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Planetary Ponderings - Part 8 of 9: Neptune

by Perry Pezzolanella, MVAS

Neptune is a beautiful, frigid world of storms and its largest moon - Triton - has perhaps one of the most exotic surfaces of any moon in the Solar System. These worlds were once so remote and obscure that it seemed there wouldn't ever be much to ponder, but when it comes to the Solar System, that is an incorrect assumption.

  • Neptune is the eighth planet from the Sun with an average distance of 2.79 billion miles and has the most circular orbit of all the planets varying no more than 40 million miles from the Sun.

  • Neptune is the first planet discovered by mathematical calculation and was first seen on September 23, 1846 by J. Galle and H. D'Arrest. U.J.J. LeVerrier and John Adams are the co-discoverers credited for the calculation.

  • Neptune is the fourth largest planet with an equatorial diameter of 30,690 miles and has 13 moons. A day on Neptune lasts 16.1 hours, but its year lasts 164.8 Earth years.

  • Neptune can come within 2.69 billion miles of Earth, appear as large as 2.4 arc-seconds, and shine as bright as magnitude +7.7. Because Neptune orbits farther from the Sun than Earth, its phase will always appear full.

  • Neptune is a ball of gas that surrounds a small rocky core. The atmosphere is thousands of miles deep and is composed of 84% hydrogen, 12% helium, 3% methane, and the rest a mixture of other gases.

  • Neptune is completely cloudy, but unlike Uranus it is usually free of obscuring haze. Methane in its atmosphere gives Neptune a blue hue since it effectively absorbs the red component of sunlight and scatters the blue. Neptune's cloud tops are as cold as -350��F, but no colder than Uranus in spite of it being further from the sun. This is because unlike Uranus there is ample heat radiating from the core. Because Neptune lacks haze, there is a wealth of weather patterns visible including dark belts, dark spots, white spots, and white streaks. The billowy white spots may be methane thunderstorms and some of the white streaks may be a blizzard of frozen methane and ammonia ice. Neptune has the fastest winds of all the planets with speeds approaching 1400 miles per hour!

  • The Great Dark Spot, discovered by Voyager in 1989, was a clearing in the cloud deck as big as the Pacific Ocean that acted as a window into the depths of Neptune's atmosphere. The Great Dark Spot was not a permanent feature. The Hubble Space Telescope did not observe it in 1994. However, other dark spots have formed and vanished.

  • As summer arrives in Neptune's southern hemisphere, the number of white clouds is increasing along with dark bands and lighter cloud belts. Neptune is looking more like a blue version of Jupiter.

  • There is no solid surface on Neptune. The atmosphere thickens into slush with increasing depth, pressure, and heat and is speculated to become a hot ocean of liquid water wrapped around a rocky core.

  • The hot ocean may be conductive enough to produce a strange magnetic field similar to Uranus' that is completely offset from the rotating core and tilted 55 degrees from the axis of rotation.

  • Aurora and lightning occur on Neptune that rival Earth but are weaker than Jupiter, Saturn, and Uranus.

  • Neptune was suspected to be surrounded by an incomplete set of dark rings in 1981 and was confirmed by the Voyager 2 spacecraft in 1989.

  • Neptune has one major moon, Triton, which is 1680 miles in diameter, but orbits Neptune backwards, leading to the speculation that it was captured by Neptune. At -392 F, Triton is the coldest place in the Solar System except for possibly Pluto. With Neptune growing closer to the Sun since the Voyager 2 encounter in 1989, the temperature has risen to -389 F. Triton has a thin atmosphere of nitrogen only1/100,000th of Earth's, but is 500 miles deep with a fine haze of methane and hydrocarbons in the bottom15 miles.

  • Triton has a huge nitrogen ice cap over a surface that is unusually smooth with few craters. Vast expanses of terrain appear to have been melted and then frozen with a mixture of methane, ammonia, and water. Seasonal evaporation of the frozen nitrogen on the daylight pole creates a wind when vapor rises and is transported to the colder, dark pole where it freezes out.

  • Triton is the only world besides the Earth to have active geysers! These geysers are powered by liquid nitrogen from the warmer interior and tower up to 5 miles high before the winds carry the plumes as much as 100 miles downwind!

  • Nereid is another odd moon that is probably a captured asteroid because it orbits Neptune as close as 839,000 miles and as far as 6 million miles. It is about 210 miles in diameter and is dark and cratered.

  • Voyager 2 flew past Neptune on August 25, 1989 for its only spacecraft encounter ever, but it was a close one at only 3000 miles! A Neptune orbiter mission is under study that would include probes to study Neptune's atmosphere and land on Triton to study the geysers by 2035.

Visions of huge storms raging in the frigid blue atmosphere of Neptune and geysers spewing liquid nitrogen miles into the sky on Triton are more than anyone could ever ponder about worlds that were once nothing more than a tiny, fuzzy bluish disc and a faint whitish star.