Neptune is a beautiful blue world giving an Earth-like feel, but it could not be more different as it is brutally cold and stormy. Triton may be one of the most exotic moons known with frigid ice geysers and a thin veil of haze. These worlds are so remote that they were once obscure with little known about them. Much has been discovered and there are more wonders to come from future missions. This roundup of facts and figures will make one a pro when talking about Neptune.
Neptune is the eighth planet from the Sun and orbits at an average distance of 2.79 billion miles. It has the most circular orbit of all the planets; it deviates no more than 40 million miles which is amazing considering its huge orbit. It will never collide with Pluto.
Neptune is the first planet discovered by mathematical calculations 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-discovers credited for the calculation.
Neptune is the fourth largest planet with an equatorial diameter of 30,690 miles and has 16 moons. It is tilted about 28º on its axis. 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 arcseconds across, and shine at magnitude +7.7, which means it is nothing more a tiny, dim, bluish-gray disc. Because Neptune orbits farther from the Sun than Earth, the phase will always be full.
Neptune is a ball of gas that surrounds a small rocky core. The atmosphere is thousands of miles deep and composed of 84% hydrogen, 12% helium, 3% methane, and the rest a mixture of frigid gases and ices containing carbon, oxygen, nitrogen, and sulfur, thus classifying it as an Ice Giant planet like Uranus.
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. The cloud tops are as cold as -350ºF, but no colder than Uranus in spite of it being nearly a billion miles further from the Sun. This is because more heat is being radiated from the core than Uranus.
The lack of haze readily reveals a wealth of weather patterns including dark belts, dark spots, white spots, and white streaks. The billowing white spots may be methane thunderstorms and some of the white streaks may be a blizzard of frozen methane and ammonia ice. Powerful storms rage on Neptune with methane raindrops as large as beach balls and winds approaching 1400 miles per hour, the fastest in the Solar System!
The great Dark Spot, discovered by Voyager 2 in 1989, was a clearing in the cloud deck as big as the Pacific Ocean that served as a window into the depths of Neptune’s atmosphere giving it the appearance of a blue Jupiter. It was not a permanent feature as 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 belts.
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 55º from the axis of rotation. Aurora and lightning also occur 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 known as ring arcs and was confirmed by Voyager 2 in 1989. There are five main rings; three are hardly more than 60 miles across and the other two between 1000 and 3000 miles across.
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. It orbits Neptune around 221,000 miles and takes 5.9 days to orbit. At -392ºF, Triton rivals Pluto as the coldest place in the Solar System. It has a thin atmosphere of nitrogen, only 1/100,000th of Earth’s, but is 500 miles deep with a fine haze layer of methane and hydrocarbons in the bottom 15 miles.
Triton has a huge nitrogen ice cap over a surface that is uniformly 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 one of few worlds besides Earth to have active geysers These geysers are powered by liquid nitrogen from the warmer interior and tower up to five miles high before the winds carry the plumes as much as 100 miles downwind.
Nereid is an 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 takes 360 days to orbit Neptune, is about 211 miles in diameter, and is dark with craters.
Voyager 2 flew past Neptune on August 25, 1989 for its only spacecraft encounter, but it was a close one at 3000 miles! Several missions have since been planned, but none approved. We are overdue for a return mission and the most logical one is an orbiter that would orbit Neptune for a minimum of four years and drop a probe into Neptune’s atmosphere. It will make multiple close passes of Triton and maybe deploy a lander or rover near a geyser.
The huge, blue stormy world of Neptune and towering geysers of liquid nitrogen on Triton are awesome to imagine - they are real, yet so far away! Both worlds are so inspiring that missions to them almost became reality before a Uranus mission. Now that is a fun fact that perfectly summarizes these amazing worlds!