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N
E P T U N E (8th planet from the Sun)
In Roman mythology
Neptune (Greek: Poseidon) was the god of the Sea.
CHAPTERS
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
8 MOONS
INTRODUCTION
I
Neptune is the fourth largest of the planets in the
solar system, and eighth major 
planet
in order of increasing distance from the sun. Neptune is, on average,
about 4.5 billion km (2.8 billion mi) from the sun. It is about 49,400
km (30,700 mi) in diameter, or about 3.8 times as wide as the earth.
Even though Neptune's volume is 72 times that of Earth's, its mass
is only 17 times Earth's mass. Like Jupiter and Saturn, Neptune is
one of the "gas giant" planets, composed of a deep atmosphere around
a liquid surface and sometimes a solid core. Neptune's atmosphere
consists of mostly hydrogen and helium, but up to three percent of
Neptune's atmosphere is made of methane, which gives the planet its
striking blue color.
Its core contains more rock and metal than the cores of other gas
giant planets. Neptune has a magnetic field, which is tilted more
than 50° to the rotation axis. Neptune rotates completely on its axis
once every 16 hours and orbits the sun once in 164.79 Earth years.
Its albedo, or reflectivity, is high (84 percent of the light falling
on it is reflected), but it is so far away from the earth its stellar
magnitude (a scale used to describe the brightness of an astronomical
object; lower numbers correspond to brighter objects, see Magnitude)
is only 7.8, which means it is never bright enough to be seen with
the naked eye from earth. When observed through a telescope, it appears
as a greenish-blue disk without any definite surface markings. However,
images from the Voyager 2 probe (see Voyager), which flew past Neptune
in 1989, and the Hubble Space Telescope, which observed Neptune in
1994, have revealed dynamic bright and dark spots in Neptune's atmosphere
that are thought to be huge storms caused by the difference in temperature
between the heat-producing core and the frigid cloud tops.
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The 1989 Voyager 2 mission produced this false-color image of Neptune showing the different components of Neptune's atmosphere. The red layer shows scattered sunlight from a haze around the planet, the blue/green indicates methane, and the white areas are high-level clouds that reflect sunlight above the atmosphere. Photo Researchers, Inc. |
Six more satellites were discovered by the Voyager 2 planetary probe in 1989. The discovery of Neptune was one of the triumphs of mathematical astronomy. To account for perturbations in the orbit of the planet Uranus, British astronomer John Couch
Adams,
by 1845, and French astronomer Urbain Jean Joseph Leverrier, by 1846,
independently calculated the existence and position of a new planet.
Using information from Leverrier, German astronomer Johann Gottfried
Galle first observed the planet in 1846. TOP OF PAGE
MOONS AND SATELLITES
II
Neptune has 8 known moons; 7 small ones and Triton.
T R I T O N
In Greek mythology, Triton is a god of the sea, the son of
Poseidon (Neptune); usually portrayed as having the head and
trunk of a man and the tail of a fish.
orbit: 354,760 km from Neptune
diameter: 2700 km
mass: 2.14e22 kg
Triton (astronomy), largest moon of the planet Neptune. Triton is the seventh known satellite from the planet. It orbits Neptune at a distance of about 355,000 km (about 213,000 mi), completing one orbit in a little under six Earth days. Triton's orbit is inclined about 30° to Neptune's orbit. Triton has a retrograde orbit, which means that it moves clockwise as seen from Neptune's north pole. Most bodies in the solar system, including all of Neptune's other moons, travel counterclockwise as seen from the north pole of the body they orbit.
There are very few craters visible; the surface is relatively young. Almost the entire southern hemisphere is covered with an "ice cap" of frozen nitrogen and methane (below right).
Triton is the only large body in the solar system with a retrograde orbit, leading scientists to theorize that Neptune captured it from an independent orbit around the sun. Triton measures about 2700 kilometers (about 1620 miles) across, making it one of the ten largest satellites in the solar system. Scientists estimate that Triton is about one-quarter ice and three-quarters rock. Triton has a complex and young surface. Planetary scientists theorize that Triton was captured by Neptune's gravity into an elliptical orbit around the planet. The moon probably rotated on its axis. Over about a billion years, Neptune's gravity slowed Triton's rotation so that it rotated only once during each orbit and forced Triton into a circular orbit. This made Triton flex, rubbing together rocks inside and producing frictional heat that reshaped Triton's surface. Triton has few craters, but many cracks in its surface. Triton also has flat ice plains and volcanolike features that measure up to 200 km (120 mi) across. Triton's southern hemisphere has a pink polar cap with a blue edge. Geysers spew dark plumes into Triton's thin atmosphere. These may be caused by sunlight vaporizing liquid nitrogen under the surface. Nitrogen ice crystals form clouds 5 to 10 km (3 to 6 mi) above Triton's surface.
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This high resolution image of Triton was obtained by Voyager 2 on August 25, 1989. It shows the moon Triton's south polar cap. One of the unusual aspect of this image is the dark streaks in the images. Perhaps they were made by geyser-like eruptions of nitrogen. The geyser eruptions could have carried darker materials from the crust. The light regions probably consist of layers of nitrogen. |
British astronomer William Lassell discovered Triton in 1846, only weeks after Neptune was discovered. Triton is named after the son of the Greek god Poseidon. Individual features on Triton bear the names of water deities outside of Greek and Roman mythology.
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MOONS AND SATELLITES
III
PROTEUS
In Greek mythology, Proteus was a sea god
who could change his shape at will.
orbit: 117,600 km from Neptune
diameter: 418 km (436 x 416 x 402)
mass: ?
Proteus (astronomy), 2nd largest satellite of the planet Neptune. Proteus is the sixth known moon from the planet. The moon orbits Neptune at a distance of about 118,000 km (about 70,600 miles), completing an orbit about once every 27 hours. Proteus's orbit is circular and parallels Neptune's equator. Proteus is irregular in shape, measuring about 436 km (about 262 mi) through its widest diameter and about 402 km (about 241 mi) through its narrowest diameter. It is about one-ninth as wide as the earth's moon. Proteus is the second-largest moon of Neptune after Triton, and is the largest irregular moon in the solar system. Most moons of its size are spherical. Planetary scientists theorize that if Proteus were any larger, its gravity would pull it into a compact sphere. Proteus's density, chemical composition, and internal structure are unknown. Proteus's irregularity, however, indicates that it is made up mostly of relatively stiff materials that can withstand the pull of gravity, so Proteus may contain more rock than ice. Proteus's surface is as dark as soot and heavily cratered. It is very similar to Saturn's distant moon Phoebe. Like Phoebe, Proteus reflects only about 6 percent of the light it receives from the sun. Some scientists theorize that Proteus and Phoebe formed in the same part of the solar system and were captured by the gravity of different planets. Proteus was discovered in 1989 by American astronomer Stephen Synnott, who found it in pictures taken by the United States Voyager 2 spacecraft. Voyager 2 flew through the Neptune moon system in August 1989. Proteus is larger than Neptune's moon Nereid, which was discovered 40 years earlier. Proteus was not found sooner because it is very dark, and because it orbits close enough to Neptune that the planet's glare hides it from viewers on Earth. The moon is named for a Greek god of the sea who could change his shape at will.
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MOONS AND SATELLITES
IV
NEREID
Nereid is any one of the sea nymphs,
the 50 daughters of Nereus and Doris.
orbit: 5,513,400 km distance from Neptune
diameter: 340 km
mass: ?
Nereid ( "NEER ee ed"
) is the outermost of Neptune's known satellites and the third largest:
Nereid is the eighth known satellite from the planet. The moon orbits
Neptune at an average distance of about 5.51 million km (about 3.33
million mi), completing an orbit about once every 360 Earth days. Nereid
is about 12 times more distant from Neptune than the next moon, Triton.
Of all moons in the solar system, Nereid has the most elliptical orbit
around its planet. When Nereid is at its closest to Neptune, its distance
from the planet is only about 1.35 million km (about 812,000 mi). At
the farthest point in its orbit around Neptune, Nereid is about 9.624
million km (about 5.77 million mi) from the planet. Nereid's orbit is
tilted 28° relative to Neptune's equator. Nereid is roughly spherical,
measuring about 340 km (about 204 mi) in diameter, or about one-tenth
as wide as the earth's moon. Nereid's internal structure, density, and
composition are unknown and little is known of the moon's surface. The
United States Voyager 2 probe, the only spacecraft to visit Neptune,
came no nearer to the moon than about 4.7 million km (about 2.9 million
miles). Only a few large craters are visible in the pictures taken by
Voyager 2. Nereid's surface reflects about 14 percent of the light that
falls on it, making it brighter than any of Neptune's other moons except
Triton. Some planetary scientists believe that Nereid has such an odd
orbit because it was originally a member of the Kuiper Belt (a number
of icy bodies orbiting the sun beyond Neptune) or a comet. Others theorize
that Nereid's orbit was perturbed when Neptune captured Triton. Nereid
may always have been a moon of Neptune, but could have been thrown into
its strange orbit by Triton's gravity when the two moons nearly collided
following Triton's capture. Nereid was discovered in 1949 by Dutch-American
astronomer Gerard Kuiper. The moon was named for a group of fifty sea
nymphs who were daughters of the Greek god Nereus and his wife, Doris.
Individual features on Nereid are named for the individual Nereids.MOONS AND SATELLITES
V
LARISSA
Larissa was a daughter of Pelasgus. Larissa is named after a
character in Greek mythology associated with the sea god Poseidon.
orbit: 73,600 km from Neptune
diameter: 193 km (208 x 178)
mass: ?
Larissa (astronomy), small satellite of the planet Neptune. Larissa is the fifth known moon from the planet. The moon orbits Neptune at a distance of about 73,600 km (about 45,700 mi), completing an orbit about once every 13 hours. Larissa's orbit is circular and parallels Neptune's equator. Larissa is irregular in shape, measuring about 208 km (about 129 mi) at its widest point and about 178 km (about 111 mi) at its narrowest. It could fit inside a large crater on Earth's moon. Larissa's density, chemical composition, and internal structure are unknown. Like the other small moons of Neptune, Larissa's surface is as dark as soot. Larissa appears egg-shaped in the best pictures taken of it by the United States spacecraft Voyager 2, but few surface details are visible. Larissa appears to be covered with craters, but no signs of internal geological activity are detectable. Larissa was first detected in 1981 by a number of astronomers around the world who set up their instruments to watch Neptune pass in front of a distant star. This interruption of starlight by a planet or moon is called an occultation, and astronomers hoped that by studying it they could learn more about Neptune's atmosphere and rings. The star vanished for several seconds before Neptune covered it, then grew bright again and vanished as Neptune covered it. Astronomers theorized that an unknown small moon of Neptune had blocked light from the star before the planet itself had done so. The first to report the detection was American astronomer David Tholen. Larissa was discovered officially, however, in 1989 by American astronomer Stephen Synnott. He found it in pictures taken by the United States Voyager 2 spacecraft.
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MOONS AND SATELLITES
VI
GALATEA
Galatea was a Sicilian Nereid loved by the Cyclops Polyphemus.
(Not related to the maiden who was originally a statue carved
by Pygmalion and who was brought to life by Aphrodite.)
orbit: 62,000 km from Neptune
diameter: 158 km
mass: ?
Galatea ("gal eh TEE eh") is the fourth of Neptune's known satellites: Discovered in 1989 by Voyager 2.
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MOONS AND SATELLITES
VII
DESPINA
Despina was a nymph, the daughter of
Poseidon (Neptune) and Demeter.
orbit: 52,600 km from Neptune
diameter: 148 km
mass: ?
Despina is the third of Neptune's known satellites:
Discovered in 1989 by Voyager 2.
TOP OF PAGE
MOONS AND SATELLITES
VIII
THALASSA
Thalassa was a daughter of Aether and Hemera.
"Thalassa" is also the Greek word for "sea".
orbit: 50,000 km from Neptune
diameter: 80 km
mass: ?
Thalassa ("tuh LASS eh") is the second of Neptune's known satellites: Discovered in 1989 by Voyager 2.
TOP OF PAGE
MOONS AND SATELLITES
IX
NAIAD
The Naiads were the nymphs who lived in and
presided over brooks, springs, and fountains.
orbit: 48,200 km from Neptune
diameter: 58 km
mass: ?
Naiad ( "NAY ed"
) is the innermost of Neptune's known satellites: The last of the satellites discovered in 1989 by Voyager 2.
Naiad, Thalassa, Despina and Galatea are all irregularly shaped.
RECOMMENDED READING
XI
COMING SOON
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WEB LINKS
XII
COMING SOON
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