Neptune and Uranus are both blue planets because the trace of methane in their atmospheres absorbs red light, leaving only blue light to be reflected or emitted. Unlike its water-rich interior, Neptune's atmosphere consists of between 16 and 22 percent helium, about 2 percent methane, and the remainder hydrogen. A few other trace molecules are thought to contribute to cloud formation, including ammonia (NH ), ammonium hydrosulfide (NH SH), and hydrogen sulfide (H S). Small amounts of water and carbon dioxide in Neptune's upper atmosphere seem to originate in interplanetary dust and meteorites. These delicate measurements have all been made with spectroscopic techniques from great distances, a testament to the ingenuity and persistence of the scientific community (for more, see the sidebar "Remote Sensing" on page 26).
Neptune has a visual magnitude of only 7.8, and so cannot be seen with the naked eye.The magnitude scale has no dimensions, but allows comparison of brightness between celestial objects.The lower the magnitude number, the brighter the object.The brightest star is Sirius, with a magnitude of —1.4.The full Moon has magnitude —12.7 and the Sun —26.7. The faintest stars visible under dark skies are around +6. During its close opposition, Mars rose to an apparent magnitude of —2.9. Neptune is therefore far dimmer than the faintest visible stars. Neptune's surface reflects almost a third of the sunlight that strikes it, so only about two-thirds of the paltry light that reaches distant Neptune is available to heat the planet. In the absence of internal heat, relying solely on heat from the Sun, Neptune's temperature at a pressure of one bar would be —375°F (—226°C). Because Neptune has an internal heat source, its mean temperature at a pressure of 1 bar is —328°F (—200°C). Beneath this level, temperatures rise with depth into the planet's interior, and above this level, temperatures also rise with height into the upper atmosphere.
Like Uranus, Neptune has an anomalously hot upper atmosphere. Uranus's atmospheric temperature reaches a high of perhaps 890°F (480°C) in the exosphere, the uppermost layer of its atmosphere. Neptune's exosphere appears to be slightly cooler, reaching only about 620°F (327°C).This heating is not well understood. Ultraviolet radiation from the Sun cannot heat the outer atmosphere to these temperatures. Though Neptune is much farther from the Sun, it has a slightly higher abundance of methane than does Uranus. Methane will help hold heat in the atmosphere and may contribute to additional solar heat retention on Neptune, but no atmospheric composition can explain the extremes of heat that have been measured.
The other gas giant planets have similarly hot exospheres, and on Jupiter the heat is thought to come from ionizing reactions tied to Jupiter's magnetic field and its auroras. Perhaps a similar process is at work on Uranus and Neptune.This process may not be able to explain the differences in temperature between Uranus and Neptune, though, because the planets have similarly strong magnetic fields and thus should accelerate ions to similar speeds and cause similar amounts of atmospheric heating. Other possibilities for exospheric heating are dissipation of gravity waves from the planet's interior, collision of supersonic jets of ions in the aurora, and other forms of heating caused by interactions between the atmosphere and the magnetic field.
Neptune has clouds, high winds, and large cyclonic storms that form and fade over months or years. The storms on Neptune are more distinct and common than those on Uranus. The variation of Neptune's surface can be seen in the image shown here, covering the entire surface of the planet. The location of the clouds is predicted based upon the temperature at which methane vapor will condense, and the levels and compositions are similar to those predicted and measured on Uranus. Methane ice clouds are expected to form at pressures less than about one bar. Between about five bars and one bar, ammonia and hydrogen sulfide ice clouds should form. At pressures greater than five bars, clouds of ammonium hydrosulfide, water, ammonia, and hydrogen sulfide form, both alone and in solution with one another. Water ice clouds are thought to form around 50 bars where the temperature is around 32°F (0°C). Neptune's deepest clouds, up to a few hundred bars pressure, are thought to consist of hydrogen sulfide (H2S) and ammonia (NH3).
Unlike Uranus, Neptune has a high-altitude, low-pressure layer of hydrocarbon haze above its methane ice clouds. Seran Gibbard of the Lawrence Livermore Laboratories and her colleagues have used highresolution images and spectral analysis (breaking the light into its constituent wavelengths) from the 32-foot (10-m) Keck II Telescope to determine the pressures and compositions of atmospheric features on Neptune. Dr. Gibbard and her team find that most cloud features exist between 0.1 and 0.24 bars and that some bright features are caused by large downwellings of methane haze rather than by methane ice clouds.
Some winds and rotation patterns on Neptune have been measured at 1,200 miles per second (2,000 km/sec), by far the highest wind speeds in the solar system. Neptune's more usual wind speeds at the cloud tops are 1,300 feet per second (400 m/sec) at the equator and 800 feet per second (250 m/sec) nearer the poles, 5,000 or more times slower than the maximum measured winds. Wind speed contrasts of this scale do not exist on Earth. Neptune's circulation pattern is similar to though simpler than the zonal patterns (parallel to latitude) on Jupiter and Saturn. Neptune has large retrograde (westward) winds at the equator, and eastward winds poleward of about 50 degrees latitude.
Voyager 2 images revealed a dark oval storm on Neptune at about the same latitude as Jupiter's Great Red Spot around 30 degrees south latitude.This storm was named the Great Dark Spot and is shown in an image (see the upper color insert on page C-4) from Voyager 2 that also contains small dark spots and a bright feature nicknamed Scooter. Over the course of 1989, the Great Dark Spot moved about 10 degrees closer to the equator. The Great Dark Spot, like the Great Red Spot, is a high-pressure, anti-cyclonic storm. Its size oscillated from 7,500 to 11,100 miles (12,000 to 18,000 km) in length and from 3,200 to 4,600 miles (5,200 to 7,400 km) in width over an eight-day period while Voyager 2 was taking images. Smaller dark spots were also seen by Voyager 2.
The two images of Neptune (see figure on page 84) were taken by Voyager 2 at a distance of about 7.5 million miles (12 million km) from
Neptune's surface was photographed twice, 17.6 hours apart, showing the relative velocities of the spots in its atmosphere. (nasa/jpl/Voyager 2)
Neptune. During the 17.6 hours between the left and right images, the Great Dark Spot, at 22 degrees south latitude (left of center), has completed a little less than one rotation of Neptune.The smaller dark spot, at 54 degrees south latitude, completed a little more than one rotation, as can be seen by comparing its relative positions in the two pictures. The velocities of the Great Dark Spot and the smaller spot differ by about 220 miles an hour (100 meters per second).
By 1994, images from the Hubble Space Telescope showed that the Great Dark Spot had disappeared. From 1994 through 1998, astronomers observed the development of new large dark spots in Neptune's northern hemisphere. The relatively rapid development and dissipation of large storms shows that Neptune's atmosphere is even less stable than Jupiter's. Though the composition of the dark material in the spots is unknown, bright attendant cirrus-like clouds are thought to be made of methane ice.
Neptune's measurable heat flow may well be tied to its extreme weather. In comparison with Uranus (very low surface heat flow), Neptune has bolder, faster, far more violent winds and weather. Since winds are driven by heating and cooling of the planet, the fastest winds in the solar system might be expected nearer the Sun, where solar heating can drive weather. That Neptune has the fastest measured winds in the solar system is therefore a surprise.
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