Neptune
Neptune
Results from the Voyager 2 flyby
Neptune is the eighth planet and most outermost planet from the sun (since August 2006, when Pluto was demoted to a dwarf planet). It is the fourth largest planet by diameter of the planets in the solar system. Neptune is about 17 times the mass of Earth, while being slightly more massive than Uranus, which is about 14 times the mass of Earth. Astronomers consider Neptune to form with Uranus a subgroup of the Jovian planets (Jupiter, Saturn, Uranus, and Neptune). Neptune and Uranus are similar in size, mass, periods of their rotation, the overall features of their magnetic fields, and ring systems. However they differ in the structure of their atmospheres (perhaps the more conspicuous features of Neptune’s clouds are caused by its significant internal energy source, which Uranus lacks), the orientations of their rotation axes, and in their satellite systems.
Neptune’s large satellite Triton—which has a very thin nitrogen atmosphere with clouds, plumes, and haze, an extremely cold surface with nitrogen, methane, carbon monoxide, and carbon dioxide ices which interact with the atmosphere, and a fairly high mean density—seems more like Pluto than the other satellites of Neptune and those of Saturn and Uranus. Not enough is known about Pluto to explore these similarities; this probably awaits future missions to Pluto, especially the New Horizons mission (or, Pluto-Kuiper Belt Mission) that NASA launched in 2006. Administered by the Applied Physics Laboratory at Johns Hopkins University (Maryland), it launched successfully on January 19, 2006, from Cape Canaveral, Florida. It was sent directly into an Earth—and solar—escape trajectory on its way to Pluto, Charon, and the Kuiper Belt (a large region of icy, rocky bodies located past the orbit of Neptune and continuing past Pluto’s orbit). It should arrive at Pluto in the summer of 2015.
Discovery
Neptune was discovered in September 1846, by German astronomer Johann Gottfried Galle (1812–1910) in Berlin, on the first night of his search for a trans-Uranian planet which had produced previously unexplained perturbations of Uranus’ orbit. These prompted French mathematician Urbain Jean Joseph Le Verrier (1811–1877) to calculate the predicted position of this undiscovered planet. He sent his calculations to Galle, who then discovered Neptune. However, there is evidence that Italian astronomer and physicist Galileo Galilei (1564–1642) had earlier fortuitously observed Neptune near Jupiter on December 28, 1612, and January 28, 1613, and French astronomer Joseph Jerome Lefrancois de Lalande (1732–1807) also had observed it in 1795. However, neither astronomer recognized Neptune as a trans-Saturnian (Uranian) planet. British astronomer William Lassell (1799–1880) discovered Neptune’s largest satellite Triton several weeks later in 1846.
Characteristics
From Triton’s 5.866 day period of revolution around Neptune and its 220,000 mi (354,300 km) mean distance from it, astronomers estimated Neptune’s mass to be 17.14 Earth masses, according to Kepler’s third law. From Neptune’s mean radius of 15,290 mi (24,625 km), a mean density (mass divided by volume) of 1.64 grams/cm3 was found. These values are similar to the ones found for Uranus. Uranus is slightly larger than Neptune, but Neptune is considerably more massive and denser than Uranus. Thus, Neptune is one
of the Jovian planets, which are characterized by large sizes and masses but low mean densities (compared with Earth). The last characteristic implies that Jovian planets have extremely thick atmospheres and are largely or mostly composed of gases.
Neptune is in a nearly circular orbit around the sun. It is at a 30.1 astronomical unit (AU) mean distance (4,500,000,000 km) from the sun. This amount makes it the most distant known Jovian planet (and probably the most distant known major planet, since recent pronouncements in 2006 have made the Pluto-Charon system a dwarf planet system) from the sun. Kepler’s third law gives 165 years for Neptune’s period of revolution around the sun. Therefore, Neptune will not have made one complete revolution around the s un since its discovery. It will do so on 2011.
Observations from Earth
Scientific knowledge about Neptune came slowly before the 1989 Voyager 2 flyby, due to its remoteness. Even when it is closest, telescopic observations show Neptune as a small, featureless bluish-green disk of only 2.35’ apparent diameter and 7.7 stellar magnitude. Spectroscopic observations showed that Neptune’s color is produced by the absorption of sunlight by methane gas in its atmosphere; this is also true for Uranus. Observations of occultations (similar to eclipses) of stars by Neptune indicated that its atmosphere is mostly composed of molecular hydrogen and helium, which are also the main components of the other Jovian planets. These gases are inconspicuous in their visible spectra.
Soon after its discovery, it was found that Triton’s orbit around Neptune is retrograde, meaning that, as seen from above Neptune’s north pole, Triton revolves around Neptune clockwise instead of counterclockwise (direct motion). If one views the solar system from above the sun’s North Pole, all the planets have direct (counterclockwise) revolutions around the sun, and most of them have direct rotations; the exceptions are Venus, Uranus, and the dwarf planet Pluto.
Infrared (at wavelengths longer than those of red light) observations of Triton since 1980 indicated the presence of an atmosphere containing methane, and the presence of nitrogen in solid or liquid form on its surface. However, its size and mass remained poorly known.
Dutch-American astronomer Gerard Peter Kuiper (1905–1973) discovered a second satellite, Nereid, in 1949. It is a small, faint (nineteenth magnitude) object in an orbit around Neptune that is very far from it (3,423,821 mi [385,513,400 km] mean distance), very elliptical (eccentricity 0.756), and highly inclined (29°to Neptune’s equator). A third satellite was suspected at about a 45,954 mi (74,000 km) distance from Neptune’s center because of a simultaneous 8.1 second decrease in the brightness of a star observed simultaneously from two locations 4 mi (6 km) apart in May 1981. In 1989, Voyager 2 discovered six new moons: Proteus (irregularly shaped, very massive, but only one-fourth Triton’s mass); Naiad, Thalassa, Despina, and Galatea, the four inner moons; and Larissa (which was originally discovered, but dismissed, in 1981). Then, between 2002 and 2003, five more moons were discovered: S/2002 N1, S/2002 N2, S/2002 N3, S/2002 N4, and S/2003 N1. As of 2006, thirteen moons, in total, have been discovered orbiting about Neptune.
From 1982, Neptune was suspected to have rings like the other Jovian planets. However, confirmation of suspected occultations of stars by rings of Neptune was not definite; occultations by possible rings were only observed sometimes. This led to the theory that Neptune had a set of incomplete arc rights that were maintained by gravitational perturbations from one or more shepherd satellites that were still undiscovered. Voyager 2 found numerous faint rings in 1989, which will be discussed later.
Efforts to determine Neptune’s rotation period from Earth-based observations using spectroscopy to find it from Doppler shifts of spectral lines across Neptune’s disk and searches for periodic brightness changes gave conflicting and almost always incorrect results.
Earth-based infrared observations of Neptune indicated by 1974 that, unlike Uranus, Neptune emits 2.4 times as much energy at infrared wavelengths as it receives from sunlight (insolation). This led to the inference that, like Jupiter and Saturn, Neptune has a significant internal heat source. This may be produced by continuing gravitational contraction or by the settling of denser materials to the center of a planet.
Results from the Voyager 2 flyby
The Voyager 2 spacecraft was launched from the Earth in 1977, and then flew by Jupiter in July 1979, Saturn in August 1981, and Uranus in January 1986. Uranus accelerated Voyager 2 toward a flyby of Neptune along a hyperbolic orbit in August 1989 (Figure 1). The season on Neptune was late spring, nearly summer, in its southern hemisphere. The closest approach (18,169 mi [29,240 km]) to Neptune’s center occurred at 3:56 Universal Time (U.T.) on August 25, 1989, about 4,900 km above the cloud tops of its north polar region. During Voyager 2 ’s encounter with Neptune from June 5, 1989, to October 2, 1989, the observations it made greatly increased scientific knowledge about the Neptune system. The plane of Neptune’s equator is tilted 29.6° to the plane of its orbit around the sun. Voyager 2 flew by Triton at a minimum distance of 24,724 mi (39,790 km) on August 25, 1989 at about 9:10 U.T. It observed Triton continuously from about 6:00 U.T. to 12:00 U.T. on that date, and discovered much
about Triton. (This information will be discussed in detail in a separate section.) The main discoveries Voyager 2 made about Neptune, its rings, and its small satellites follow.
Neptune’s magnetic field
Like the Earth and the other Jovian planets, Neptune has a strong magnetic field and a sizeable
magnetosphere (the region of space where Neptune’s magnetic field is dominant over the interplanetary field). Voyager 2 did not find definite evidence for the existence of Neptune’s magnetic field until eight days before its closest approach to Neptune, when radio and plasma wave observations of magnetospheric phenomena were obtained. Voyager 2 entered Neptune’s magnetosphere about seven hours before its closest approach to Neptune and remained inside the magnetosphere for about three days. The center of Neptune’s magnetic field is offset by 0.55 Neptune radius (0.55NR) from Neptune’s center, and its magnetic axis is tilted 47° with respect to Neptune’s rotation axis (Neptune’s magnetic poles are 47° from its poles of rotation; Figure 2). In this way, Neptune’s magnetic field is similar to that of Uranus (see Uranus), and is unlike the magnetic fields of the Earth, Jupiter, and Saturn, in which the offsets from the planets’ centers and the tilts to their rotation axes are much smaller.
Voyager 2 also detected radiation belts of charged particles trapped in Neptune’s magnetosphere. They include protons, electrons, charged molecular hydrogen, and charged heliumions. The particle density was found to be even lower than that found in Uranus’s magnetosphere. Neptune’s rings and small nearby satellites affect the structure of the magnetosphere near Neptune. Triton affects the outer part of the magnetosphere; among other things, it is presumed to be the main source of the charged nitrogenions that are found there.
Neptune’s rotation period
The tilt of Neptune’s magnetic field to its rotation axis and the field’s offset from Neptune’s center cause fluctuations of its magnetic field that are associated with the rotation of Neptune’s interior. From measurements of these fluctuations by Voyager 2, the rotation period of Neptune’s interior of 16 hours, seven minutes was determined; this was the first reliable determination of Neptune’s rotation period. The 12 to 21 hour rotation periods found for the atmospheric features observed by Voyager 2 differ from the above value because of winds.
Atmospheric features
The Voyager 2 images show that there were many more prominent features in Neptune’s atmosphere in 1989 than were seen in Uranus’ atmosphere in 1986: a Great Dark Spot at 20° South Latitude, a Small Dark Spot at 55° South Latitude that was encompassed by a circumpolar dark band from 45° South to 70° South Latitudes, another dark band from 6° North to 25° North Latitudes, and several elongated white clouds. One of these rapidly moving white clouds was given the humorous nickname Scooter. Evolution of the cloud features in Neptune’s atmosphere was observed by Voyager 2, suggesting that the atmosphere was quite dynamic. Except for their bluish color, Voyager 2 images of Neptune seem more like those of Jupiter than Uranus.
The temperature at the aerosol layer in Neptune’s atmosphere is about -346°F (-210°C), which is close to the temperature at the main cloud level in Uranus’ atmosphere, and the effective temperatures of the atmospheres of both Uranus and Neptune were found to be close to this temperature. One would expect Neptune’s visible troposphere and lower stratosphere to be about 59°F (15°C) colder than those of Uranus because of Neptune’s greater distance form the sun (30.1 AU vs. 19.2 AU); instead, the temperatures of these parts of the atmospheres of both planets are found to be about the same. Neptune’s atmosphere seems to be considerably warmer than it would be if it received all or nearly all its heating from sunlight, as seems to be the case for Uranus. This is another indication that Neptune has a powerful internal heat source, unlike Uranus, which has at most a weak internal heat source (compatible with radioactivity in its interior) or none at all. Voyager 2 infrared observations confirmed this; the emission to insolation ratio was found to be 2.6 from them instead of the value 2.4 found earlier. This internal heat source may be what drives the active features in Neptune’s visible atmosphere, which are much less noticeable in Uranus’ atmosphere.
Neptune’s atmosphere was found to be similar to that of Uranus in that it seems to have little temperature change with latitude. This probably indicates enormous heat capacities for both atmospheres. Also, Neptune has a hot (about 900°F [482°C]) ionosphere and an exosphere that consists mainly of a hydrogen thermal corona; both these atmospheric components seem similar to those of Uranus. However, Neptune’s stronger gravity and slightly colder stratosphere cause much lower particle densities in Neptune’s upper atmosphere than are found at the same heights above the cloud layers in Uranus’ atmosphere.
There is evidence of methane-hydrocarbon recycling in Neptune’s atmosphere, and the methane there is likely broken down by sunlight. After breakdown, resultant hydrocarbons sink into Neptune’s atmosphere. The hydrocarbons likely decomponse on their trip downward and release carbon which recombines with methane at depth in the upper atmosphere.
Neptune’s atmosphere has distinctive visible structures, including cloud bands, spots (similar to spots that are storms on Jupiter and Saturn), and methane ice clouds. Cloud bands circle the planet at given latitudes. Spots are stormy areas that have counter-clockwise rotation because of shear between cloud bands. The Great Dark Spot occurs at about 20 degrees south latitude and the Lesser Dark Spot at about 50 degrees south. Methane ice clouds are white and appear approximately 31 mi (50 km) above dark spots, but do not rotate with the spots.
Neptune’s internal structure
Neptune’s upper atmosphere (what is seen on Earth) is a mixture of hydrogen, helium, methane and traces of acetylene (C2H2), carbon dioxide, and other gasses. Only 10% of the planet’s mass is in this outermost layer (approximately 3,100 mi or 5,000 km thick). Under the upper atmosphere lies a lower atmosphere of molecular (gaseous) hydrogen and helium, plus some ices (approximately 6,200 mi or 10,000 km thick). Below the atmosphere lies the mantle, a water ice and rock mixture that perhaps contains methane ice and ammonia ice mixed in. A core is at the center of the planet’s mass, and it is likely a body with a 6,200-mi radius and represents 45% of the planet’s mass that is composed of silicate rock and water ice. Like the other Jovian planets (Jupiter, Saturn, and Uranus), Neptune has a distinctive structure quite different from the terrestrial planets like Earth.
Neptune’s ring system
Voyager 2 confirmed the suspected existence of a ring system around Neptune, consisting of four rings with imbedded ring arcs within some of them. A ring is a continuous band of particles in a narrow orbital belt around the planet. A ring arc is part of a ring wherein particles are more heavily concentrated and the ring is thicker. The leading edge of a ring arc is rather vague and the trailing edge is sharp. There are names for the rings, in order from nearest the planet, they are: Galle (26,040 mi [41,900 km] from Neptune); Leverrier (33,060 mi [53,200 km]); Lassell (34,420 mi [55,400 km]); Arago (35,790 mi [57,600 km]); Liberte (38,080 mi [62,900 km]); Egalite (38,080 mi [62,900 km]); Fraternite (38,080 mi [62,900 km]); Courage (38,080 mi [62,900 km]); and Adams (39,100 mi [62,930 km]).
The rings revolve directly around Neptune, are optically thinner and dustier (with much smaller average particle size) than Uranus’ rings, are very dark with albedos (ratios of reflected to incident light) less than 0.07, and lie in or near the plane of Neptune’s equator. Neptune’s outer rings are closely associated with some of the six small satellites of Neptune discovered by Voyager 2 (see below). The cause of the clumping of many of the particles in the rings into compact ring arcs is still not definitely known. Gravitational perturbations by nearby satellites, for example Galatea affecting the Adams ring, are suspected of producing gravitational resonances that produce the arcs. The perturbed motions of ring particles can lead to collisions of them that may be the main source of the copious and extensive fine (micron-sized) dust observed in Neptune’s rings. The plasma wave instruments on Voyager 2 have detected a halo of dust around Neptune that extends far from the ring (equatorial) plane; it was detected even above Neptune’s north polar region.
In 2005, astronomers using Earth-based equipment discovered that the rings of Neptune are very unstable, more than previously thought. In fact, the Liberty Ring is expected to disappear sometime within the twenty-first century.
Like the case for Uranus’ rings, the origin and evolution of Neptune’s rings are unknown. Are they the result of earlier tidal disruption of other nearby satellites? Are they a transitory phenomenon, or will they persist for millions or billions of years? Comparison of the positions of the arcs in the Adams ring observed by Voyager 2 in 1989 with their positions extrapolated back in time to 1984 and 1985 shows that they match the positions of three occultations of stars observed in those years. This indicates that the arcs in the Adams ring are stable over time intervals of at least five years.
Satellites
Neptune’s satellites may be classified into groups according to (1) orbital distances, (2) orbital properties, and (3) composition. By orbital distances, there are (a) intra-ring satellites (note—these are not shepherd satellites like those around Saturn) Naiad, Thalassa, Despina, and Galatea and (b) extra-ring satellites (Larissa, Proteur, Triton, and Nereid). By orbital properties, there are (a) inclined, elliptically orbiting, pro-grade satellites (Nereid and Galatea), (b) equatorial, circular-orbiting, prograde satellites (Naiad, Thalassa, Despina, Larissa, and Proteus), and (c) an inclined, circular-orbiting, retrograde satellite (Triton). By composition, there are (a) irregular, dark soil-covered icy bodies (perhaps captured comets), including Naiad, Thalassa, Despina, Galatea, Larissa, and Proteus, (b) a regular (nearly spherical) methane ice bodies (Triton), and (c) an irregular methane ice body (Nereid). (Little is known about the latest satellites discovered in 2002–2003.)
Voyager 2 discovered six more small satellites (all under 125 mi [200 km] diameter, except Proteus, 250 mi [400 km] diameter), which all have direct orbital motion around Neptune and are much closer to it than Triton. From 2002 to 2003, five more moons were discovered from Earth-based observations. This brings the total number of known satellites of Neptune (excluding the vast number of ring particles) to thirteen. The four innermost satellites Naiad, Thalassa, Despina, and Galatea are within the ring system and must interact with it. Galatea’s possible effect in producing and maintaining the arcs in the Adams ring has already been described; it orbits just 560 mi (900 km) inside that ring. Despina orbits Neptune about 434 mi (700 km) inside the Leverrier ring and may contribute to its stability, although arcs are not obvious in that ring.
All of Neptune’s six innermost satellites are very dark, having albedos of 0.065 or less. Proteus, the largest and outermost of the satellites, had details on its surface imaged by spacecraft and Earth-based equipment. It is heavily cratered, with a 93 mi (150 km) diameter crater visible there. Proteus turns out to be somewhat larger than Nereid and has a mean radius of 250 mi (400 km); images of Proteus indicate it to be decidedly non-spherical. The images indicate also that Proteus is tidally locked to Neptune (that is, its rotation period equals its period of revolution around Neptune and keeps the same hemisphere always turned towards Neptune), as is Triton.
Larissa also was imaged with sufficient resolution for its surface to be studied in some detail. It is also decidedly non-spherical and dark. The sizes, shapes, albedos, and states of rotation of the four innermost newly discovered satellites are less definite. Nereid, Neptune’s outermost satellite, was observed by Voyager 2, which determined its size (105 mi [170 km] radius) and albedo (0.15 to 0.20), which indicate that its surface seems more like those of Uranus’ icy satellites Umbriel, Oberon, and the darker areas on Miranda than the surfaces of the six other satellites. Limited information was obtained from the probe’s observations because Nereid was more than 2,800,000 mi (4,600,000 km) from Voyager 2 at closest approach, and it gave no definite information about Nereid’s rotation period (it is probably not tidally locked to Neptune), pole position, or its mass. Finally, from the discovery observations of Larissa and its period of revolution around Neptune, it seems that Larissa was probably the satellite that produced the 8.1 second decrease of the brightness of a star in 1981, which was mentioned above.
Neptune’s history
The history of Neptune and its system of rings and satellites was likely profoundly affected by the capture of Triton. It is thought that Neptune had an earlier system of satellites and perhaps rings, but the capture of Triton disrupted Neptune’s original system causing possible ejection of some satellites, reformation of others (i.e., the inner satellites) and an orbital change for Nereid. Triton’s origin was somewhere in the outer solar system, where it persisted until some time after the end of heavy bombardment (about four billion years ago) when it was captured by Neptune. At that time there may have been catastrophic interaction with pre-existing Neptunian satellites.
Triton
Triton is the only satellite of Neptune whose mass could be determined from its gravitational perturbation of Voyager 2’s hyperbolic flyby orbit. In addition to having its rotation tidally locked to its orbital revolution around Neptune, Triton’s rotation axis is almost aligned (to within 1°) with the perpendicular to Neptune’s orbital plane around the sun. The fact that Triton’s orbital plane around Neptune is tilted 23° to Neptune’s equatorial plane and the nodes (the intersection points of Triton’s orbit with Neptune’s equator) process along the equator, making a 360° circuit around it in 688 years, causes the seasons on Triton to be much more complicated than they are on Neptune. A graph of the latitude of the subsolar point—the point directly below the sun—on Triton from the present to the year 3000 is shown in Figure 3.
The subsolar point was at about 45° South Latitude at the Voyager 2 encounter, and since then has moved further south toward its farthest south value of 52° South Latitude.
The main discoveries that Voyager 2 made about Triton are the following.
Triton’s mass
Triton’s mass was found to be 2.14722 kg (0.00358 Earth mass), and its radius 840 mi (1,352 km); from this, its mean density was found to be 2.05 grams/cm3. This makes Triton the densest satellite yet found beyond Jupiter, although Saturn’s satellite Titan is considerably larger and more massive.
Triton’s atmosphere
Triton has a thin atmosphere of mainly molecular nitrogen with a trace (0.01%) of methane. Radio observations during the occultation of Voyager 2 by Triton found a value of 1.5 Pascal for the atmospheric surface pressure (the pressure in the Earth’s atmosphere about 75 mi [120 km] above sea level). Clouds are observed in the lower few miles of Triton’s atmosphere, and haze is found as high as 31 mi (50 km) above its surface. Four mysterious plumes were observed to rise as high as eight kilometers above the surface and then extend up to 93 mi (150 km) horizontally. The clouds and plumes showed the presence of winds in Triton’s atmosphere. The mechanism that powers the plumes is uncertain; a greenhouse mechanism (which traps sunlight in a transparent solid nitrogen layer and then vaporizes some nitrogen), geothermal heat, and also dust devils in Triton’s atmosphere have all been suggested.
Triton’s surface
Triton’s surface is very bright; its average albedo is about 0.8, while those for individual regions vary from 0.6 to nearly 1.0. Yellowish, pinkish, and peach-colored regions are seen on Triton’s generally whitish background; these may show the presence of organic compounds produced by reactions of molecular nitrogen and methane in the presence of solar radiation, high-energy electrons, or of cosmic rays. Voyager 2 infrared observations gave a -391°F (-235°C) for Triton’s surface, which varied very little across it. This is the coldest temperature observed for the surface of any solar system satellite or planet. Nitrogen and methane ice were both identified on the surface; at 1.5 Pascal atmospheric surface pressure the atmosphere is in vapor equilibrium with nitrogen surface ice. Triton’s surface has few craters (compared with the outer satellites of Saturn and Uranus), and it lacks craters larger than 19 mi (30 km) in diameter. The crater density is considerably lower than that on the most heavily cratered regions on Uranus’ satellite Miranda. Many regions show evidence of resurfacing. Many darker streaks are seen on Triton’s surface; they may be debris that has been deposited by earlier plumes.
Triton’s surface can be subdivided into four distinctive terrains including (1) an old, highly fractured terrain (dark plains crossed by intersecting networks of fractures in the crustal ice, (2) a smooth, volcanic plains terrain where icy lava have resurfaced vast tracts (includes irregular depressions that are likely volcanic caldera), (3) two polar ice terrains covered with nitrogen snow and frost (constant change due to tilt of axis of Triton and highly inclined orbital path), and (4) dark streak terrain formed by upon polar ice terrain due to volcanic activity spewing out dark methane particles.
Earth-based observations in 1991 and 1992 made in near-infrared wavelengths with the United Kingdom Infrared Telescope Facility on Mauna Kea, Hawaii, showed evidence of carbon monoxide and carbon dioxide ices on Triton’s surface as well as those of nitrogen and methane. They confirmed the 391°F temperature of Triton’s surface and found that there is less than 10% carbon monoxide ice dissolved in the nitrogen ice on Triton’s surface.
In size, mass, and mean density, Triton appears to be a larger and more massive variant of Pluto, since their mean densities are both nearly 2.1 grams/cm3. A plausible model for Triton’s interior is one with a rocky core of about 621 mi (1,000 km) radius surrounded by a 217 mi (350 km) thick water ice mantle, above which there is a crust of nitrogen, methane, carbon monoxide, and carbon dioxide ices that is only a few miles thick.
Very little is now known about Neptune’s interior except that its powerful internal energy source and strong magnetic field imply a field-generating region in Neptune’s interior that extends through most of it and that most of Neptune’s interior is a fluid having a high internal temperature. Neptune’s mean density indicates that it probably has a small rocky core surrounded by a hot fluid shell. A second shell comprised of gaseous and icy water, ammonia, and carbon compounds surrounds this first shell. Neptune’s oblateness (polar flattening) of 0.0171 indicates that its interior is
Key Terms
Doppler shift— The shift in wavelength of a spectroscopic line from its zero velocity wavelength to longer (redder) wavelengths if the source of the line is moving away from the observer or to shorter (bluer) wavelengths if the source is approaching the observer. It is used to measure velocity along the line of sight (radial velocity).
Pascal— The Pascal (Pa) is the metric unit of pressure (force per unit area). One Pascal is defined as a pressure of one Newton per square meter. The standard sea level atmospheric pressure on Earth is 101,200 Pascals.
Precession— What happens when a turning moment (torque) is applied to a body with angular momentum. Instead of turning in the direction of the turning moment, the body’s angular momentum will turn in a plane perpendicular to the one in which the turning moment acts.
Shepherd satellite— A planetary satellite whose gravitational perturbations on a particle tend to keep it in a stable orbit around the planet.
considerably denser than that of Uranus with oblateness of 0.023. The precession of Triton’s orbit gives some information about Neptune’s internal structure. However, it is too far from Neptune (14.4 Neptune radii) for detailed mapping of Neptune’s interior and gravity field. Accurate orbits are needed for the nearest newly discovered satellites and rings to do this; they are not yet available. Even if such orbits are determined, the information from them may not allow a unique model to be selected for Neptune’s interior; this cannot yet be done for Uranus, where much more information about its close satellites and rings is available.
The NASA Hubble Space Telescope observed Neptune in 1994 from Earth orbit and detected cloud features in its atmosphere that appeared to be different from those observed by Voyager 2 in 1989. The main change noticed was that the Great and Small Dark Spots seemed to have disappeared by 1994. However, later in the year, another spot near the location of the Great Dark Spot appeared. These spots were therefore of much shorter duration than Jupiter’s Great Red Spot, to which these large dark spots in Neptune’s atmosphere have sometimes been compared.
NASA proposed in 2003 that a Neptune Orbiter with Probes mission be done in connection with the Jet Propulsion Laboratory (California Institute of Technology). As of 2005, the mission is planned to launch around 2016. If launched, the spacecraft would take from eight to 12 years to travel to Neptune. While at the planet, the spacecraft will study Neptune’s weather and atmosphere, its ring system and moons, and other important characteristics of the planet that are under question.
See also Celestial mechanics; Kepler’s laws; Space probe.
Resources
BOOKS
de Pater, Imke and Jack J. Lissauer. Planetary Sciences. Cambridge, UK: Cambridge University Press, 2001.
Morrison, D., and Tobias Owen. The Planetary System. 3rd ed. Addison-Wesley Publishing, 2002.
Sobel, Dava. The Planets. New York: Viking, 2005. Taylor, F.W. The Cambridge Photographic Guide to the Planets. Cambridge University Press, 2002.
OTHER
Arnett, B. SEDS, University of Arizona. “The Eight Planets, a Multimedia Tour of the Solar System.” (August 25, 2006) <http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html> (accessed October 19, 2006).
Jet Propulsion Laboratory, National Aeronautics and Space Administration. “Welcome to the Planets: Neptune.” <http://pds.jpl.nasa.gov/planets/choices/neptune1.htm> (accessed October 19, 2006). SpaceKids, National Aeronautics and Space Administration.
“Tour the Solar System and Beyond.” <http://spacekids.hq.nasa.gov/osskids/animate/mac.html> (accessed October 19, 2006).
Frederick R. West
David T. King, Jr.
Neptune
Neptune
Neptune is the eighth planet from the Sun and about four times the size of Earth . Astronomers consider Neptune to form with Uranus a subgroup of the Jovian planets (Jupiter , Saturn , Uranus, and Neptune). Neptune and Uranus are similar in size, mass , periods of their rotation , the overall features of their magnetic fields, and ring systems. However they differ in the structure of their atmospheres (perhaps the more conspicuous features of Neptune's clouds are caused by its significant internal energy source, which Uranus lacks), the orientations of their rotation axes, and in their satellite systems.
Neptune's large satellite Triton, which has a very thin nitrogen atmosphere with clouds, plumes, and haze, an extremely cold surface with nitrogen, methane, carbon monoxide , and carbon dioxide ices which interact with the atmosphere, and a fairly high mean density , make it seem more like Pluto than the other satellites of Neptune and those of Saturn and Uranus. Not enough is known about Pluto to explore these similarities; this probably awaits future missions to Pluto, especially the New Horizons mission that NASA hopes to launch in 2006.
Discovery
Neptune was discovered in September, 1846, by Johann Galle in Berlin, Germany, on the first night of his search for a trans-Uranian planet which had produced previously unexplained perturbations of Uranus's orbit . These prompted Urbain Leverrier in France to calculate the predicted position of this undiscovered planet; he sent his calculations to Galle, who then discovered Neptune. However, there is evidence that Galileo Galilei had earlier fortuitously observed Neptune near Jupiter on December 28, 1612, and January 28, 1613, and J. Lalande also had observed it in 1795, but neither astronomer recognized Neptune as a trans-Saturnian (Uranian) planet. Lassell discovered Neptune's largest satellite Triton several weeks later in 1846.
Characteristics
From Triton's 5.866 day period of revolution around Neptune and its 220,000 mi (354,300 km) mean distance from it, astronomers estimated Neptune's mass to be 17.14 Earth masses, according to Kepler's third law. From Neptune's mean radius of 15,290 mi (24,625 km), a mean density (mass divided by volume ) of 1.64 grams/cm3 was found. These values are similar to the ones found for Uranus. Uranus is slightly larger than Neptune, but Neptune is considerably more massive and denser than Uranus. Thus, Neptune is one of the Jovian planets, which are characterized by large sizes and masses but low mean densities (compared with Earth). The last characteristic implies that Jovian planets have extremely thick atmospheres and are largely or mostly composed of gases.
Neptune is in a nearly circular orbit around the Sun at a 30.1 astronomical unit (a.u.) mean distance (4,500,000,000 km) from it, making it the most distant known Jovian planet (and probably the most distant known major planet, since recent findings indicate that the Pluto-Charon system is too small to be considered a major planet) from the Sun. Kepler's third law gives 165 years for Neptune's period of revolution around the Sun. Therefore, Neptune will not have made one complete revolution around the Sun since its discovery until 2011.
Observations from Earth
Our knowledge about Neptune came slowly before the 1989 Voyager 2 flyby, due to its remoteness. Even when it is closest, telescopic observations show Neptune as a small, featureless bluish-green disk of only 2.35" apparent diameter and 7.7 stellar magnitude. Spectroscopic observations showed that Neptune's color is produced by the absorption of sunlight by methane gas in its atmosphere; this is also true for Uranus. Observations of occultations (similar to eclipses ) of stars by Neptune indicated that its atmosphere is mostly composed of molecular hydrogen and helium, which are also the main components of the other Jovian planets. These gases are inconspicuous in their visible spectra.
Soon after its discovery, it was found that Triton's orbit around Neptune is retrograde, meaning that, as seen from above Neptune's north pole, Triton revolves around
Neptune clockwise instead of counterclockwise (direct motion ). If one views the solar system from above the Sun's north pole, all the planets have direct (counterclockwise) revolutions around the Sun, and most of them have direct rotations; the exceptions are Venus , Uranus, and possibly Pluto. Infrared (at wavelengths longer than those of red light ) observations of Triton since 1980 indicated the presence of an atmosphere containing methane, and the presence of nitrogen in solid or liquid form on its surface. However, its size and mass remained poorly known.
G.P. Kuiper discovered a second satellite, Nereid, in 1949. It is a small, faint (nineteenth magnitude) object in an orbit around Neptune that is distant (3,423,821 mi [385,513,400 km] mean distance) from it, very elliptical (eccentricity 0.756), and highly inclined (29° to Neptune's equator). A third satellite was suspected at about a 45,954 mi (74,000 km) distance from Neptune's center because of a simultaneous 8.1 second decrease in the brightness of a star observed simultaneously from two locations 4 mi (6 km) apart in May 1981.
From 1982, Neptune was suspected to have rings like the other Jovian planets. However, confirmation of suspected occultations of stars by rings of Neptune was not definite; occultations by possible rings were only observed sometimes. This led to the theory that Neptune had a set of incomplete arc rights which were maintained by gravitational perturbations from one or more" shepherd satellites" which were still undiscovered.
Efforts to determine Neptune's rotation period from Earth-based observations using spectroscopy to find it from Doppler shifts of spectral lines across Neptune's disk and searches for periodic brightness changes gave conflicting and almost always incorrect results.
Earth-based infrared observations of Neptune indicated by 1974 that, unlike Uranus, Neptune emits 2.4 times as much energy at infrared wavelengths as it receives from sunlight (insolation). This led to the inference that, like Jupiter and Saturn, Neptune has a significant internal heat source. This may be produced by continuing gravitational contraction or by the settling of denser materials to the center of a planet.
Results from the Voyager 2 flyby
The Voyager 2 spacecraft was launched from Earth in 1977, and then flew by Jupiter in July 1979, Saturn in August 1981, and Uranus in January 1986. Uranus accelerated Voyager 2 toward a flyby of Neptune along a hyperbolic orbit in August 1989. The season on Neptune was late spring, nearly summer, in its southern hemisphere. The closest approach (18,169 mi [29,240 km]) to Neptune's center occurred at 3:56 Universal Time (U.T.) on August 25, 1989, about 4,900 km above the cloud tops of its north polar region. During Voyager 2's encounter with Neptune from June 5, 1989, to October 2, 1989, the observations it made greatly increased our knowledge about the Neptune system. The plane of Neptune's equator is tilted 29.6° to the plane of its orbit around the Sun. Voyager 2 flew by Triton at a minimum distance of 24,724 mi (39,790 km) on August 25, 1989 at about 9:10 U.T. It observed Triton continuously from about 6:00 U.T. to 12:00 U.T. on that date, and discovered much about Triton that will be discussed in detail in a separate section. The main discoveries Voyager 2 made about Neptune, its rings, and its small satellites follow.
Neptune's magnetic field
Like Earth and the other Jovian planets, Neptune has a strong magnetic field and a sizeable magnetosphere (the region of space where Neptune's magnetic field is dominant over the interplanetary field). Voyager 2 did not find definite evidence for the existence of Neptune's magnetic field until eight days before its closest approach to Neptune, when radio and plasma wave observations of magnetospheric phenomena were obtained. Voyager 2 entered Neptune's magnetosphere about seven hours before its closest approach to Neptune and remained inside the magnetosphere for about three days. The center of Neptune's magnetic field is offset by 0.55 Neptune radius (0.55Nr) from Neptune's center, and its magnetic axis is tilted 47° with respect to Neptune's rotation axis (Neptune's magnetic poles are 47° from its poles of rotation). In this way, Neptune's magnetic field is similar to that of Uranus (see Uranus), and is unlike the magnetic fields of Earth, Jupiter, and Saturn, in which the offsets from the planets' centers and the tilts to their rotation axes are much smaller.
Voyager 2 also detected radiation belts of charged particles trapped in Neptune's magnetosphere. They include protons, electrons, charged molecular hydrogen, and charged heliumions. The particle density was found to be even lower than that found in Uranus's magnetosphere. Neptune's rings and small nearby satellites affect the structure of the magnetosphere near Neptune. Triton affects the outer part of the magnetosphere; among other things, it is presumed to be the main source of the charged nitrogenions that are found there.
Neptune's rotation period
The tilt of Neptune's magnetic field to its rotation axis and the field's offset from Neptune's center cause fluctuations of its magnetic field that are associated with the rotation of Neptune's interior. From measurements of these fluctuations by Voyager 2, the rotation period of Neptune's interior of 16 hours, seven minutes was determined; this was the first reliable determination of Neptune's rotation period. The 12-21 hour rotation periods found for the atmospheric features observed by Voyager 2 differ from the above value because of winds.
Atmospheric features
The Voyager 2 images show that there were many more prominent features in Neptune's atmosphere in 1989 than were seen in Uranus's atmosphere in 1986: a Great Dark Spot at 20° South Latitude, a Small Dark Spot at 55° South Latitude which was encompassed by a circumpolar dark band from 45° South to 70° South Latitudes, another dark band from 6° North to 25° North Latitudes, and several elongated white clouds. One of these rapidly-moving white clouds was given the humorous nickname Scooter. Evolution of the cloud features in Neptune's atmosphere was observed by Voyager 2, suggesting that the atmosphere was quite dynamic. Except for their bluish color, Voyager 2 images of Neptune seem more like those of Jupiter than Uranus.
The temperature at the aerosol layer in Neptune's atmosphere is about -346°F (-210°C), which is close to the temperature at the main cloud level in Uranus's atmosphere, and the effective temperatures of the atmospheres of both Uranus and Neptune were found to be close to this temperature. One would expect Neptune's visible troposphere and lower stratosphere to be about 59°F (15°C) colder than those of Uranus because of Neptune's greater distance form the Sun (30.1 a.u. vs. 19.2 a.u.); instead, the temperatures of these parts of the atmospheres of both planets are found to be about the same. Neptune's atmosphere seems to be considerably warmer than it would be if it received all or nearly all its heating from sunlight, as seems to be the case for Uranus. This is another indication that Neptune has a powerful internal heat source, unlike Uranus, which has at most a weak internal heat source (compatible with radioactivity in its interior) or none at all. Voyager 2 infrared observations confirmed this; the emission to insolation ratio was found to be 2.6 from them instead of the value 2.4 found earlier. This internal heat source may be what drives the active features in Neptune's visible atmosphere, which are much less noticeable in Uranus's atmosphere.
Neptune's atmosphere was found to be similar to that of Uranus in that it seems to have little temperature change with latitude. This probably indicates enormous heat capacities for both atmospheres. Also Neptune has a hot (about 900°F [482°C]) ionosphere and an exosphere that consists mainly of a hydrogen thermal corona; both these atmospheric components seem similar to those of Uranus. However, Neptune's stronger gravity and slightly colder stratosphere cause much lower particle densities in Neptune's upper atmosphere than are found at the same heights above the cloud layers in Uranus's atmosphere.
There is evidence of methane-hydrocarbon recycling in Neptune's atmosphere, and the methane there is likely broken down by sunlight. After breakdown, resultant hydrocarbons sink into Neptune's atmosphere. The hydrocarbons likely decomponse on their trip downward and release carbon which recombines with methane at depth in the upper atmosphere.
Neptune's atmosphere has distinctive visible structures, including cloud bands, spots (similar to spots that are storms on Jupiter and Saturn), and methane ice clouds. Cloud bands circle the planet at given latitudes. Spots are stormy areas that have counter-clockwise rotation as a result of shear between cloud bands. The Great Dark Spot occurs at about 20 degrees south latitude and the Lesser Dark Spot at about 50 degrees south. Methane ice clouds are white and appear approximately 31 mi (50 km) above dark spots, but do not rotate with the spots.
Neptune's internal structure
Neptune's upper atmosphere (what we see) is a mixture of hydrogen, helium, methane and traces of acetylene (C2H2), carbon dioxide, and other gasses. Only 10% of the planet's mass is in this outermost layer (approximately 3,100 mi or 5,000 km thick). Under the upper atmosphere lies a lower atmosphere of molecular (gaseous) hydrogen and helium, plus some ices (approximately 6,200 mi or 10,000 km thick). Below the atmosphere lies the mantle, a water ice and rock mixture that perhaps contains methane ice and ammonia ice mixed in. A core is at the center of the planet's mass, and it is likely a body with a 6,200-mi radius and represents 45% of the planet's mass that is composed of silicate rock and water ice. Like the other Jovian planets (Jupiter, Saturn, and Uranus), Neptune has a distinctive structure quite different from the terrestrial planets like Earth.
Neptune's ring system
Voyager 2 confirmed the suspected existence of a ring system around Neptune, consisting of four rings with imbedded ring arcs within some of them. A ring is a continuous band of particles in a narrow orbital belt around the planet. A ring arc is part of a ring wherein particles are more heavily concentrated and the ring is thicker. The leading edge of a ring arc is rather vague and the trailing edge is sharp. There are names for the rings, in order from nearest the planet, they are: Galle (26,100 mi [42,000 km] from Neptune); Lassell (30.500 mi [49,000 km]); Arago (39,032 mi [53,000 km]); and Adams (39,150 mi [63,000 km]). The Lassell ring contains a named ring arc (Leverrier) at 31,690 mi (51,000 km) from Neptune's surface. The rings revolve directly around Neptune, are optically thinner and dustier (with much smaller average particle size) than Uranus's rings, are very dark with albedos (ratios of reflected to incident
light) less than 0.07, and lie in or near the plane of Neptune's equator. Neptune's outer rings are closely associated with some of the six small satellites of Neptune discovered by Voyager 2 (see below). The cause of the clumping of many of the particles in the rings into compact ring arcs is still not definitely known. Gravitational perturbations by nearby satellites, for example Galatea affecting the Adams ring, are suspected of producing gravitational resonances that produce the arcs. The perturbed motions of ring particles can lead to collisions of
them that may be the main source of the copious and extensive fine (micron-sized) dust observed in Neptune's rings. The plasma wave instruments on Voyager 2 have detected a "halo" of dust around Neptune that extends far from the ring (equatorial) plane; it was detected even above Neptune's north polar region.
Like the case for Uranus's rings, the origin and evolution of Neptune's rings are unknown. Are they the result of earlier tidal disruption of other nearby satellites? Are they a transitory phenomenon, or will they persist for millions or billions of years? Comparison of the positions of the arcs in the Adams ring observed by Voyager 2 in 1989 with their positions extrapolated back in time to 1984 and 1985 shows that they match the positions of three occultations of stars observed in those years. This indicates that the arcs in the Adams ring are stable over time intervals of at least five years.
Satellites
Neptune's satellites may be classified into groups according to (1) orbital distances, (2) orbital properties, and (3) composition. By orbital distances, there are (a) intra-ring satellites (note—these are not shepard satellites like those around Saturn) Naiad, Thalassa, Despina, and Galatea and (b) extra-ring satellites (Larissa, Proteur, Triton, and Nereid). By orbital properties, there are (a) inclined, elliptically orbiting, prograde satellites (Nereid and Galatea), (b) equatorial, circular-orbiting, prograde satellites (Naiad, Thalassa, Despina, Larissa, and Proteus), and (c) an inclined, circular-orbiting, retrograde satellite (Triton). By composition, there are (a) irregular, dark soil-covered icy bodies (perhaps captured comets ), including Naiad, Thalassa, Despina, Galatea, Larissa, and Proteus, (b) a regular (nearly spherical) methane ice bodies (Triton), and (c) an irregular methane ice body (Nereid).
Voyager 2 discovered six more small satellites (all under 125 mi [200 km] diameter, except Proteus, 250 miles [400 km] diameter), which all have direct orbital motion around Neptune and are much closer to it than Triton. This brings the total number of known satellites of Neptune (excluding the vast number of ring particles) to eight. The four innermost satellites Naiad, Thalassa, Despina, and Galatea are within the ring system and must interact with it. Galatea's possible effect in producing and maintaining the arcs in the Adams ring has already been described; it orbits just 560 mi (900 km) inside that ring. Despina orbits Neptune about 434 mi (700 km) inside the Leverrier ring and may contribute to its stability, although arcs are not obvious in that ring.
All of Neptune's six innermost satellites are very dark, having albedos of 0.065 or less. Proteus, the largest and outermost of the newly discovered satellites, had details on its surface imaged. It is heavily cratered, with a 93 mi (150 km) diameter crater visible there. Proteus turns out to be somewhat larger than Nereid and has a mean radius of 250 mi (400 km); images of Proteus indicate it to be decidedly non-spherical. The images indicate
also that Proteus is tidally locked to Neptune (that is, its rotation period equals its period of revolution around Neptune and keeps the same hemisphere always turned towards Neptune), as is Triton.
Larissa also was imaged with sufficient resolution for its surface to be studied in some detail. It is also decidedly non-spherical and dark. The sizes, shapes, albedos, and states of rotation of the four innermost newly discovered satellites are less definite. Nereid, Neptune's outermost satellite, was observed by Voyager 2, which determined its size (105 mi [170 km] radius) and albedo (0.15 to 0.20), which indicate that its surface seems more like those of Uranus's icy satellites Umbriel, Oberon, and the darker areas on Miranda than the surfaces of the six newly discovered satellites. Limited information was obtained from the probe's observations because Nereid was more than 2,800,000 mi (4,600,000 km) from Voyager 2 at closest approach, and it give no definite information about Nereid's rotation period (it is probably not tidally locked to Neptune), pole position, or its mass. Finally, from the discovery observations of Larissa and its period of revolution around Neptune, it seems that Larissa was probably the satellite which produced the 8.1 second decrease of the brightness of a star in 1981 which was mentioned above.
Neptune's history
The history of Neptune and its system of rings and satellites was likely profoundly affected by the capture of of Triton. It is thought that Neptune had an earlier system of satellites and perhaps rings, but the capture of Triton disrupted Neptune's original system causing possible ejection of some satellites, reformation of others (i.e., the inner satellites) and an orbital change for Nereid. Triton's origin was somewhere in the outer solar system, where it persisted until some time after "heavy bombardment" end (about four billion years ago) when it was captured by Neptune. At that time there may have been catastrophic interaction with pre-existing Neptunian satellites.
Triton
Triton is the only satellite of Neptune whose mass could be determined from its gravitational perturbation of Voyager 2's hyperbolic flyby orbit. In addition to having its rotation tidally locked to its orbital revolution around Neptune, Triton's rotation axis is almost aligned (to within 1°) with the perpendicular to Neptune's orbital plane around the Sun. The fact that Triton's orbital plane around Neptune is tilted 23° to Neptune's equatorial plane and the nodes (the intersection points of Triton's orbit with Neptune's equator) process along the equator, making a 360° circuit around it in 688 years, causes the seasons on Triton to be much more complicated than they are on Neptune. A graph of the latitude of the subsolar point-the point directly below the Sun-on Triton from the present to the year 3000 is shown in this figure.
The subsolar point was at about 45° South Latitude at the Voyager 2 encounter, and since then has moved further south toward its farthest south value of 52° South Latitude.
The main discoveries that Voyager 2 made about Triton are the following.
Triton's mass
Triton's mass was found to be 2.141.1022 kg (0.00358 Earth mass), and its radius 840 mi (1,352 km); from this, its mean density was found to be 2.07 grams/cm3. This makes Triton the densest satellite yet found beyond Jupiter, although Saturn's satellite Titan is considerably larger and more massive.
Triton's atmosphere
Triton has a thin atmosphere of mainly molecular nitrogen with a trace (.01%) of methane. Radio observations during the occultation of Voyager 2 by Triton found a value of 1.5 Pascal for the atmospheric surface pressure (the pressure in Earth's atmosphere about 75 mi [120 km] above sea level ). Clouds are observed in the lower few miles of Triton's atmosphere, and haze is found as high as 31 mi (50 km) above its surface. Four mysterious plumes were observed to rise as high as eight km above the surface and then extend up to 93 mi (150 km) horizontally. The clouds and plumes showed the presence of winds in Triton's atmosphere. The mechanism which powers the plumes is uncertain; a "greenhouse" mechanism (which traps sunlight in a transparent solid nitrogen layer and then vaporizes some nitrogen), geothermal heat, and also "dust devils" in Triton's atmosphere have all been suggested.
Triton's surface
Triton's surface is very bright; its average albedo is about 0.8, while those for individual regions vary from 0.6 to nearly 1.0. Yellowish, pinkish, and peach-colored regions are seen on Triton's generally whitish background; these may show the presence of organic compounds produced by reactions of molecular nitrogen and methane in the presence of solar radiation, high energy electrons, or of cosmic rays. Voyager 2 infrared observations gave a -391°F (-235°C) for Triton's surface, which varied very little across it. This is the coldest temperature observed for the surface of any solar system satellite or planet. Nitrogen and methane ice were both identified on the surface; at 1.5 Pascal atmospheric surface pressure, the atmosphere is in vapor equilibrium with nitrogen surface ice. Triton's surface has few craters (compared with the outer satellites of Saturn and Uranus), and it lacks craters larger than 19 mi (30 km) in diameter. The crater density is considerably lower than that on the most heavily cratered regions on Uranus's satellite Miranda. Many regions show evidence of resurfacing. Many darker streaks are seen on Triton's surface; they may be debris that has been deposited by earlier plumes.
Triton's surface can be subdivided into four distinctive terrains including (1) an old, highly fractured terrain (dark plains crossed by intersecting networks of fractures in the crustal ice, (2) a smooth, volcanic plains terrain where icy lava have resurfaced vast tracts (includes irregular depressions that are likely volcanic caldera), (3) two polar ice terrains covered with nitrogen snow and frost (constant change due to tilt of axis of Triton and highly inclined orbital path), and (4) dark streak terrain formed by upon polar ice terrain due to volcanic activity spewing out dark methane particles.
Earth-based observations made in 1991 and 1992 made in near-infrared wavelengths with the United Kingdom Infrared Telescope Facility on Mauna Kea, Hawaii showed evidence of carbon monoxide and carbon dioxide ices on Triton's surface as well as those of nitrogen and methane. They confirmed the 391°F temperature of Triton's surface and found that there is less than 10% carbon monoxide ice dissolved in the nitrogen ice on Triton's surface.
This summarizes the present state of our knowledge about Triton. In size, mass, and mean density, Triton appears to be a larger and more massive variant of Pluto, since their mean densities are both nearly 2.1 grams/cm3. A plausible model for Triton's interior is one with a rocky core of about 621 mi (1,000 km) radius surrounded by a 217 mi (350 km) thick water ice mantle, above which there is a crust of nitrogen, methane, carbon monoxide, and carbon dioxide ices which is only a few miles thick.
Returning to Neptune, very little is now known about its interior except that its powerful internal energy source and strong magnetic field imply a field-generating region in Neptune's interior which extends through most of it, and that most of Neptune's interior is a fluid having a high internal temperature. Neptune's mean density indicates that it probably has a small rocky core surrounded by a hot fluid shell; this is surrounded by a second shell comprised of gaseous and icy water, ammonia and carbon compounds. Neptune's oblateness (polar flattening) of 0.0171 indicates that its interior is considerably denser than that of Uranus with oblateness of 0.023. The precession of Triton's orbit gives some information about Neptune's internal structure. However, it is too far from Neptune (14.4 Neptune radii) for detailed mapping of Neptune's interior and gravity field. Accurate orbits are needed for the nearest newly discovered satellites and rings to do this; they are not yet available. Even if such orbits are determined, the information from them may not allow a unique model to be selected for Neptune's interior; this cannot yet be done for Uranus, where much more information about its close satellites and rings is available.
The Hubble Space Telescope observed Neptune in 1994 from Earth orbit and detected cloud features in its atmosphere which appeared to be different from those observed by Voyager 2 in 1989. The main change noticed was that the Great and Small Dark Spots seemed to have disappeared by 1994. These spots were therefore of much shorter duration than Jupiter's Great Red Spot, to which these large dark spots in Neptune's atmosphere have sometimes been compared.
See also Celestial mechanics; Kepler's laws; Space probe.
Resources
books
Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew L. Chaikin. The New Solar System. Cambridge: Cambridge Univ. Press, 1999.
de Pater, Imke and Jack J. Lissauer. Planetary Sciences. Cambridge, UK: Cambridge University Press, 2001.
Morrison, D., and Tobias Owen. The Planetary System. 3rd ed. Addison-Wesley Publishing, 2002.
Taylor, F.W. The Cambridge Photographic Guide to the Planets. Cambridge University Press, 2002.
periodicals
Beatty, J. Kelly. "Hubble's Worlds." Sky & Telescope, 89, No. 2,(1995): 25.
Beatty, J. Kelly. "Welcome to Neptune." Sky and Telescope Magazine, 78, No. 4 (1989): 358-359.
Chapman, Clark R. "Voyager at Neptune: Sorting Out the Early Results." The Planetary Report 9, No. 6 (1989): 12-15.
"Galileo Saw Neptune." Sky & Telescope 60, No. 5 (1980): 363.
Lunine, J.I., et al. "Voyager at Triton." Science 250, No. 386 (1990): 410-443.
Porco, Carolyn C., et al. "Voyager 2 at Neptune." The Planetary Report 12, No. 2, (1992): 4-23.
Stone, E.C., et al. "Voyager 2 Encounter with the Neptunian System." Science 246 (1989): 1417-1500.
Stone, E.C., et al. "The Voyager Encounter with Neptune." Journal of Geophysical Research 96, Supplement, 18 (1991): 906-19, 268.
"Voyager's Last Picture Show." Sky & Telescope 78, No. 5, (1989): 463-470.
other
Arnett, B. SEDS, University of Arizona. "The Nine Planets, a Multimedia Tour of the Solar System." November 6, 2002 [cited February 8, 2003] <http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html>.
Frederick R. West David T. King, Jr.
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Doppler shift
—The shift in wavelength of a spectroscopic line from its zero velocity wavelength to longer (redder) wavelengths if the source of the line is moving away from the observer or to shorter (bluer) wavelengths if the source is approaching the observer. It is used to measure velocity along the line of sight (radial velocity).
- Precession
—What happens when a turning moment (torque) is applied to a body with angular momentum. Instead of turning in the direction of the turning moment, the body's angular momentum will turn in a plane perpendicular to the one in which the turning moment acts.
- Shepherd satellite
—A planetary satellite whose gravitational perturbations on a particle tend to keep it in a stable orbit around the planet. The Pascal (Pa) is the metric unit of pressure (force per unit area). One Pascal is defined as a pressure of one Newton per square meter. The standard sea level atmospheric pressure on Earth is 101,200 Pascals.
Neptune
Neptune
Neptune, the eighth planet away from the Sun, was discovered in 1846 by German astronomer Johann Galle, who based his finding on the mathematical predictions of French astronomer Urbain Le Verrier and English astronomer John Couch Adams. Because Neptune is so far way from the Sun—about 2.8 billion miles (4.5 billion kilometers)—it is difficult to observe. Very little was known about it until fairly recently. In August 1989, the U.S. space probe Voyager 2 flew by Neptune, finally providing some answers about this mysterious, beautiful globe.
Neptune is a large planet, with a mass 17 times that of Earth. The diameter at its equator is roughly 30,700 miles (49,400 kilometers). Neptune spins slightly faster than Earth—its day is equal to just over 19 Earth hours. It completes one revolution around the Sun in about 165 Earth years.
Since it is the color of water, Neptune was named for the Roman god of the sea. Its blue-green color, however, is due to methane gas. The thick outer atmospheric layer of hydrogen, helium, and methane is extremely cold: −350°F (−212°C). Below the atmosphere lies an ocean of ionized (electrically charged) water, ammonia, and methane ice. Underneath the ocean, which reaches thousands of miles in depth, is a rocky iron core.
Neptune is subject to the fiercest winds in the solar system. It has a layer of blue surface clouds that whip around with the wind and an upper layer of wispy white clouds of methane crystals that rotate with the planet. At the time of Voyager 2 's encounter, three storm systems were evident on its surface. The most prominent was a dark blue area called the Great Dark Spot, which was about the size of Earth. Another storm, about the size of our moon, was called the Small Dark Spot. Then there was Scooter, a small, fast-moving white storm system that seemed to chase the other storms around the planet. Its true nature remains a mystery.
In 1994, however, observations from the Hubble Space Telescope showed that the Great Dark Spot had disappeared. Astronomers theorize the spot either simply dissipated or is being masked by other aspects of the atmosphere. A few months later, the Hubble Space Telescope discovered a new dark spot in Neptune's northern hemisphere. This discovery has led astronomers to conclude that the planet's atmosphere changes rapidly, which might be due to slight changes in the temperature differences between the tops and bottoms of the clouds.
Neptune's magnetic field
A magnetic field has been measured on Neptune, tilted from its axis at a 48-degree angle and just missing the center of the planet by thousands of miles. This field is created by water beneath the surface that measures 4,000°F (2,204°C), water so hot and under so much pressure that it generates an electrical field.
Voyager 2 found that Neptune is encircled by at least four very faint rings, much less pronounced than the rings of Saturn, Jupiter, or Uranus. Although astronomers are not quite sure, they believe these rings are composed of particles, some of which measure over a mile across and are considered moonlets. These particles clump together in places, creating relatively bright arcs. This originally led astronomers to believe that only arcs—and not complete rings—were all that surrounded the planet.
The moons of Neptune
Neptune has eight moons, six of which were discovered by Voyager 2. The largest, Triton, was named for the son of the mythical Neptune. Triton was discovered a month after Neptune itself. It is 1,681 miles (3,705 kilometers) in diameter and has a surface temperature of −400°F (−240°C), making it the coldest place in the solar system. It has a number of unusual qualities. First, this peach-colored moon orbits Neptune in the opposite direction of all the other planets' satellites, and it rotates on its axis in the opposite direction that Neptune rotates. In addition, Voyager found that Triton has an atmosphere with layers of haze, clouds, and wind streaks. All of this information has led astronomers to conclude that Triton was captured by Neptune long ago from an independent orbit around the Sun.
The second Neptunian moon, a faint, small body called Nereid, was discovered in 1949 by Dutch astronomer Gerald Kuiper. The other six moons range from 30 miles (50 kilometers) to 250 miles (400 kilometers) in diameter.
[See also Solar system; Space probe ]
Neptune
Neptune
Neptune is the most distant giant planet, circling the Sun at an average distance of almost 6 billion kilometers (3.7 billion miles; thirty-nine times the distance from Earth to the Sun). Neptune is a near twin to Uranus in size (with a radius of 24,764 kilometers [15,354 miles] at the equator), in composition (about 80 percent hydrogen, 15 percent helium, and 3 percent methane, with other trace elements), and in internal structure (a rocky core surrounded by a methane-and ammonia-rich watery mantle topped by a thick atmosphere).
The icy particles in the upper cloud decks of Neptune differ slightly from those of Uranus. Their color, combined with the atmospheric methane that absorbs red light, gives Neptune a rich sky-blue tint compared with the more greenish Uranus. Neptune has the strongest internal heat source of all the giant planets, radiating almost three times more heat than one would expect. Like Jupiter and Saturn, which radiate about twice as much energy than expected, Neptune is thought to have excess heat from the time of the planet's formation and from continued gravitational contraction . Neptune's rotational axis is inclined only 29 degrees, compared with Uranus's more than 90 degrees.
A Saga of Discovery
The discovery of Neptune was a mathematical triumph and a political nightmare. After Uranus was discovered in 1781, astronomers inferred the presence of another planet from the shape of the Uranian orbit. In England, astronomer John Adams made meticulous but unpublished calculations of the planet's likely position in 1845. Shortly thereafter, French astronomer Urbain Leverrier independently determined the suspected planet's position, which nearly matched Adams's prediction. After Leverrier's work was published in 1846, English astronomers realized that Adams's work warranted a more serious look. But by then, the French astronomer had sent his prediction to observers in Berlin. Almost immediately, German astronomer Johann Galle discovered Neptune near the predicted location. For years, debates raged across national boundaries over who deserved credit for the discovery of Neptune. We now credit both Leverrier and Adams for the prediction and recognize Galle for the actual observation.
Unusual Cloud Features
In 1989, Voyager 2 flew by Neptune and detected numerous cloud features. The biggest was the Great Dark Spot, a hurricane-like storm that was about half the size of Earth. The next to be discovered was a small white spot, which appeared to race rapidly around the planet when compared with the lumbering Great Dark Spot. It was named the "Scooter." Many more spots were found, many of which were rotating even faster than Scooter. A small dark spot in the south developed a bright core, and a bright clump near the south pole was observed to be composed of many fast-moving bright patches.
Rotation and Magnetic Field
Voyager 2 measured Neptune's 16.11-hour internal rotation period by monitoring the planet's magnetic field. The atmosphere rotates with periods ranging from over 18 hours near the equator to faster than 13 hours near the poles. In fact, the winds of Neptune are among the fastest in the solar system; only Saturn's high-speed equatorial jet is faster. Like the Uranian magnetic field, Neptune's magnetic field is also offset from the planet's center and significantly tilted with respect to the planet's rotation axis. Neptune's field is about 60 percent weaker than that of Uranus.
The Moons of Neptune
Neptune's largest moon, Triton, has a retrograde and highly inclined orbit. This suggests the moon may have been captured rather than formed around Neptune. Triton has a thin atmosphere of primarily nitrogen gas, thought to be in equilibrium with the nitrogen ice covering Triton's surface. Because of Triton's unusual orbit, however, the surface ice is thought to change with time, leading to the possibility that Triton's atmosphere also varies. Recent occultations (observations of stars glimmering through Triton's tenuous atmosphere) suggest that Triton's atmosphere may have expanded by nearly a factor of two since the Voyager 2 encounter. Triton's northern hemisphere looks much like the surface of a cantaloupe. The southern hemisphere is dominated by a polar ice cap, probably composed of nitrogen. In the highest resolution images, active geysers (ice volcanoes) were seen spewing columns of dark material many kilometers into the thin atmosphere. Triton's surface has relatively few impact craters , suggesting that it is young.
Nereid (the only other Neptune moon known prior to the Voyager 2 mission) also has an unusual orbit that is highly elliptical and tilted nearly 30 degrees, again suggesting a capture origin. Little is known about it other than its irregular shape. Voyager 2 discovered six additional moons around Neptune. These are all in circular prograde orbits near Neptune's equatorial plane, and they probably formed in place. One of these, Proteus, is larger than Nereid; it had not been discovered prior to the Voyager 2 encounter because it is so close to Neptune. Proteus is irregular in shape. A particularly large impact crater suggests that it came close to destruction in an earlier collision.
The Rings of Neptune
Astronomers used occultations to search for rings around Neptune, because that technique had been successful for discovering the rings of Uranus. The results were odd: some events seemed to clearly show rings, but others clearly did not. The Voyager 2 encounter solved the puzzle. There were three complete rings, but the rings were variable in their thicknesses (the three distinct rings were named Adams, Leverrier, and Galle, after the astronomers who were involved in the discovery saga). The thickest parts—dubbed rings arcs—were seen during occultations; the other parts of the rings were too thin to be detected. Scientists are not sure what causes Neptune's rings arcs. Some of the smallest moons appear to "shepherd" the inner edges of two of the rings, but no moons were found at locations that would explain the clumps through a shepherding mechanism. Despite their clumpiness, Neptune's rings are very circular, unlike the rings of Uranus.
Recent Hubble Space Telescope images have continued to show remarkable changes in Neptune's atmosphere: the Great Dark Spot discovered by Voyager 2 in 1989 had disappeared, and a new Great Dark Spot developed in the northern hemisphere. From the dynamics of Neptune's clouds, to the expanding Triton atmosphere, to the forces creating the clumpy rings, many interesting puzzles remain to be solved in the Neptune system.
see also Exploration Programs (volume 2); Nasa (volume 3); Robotic Exploration of Space (volume 2); Uranus (volume 2).
Heidi B. Hammel
Bibliography
Cruikshank, Dale P., ed. Neptune and Triton. Tucson: University of Arizona Press,1995.
Moore, Patrick. The Planet Neptune. Chichester, UK: Ellis Horwood, 1988.
Standage, Tom. The Neptune File: A Story of Astronomical Rivalry and the Pioneers of Planet Hunting. New York: Walker and Company, 2000.
Neptune
Neptune
Neptune
In Roman mythology, Neptune was an early Italian water deity who became identified with the Greek god Poseidon after about 400 b.c. Unlike Poseidon, who appeared in many Greek myths and legends, Neptune played a relatively minor role in Roman mythology In ancient Roman art, he is generally shown holding a trident, a traditional weapon of fishermen in the Mediterranean region.
Possibly called Nethunus by the Etruscans of ancient Italy, the early Neptune was linked to freshwater rivers, lakes, and springs. After identification with Poseidon, he also became the god of the sea. His wife, Salacia, a goddess of springwater, became associated with Amphitrite, the wife of Poseidon and queen of the sea.
deity god or goddess
trident three-pronged spear, similar to a pitchfork
patron special guardian, protector, or supporter
The ancient Romans held an annual festival to honor Neptune in July, a time when the hot, dry weather of Italy made water scarce. The purpose of the festival was probably to appease the god and help ensure that water would soon be abundant again. Closely associated with horses, as was Poseidon, Neptune may also have been worshiped by the Romans as a god of horses and patron of horse racing.
See also Poseidon; Roman Mythology.
*See Names and Places at the end of this volume for further information.
Neptune
http://lpl.arizona.edu/nineplanets/nineplanets/neptune.html; http://wr.usgs.gov
Neptune
Nep·tune / ˈnept(y)oōn/ 1. Roman Mythol. the god of water and of the sea. Greek equivalent Poseidon.2. Astron. a distant planet of the solar system, eighth in order from the sun, discovered in 1846.
Neptune
From the early 19th century, the traditional shipboard ceremony held when crossing the equator has included a sailor dressed as Neptune.
Neptunism was a theory propounded by A. G. Werner that the rocks of the earth's crust were formed primarily by crystallization from the sea, rather than by solidification of magma. The theory was popular at the end of the 18th and the beginning of the 19th century, but is now rejected.