Neptune is physically similar to Uranus (review Figures 7-34 and 7-36). Neptune has 17.1 times Earth’s mass, 3.88 times Earth’s diameter, and a density of 1640 kg/m³ (2760 lb/yd³). Its rapid rotation draws its clouds into belts and zones. Unlike on Uranus, however, cloud features can readily be discerned on Neptune. Its whitish, cirruslike clouds (Figure 7-41b) consist of methane ice crystals. As on Uranus, the methane absorbs red light, leaving the planet’s belts and zones with a banded, bluish appearance (Figure 7-41). Like the other giant planets, the atmosphere of Neptune experiences differential rotation. The winds on Neptune blow at speeds of up to 2000 km/h (1240 mi/h)—among the fastest known in the solar system. Neptune’s rotation axis is tilted nearly 30° from the plane of its orbit around the Sun and Hubble Space Telescope observations of Neptune in 2003 showed seasonal changes near its poles.

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Neptune’s discovery is storied because it illustrates a scientific prediction leading to an expected discovery. Recall from Section 2-8 that, in 1781, the British astronomer William Herschel discovered Uranus. The planet’s position was carefully plotted and, by the 1840s, it was clear that even considering the gravitational effects of all of the known bodies in the solar system, Uranus was not following the path predicted by Newton’s and Kepler’s laws. Either the theories behind these laws were wrong or there had to be another, yet-to-be-discovered body in the solar system pulling on Uranus.

Independent, nearly simultaneous, calculations by an English mathematician, John Adams, and a French astronomer, Urbain Leverrier, predicted the same location for the alleged planet. That planet, Neptune, was located in 1846 by the German astronomer Johann Galle, within a degree or two of where it had to be to have the observed influence on Uranus.

In August 1989, nearly 150 years after Neptune’s discovery, Voyager 2 arrived at the planet to cap one of NASA’s most ambitious space missions. Scientists were overjoyed at the detailed, close-up pictures and wealth of data about Neptune sent back to Earth by the spacecraft.

At the time Voyager 2 passed it, a giant storm raged in Neptune’s atmosphere. Called the Great Dark Spot, the storm was about half as large as Jupiter’s Great Red Spot. The Great Dark Spot (Figure 7-42) was located at about the same latitude on Neptune and occupied a similar proportion of Neptune’s surface as the Great Red Spot does on Jupiter. Although these similarities suggested that similar mechanisms created the spots, the Hubble Space Telescope in 1994 showed that the Great Dark Spot had disappeared. Then, in April 1995, another spot developed in the opposite hemisphere.

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Neptune’s interior is believed to be very similar in composition and structure to that of Uranus: a rocky core surrounded by ammonia and methane-laden water (see Figure 7-36). Neptune’s surface magnetic field is about 40% that of Earth. As with Uranus, Neptune’s magnetic axis (the line connecting its north and south magnetic poles) is tilted sharply from its rotation axis. In this case, the tilt is 47°. Also like Uranus, Neptune’s magnetic axis does not pass through the center of the planet (see Figure 7-37).

We saw in Section 7-2 that the magnetic fields of Jupiter and Saturn are believed to be generated by the motions of their liquid metallic hydrogen. However, Uranus and Neptune lack this material, and their magnetic fields have a different origin. These fields are believed to exist because molecules such as ammonia, which dissolve in their water layers, lose electrons (become ionized). These ions, moving with the planets’ rotating, fluid interiors, create the same dynamo effect and, hence, also the magnetic field.

Like Uranus, Neptune is surrounded by a system of thin, dark rings (Figure 7-43). At these distances from the Sun it is so cold that both planets’ ring particles retained methane ice. Scientists speculate that eons of radiation damage have converted this methane ice into darkish carbon compounds, thus accounting for the low reflectivity of the rings.

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Neptune has 14 known moons. Thirteen have irregular shapes and highly elliptical orbits, which suggest that Neptune captured them. Triton, discovered in 1846, is spherical and was quickly observed to have a nearly circular, retrograde orbit around Neptune. It is difficult to imagine how a planet rotating one way and a satellite revolving the other way could form together. Indeed, only the small outer satellites of Jupiter and Saturn have retrograde orbits, and these bodies are probably captured asteroids. Some scientists have therefore suggested that Triton was also captured 3 or 4 billion years ago by Neptune’s gravity. Upon being captured, Triton was most likely in a highly elliptical orbit. However, the tides that the moon creates on Neptune’s liquid surface would, in turn, have made Triton’s orbit more circular.

Triton’s average density of about 2100 kg/m³ (3540 lb/yd³) indicates that it is an equal mix of rock and ice. It has a thin nitrogen, methane, and carbon dioxide atmosphere, which changes density with the seasons on that world. When Triton was younger, the tidal force exerted on it by Neptune due to the moon’s initially elliptical orbit caused Triton to stretch and flex, providing enough energy to melt much of its interior and obliterate its original surface features, including craters. Triton’s south polar region is shown in Figure 7-44. Note that very few craters are visible. While cratering continued, so did resurfacing. Calculations indicate that Triton’s present surface is only about 100 million years old.

Triton does exhibit some surface features seen on other icy worlds, such as long cracks resembling those on Europa and Ganymede. Other features unique to Triton are quite puzzling. For example, the top part of Figure 7-44 reveals a wrinkled terrain that resembles the skin of a cantaloupe. Triton also has a few frozen lakes like the one shown in Figure 7-45. Some scientists have speculated that these lakelike features are the calderas of extinct cryovolcanoes. A mixture of methane, ammonia, and water, which can have a melting point far below that of pure water, could have formed a kind of cold lava on Triton.

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Voyager instruments measured a surface temperature of 36 K (−395°F), making Triton the coldest world that our probes have ever visited. Nevertheless, Voyager cameras did glimpse two towering plumes of gas extending up to 8 km (5 mi) above the satellite’s surface. These are apparently jets of nitrogen gas warmed by interior radioactive decay and escaping through vents or fissures.

Triton continues to create tides on Neptune. Whereas the tides on Earth cause our Moon to spiral outward, the tides on Neptune cause Triton (in its retrograde orbit) to spiral inward. Within the next quarter of a billion years, Triton will reach the Roche limit, at which time pieces of Triton will then literally float into space until the entire moon is demolished! By destroying Triton, Neptune will create a new ring system that will be much more substantial than its present one (Figure 7-46).