Since the demotion of Pluto, Neptune is the furthest of the planets in our solar system from the Sun. It is also the only planet that was discovered as a result of mathematical calculation. In the first half of the nineteenth century, observations of Uranus showed that it was deviating slightly from its predicted path. After several attempts, in 1846 Neptune was finally discovered—within a degree of its predicted position—by an astronomer who took the calculations seriously enough to look.
Neptune is slightly smaller, yet slightly more massive than Uranus, and has a correspondingly higher gravity. In fact, Neptune is the only other planet aside from Jupiter to have a higher “surface” gravity than Earth. Of course, like its cousin Uranus, it does not have a solid surface. Also, like Uranus, it does not have a yellow color like Jupiter or Saturn. Neptune has a chemical composition similar to Uranus, with higher concentrations of methane and other hydrocarbons than Jupiter or Saturn. However, whereas Uranus is bluish green, Neptune is dark blue, with clearly distinct atmospheric bands. Scientist are not completely sure as to why the color difference; theories postulate some unknown atmospheric component, or possibly differences caused by distance from the Sun or the fact that Neptune is not tilted on its side rotationally.
Modeling predicts that Neptune should have a rocky core slightly more massive than the Earth, surrounded by a mantle of “ice” composed primarily of water, ammonia and methane, and topped off by the actual atmosphere. The top regions of the atmosphere contain clouds of varying composition. The highest ones are made of methane. Lower down, where the pressure increases to between 1 and 5 atmospheres, clouds of ammonia and hydrogen sulfide are found. Deeper layers add clouds of ammonium sulfide and water. Interestingly, in spite of being the furthest planet from the Sun and receiving the least amount of solar energy, the winds on Neptune are the fastest found on any planet, with speeds of up to 2100 kilometers per hour having been recorded. The energy driving these winds probably comes from deep within the planet; the core temperature being over 5000 degrees Celsius.
One interesting theoretical possibility is that it may rain methane on Neptune—and that the raindrops could be up to a meter in diameter!
Like Jupiter, Neptune also displays large “spots” in its atmosphere. However, they are very dark instead of red. This is probably because the actual storms themselves occur deeper within the atmosphere, and as a side effect leave a hole in the upper cloud layers. The darkness results from seeing deeper within the atmosphere of the planet. However, at times white spots will form within the dark spots; these are believed to be associated methane clouds in the upper atmosphere.
Neptune has one large satellite—Triton—as well as several smaller ones. And like the rest of the giant planets, it also has a faint ring structure, consisting of five faint but distinct, narrow rings.
Triton is Neptune's largest moon, and the only one of the 7 “giant satellites” in the Solar System to have a retrograde orbit. As such, it is the only large moon to also be classified as an “irregular satellite”, a designation usually only given to small, lumpy moons with eccentric, inclined and/or retrograde orbits. Because of this retrograde orbit, Triton is believed to have originated in the Kuiper Belt (see Trans-Neptunian Objects), instead of as a part of the Neptune system. Interactions with other Kuiper Belt objects ejected it into the planetary space of the Solar System, where it was captured by Neptune.
Like most large objects in the Solar System, Triton is differentiated, with a rocky core at the center, a layer of water ice surrounding the core, and the surface crust. It's relatively high density in comparison with most other icy satellites indicates that the rocky core composes a somwehat larger percentage of its mass. The capture by Neptune would have produced a large amount of tidal heating, and probably left Triton essentially liquid for a long time. Today, the surface is frozen, with over half of it being covered with nitrogen ice. The south pole is covered with a large cap made up of nitrogen and methane ices. Nitrogen geysers dot its surface. Freeze-thaw cycles have left large ridges and valleys across a large amount of Triton's surface.
One unique feature of Triton's surface is the so-called “cantaloupe terrain”, which was given this name because it resembles the skin of a cantaloupe. It has few craters, and is composed of a mixture of fissures and depressions. There are various theories as to its origin, but nothing definite has been concluded.
Triton is the only other moon in the Solar System besides Titan (see Titan on the page about Saturn) to possess an atmosphere capable of sustaining clouds. It is very thin by Earth standards, only about a thousandth the density of Mars' atmosphere. This atmosphere is primarily nitrogen, with trace amounts of carbon monoxide and small amounts of methane near the surface. Evaporation of nitrogen ice, along with nitrogen geysers spewing gas, maintain and replenish the thin atmosphere.
There are only a few impact craters on Triton. This is consistent with the idea that it was captured at some time in the past, and that the tidal heating caused by the capture melted the moon for a considerable length of time, thus erasing any previous craters. Most of the craters that Voyager 2 saw were concentrated on the leading hemisphere, and probably resulted from material in prograde orbit around Neptune colliding with the backward-orbiting moon.
Triton is the coldest place yet visited by space probes.
The smaller moons of Neptune can be divided into two distinct groups. The Regular Moons all have fairly circular, prograde orbits which lie inside the orbit of Triton. Some of these moons are associated with Neptune's rings, providing material and maintaining their stability. Interestingly, the outermost of these regular satellites, Proteus, was not discovered until Voyager 2 flew past Neptune, even though it is larger than Nereid, which had been known since 1949. The reason it was not seen from Earth was because these satellites are all so close to Neptune that they were lost in the glare of the planet.
All of these inner satellites are dark objects. Their spectra indicate that they are composed of water ice, contaminated by some dark material.
The remaining Irregular Moons all orbit well beyond Triton. They tend to have inclined, eccentric and/or retrograde orbits. The innermost, Nereid, shows the spectra of water ice on its surface. It also shows irregular variations in magnitude, suggesting that it may be tumbling or precessing in its orbit. One theory states that it was originally a regular moon, but that its orbit was disrupted when Triton was captured.
Two of the irregular satellites, Psamathe and Neso, have the largest orbits of any moons in the solar system. It takes them around 25 years to complete one revolution around Neptune.
Neptune is the second planet discovered telescopically. Unlike Uranus, it is not at all visible to the naked eye, nor was its discovery an accident. Like Uranus, it has been extensively studied through telescopes since its discovery. Its large moon Triton was spotted almost immediately after the planet itself was discovered, but the rest of its satellites had to wait until the more sophisticated instruments of the 20th century.
Interestingly, examinations of Galileo's drawings of his observations of Jupiter show that he actually saw Neptune! But because of its slow orbital speed he did not detect any motion and assumed that it was just another star.
Only one spacecraft has visited Neptune. Voyager 2 flew past in 1989, befor heading out into the Great Beyond. At this time, no further probes are planned, although some scientists have suggested Triton as an interesting world to visit again.
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