered Uranus, an explanation was attempted to explain its orbit - it seems the orbit of Uranus was a result of another planet beyond. By applying the usual orbital dynamics, the plot of the orbit was made and a telescope was aimed in that direction. Sure enough, there was the planet Neptune.

Math can be our friend!

Neptune - A Quick Summary: (More information can be found on the Neptune Fact Sheet and the Neptunian Rings Fact Sheet)

Average Distance from Sun: 4.498 x 109 km
Eccentricity of Orbit: 0.010
Average Orbital Speed: 5.5 km/s
Orbital Period: 164.86 years
Rotational Period: 16.11 hours
Inclination of Equator to Orbit 29.56
Diameter: 49,528 km
Mass: 1.024 x 1026 kg


Average Density: 1638 kg/m3


Escape Speed: 23.5 km/s 
Albedo: 0.51
Average Cloud-Top Temperature: -218 C
Atmospheric Composition 79% hydrogen
18% helium
3% methane

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The Atmosphere and Interior:

Unlike Uranus, Neptune features Jupiter-like bands in the atmosphere. The blue color is the result if a higher percentage of methane in the atmosphere. There are three prominent features on Neptune:

  • The Great Dark Spot
  • Scooter
  • White methane clouds

Like Jupiter, Neptune has a storm called the Great Dark Spot (who thinks of these names?) and a smaller storm below the Great Dark Spot called "scooter." All the images above show both of these storms. The Great Dark Spot rotates east-west while scooter rotates west-east, and at a faster speed - hence the name. The diagram below demonstrates the Gas Giant atmospheres, including Neptune:

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

In addition to the storms, Neptune also sports occasional high-altitude methane-ice clouds. We know they are high up as shadows are cast underneath, as in the image below:

The Hubble Space Telescope is able to continue study on Neptune, and the image below demonstrates the storm activity on the planet. Neptune's atmosphere is a dynamic place.

The image below demonstrates the interiors of the Gas Giants:

Major differences between Neptune 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 Neptune is off-center and tilted by 47 versus the average tilt of about 12.

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The Rings:

The rings of Neptune are very thin, and are possible composed of methane-ice. There are possibly 3 rings. Collectively, they are called the N-Rings - N meaning Neptune.

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.

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