Geology of the Solar System 13: Neptune


There is a shocking lack of anaglyph images for the outer Jovian planets.

Neptune rotates on its axis once every 16 Earth hours, and it takes 165 Earth years to orbit the Sun. It is the most dense of the Jovian planets at 1.64 g/cm3, and has a crazy magnetic field similar to Uranus’, tilted and not passing through the center of the planet.  This could also be because Neptune is experiencing a reversal of its polarity, but may be because the field is created by convection near the surface instead of in the interior.  Alternative mechanisms for such a magnetic structure are being explored because considering the fact that poles will remain stable for many thousands of years before switching again, it is extremely unlikely that two planets right next to each other are experiencing a reversal of polarity at exactly the same time.

Its atmosphere is banded like Jupiter’s, with spots and cyclones, and is composed mainly of hydrogen, helium, and methane. Its structure is exactly like every other Jovian planet discussed, so I won’t bother explaining it or posting an image. The only point to note is the higher content of carbon and carbon monoxide in Neptune and Uranus compared with Saturn and Jupiter.


One of 8 satellites, it is the innermost moon of Neptune, and orbits the opposite direction we would expect it to. Its orbit is currently circular, but would have been elliptical at one time as the retrograde orbit suggests it did not originate there but was captured by Neptune’s gravitational pull, and captured objects tend to orbit elliptically.  Such an orbit would have caused much tidal heating early in its history, before the circular orbit was established.

There is no evidence of water ice, but there is some methane and ammonia ices.  There is a polar ice cap on the south pole, which should vapourize soon as it is facing the Sun. Because the tilt of the body creates seasons like on Earth, the volatiles produced from this will likely travel to the north pole instead, where they will condense into another ice cap. It has two types of terrain, fractured plains that resemble a cantaloup in appearance, and flooded volcanic plains. There is also some active volcanism, and a very tenuous sort-of-atmosphere.


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