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Neptune is the outermost planet of the gas
giants. It has an equatorial diameter of 49,500 kilometers (30,760
miles) and is the eighth planet from the sun. If Neptune were hollow, it
could contain nearly 60 Earth's. Neptune orbits the Sun every 165 years.
It has eight moons, six of which were found by Voyager 2. A day on
Neptune is 16 hours and 6.7 minutes. Neptune was discovered on September
23, 1846 by Johann Gottfried Galle, of the Berlin Observatory. Neptune
got its named from the Roman God of the Sea.
Much of what is know today about Neptune was discovered in 1989 by the
U.S Voyager 2 spacecraft during its 1989 flyby f Neptune. Neptune as
compared to Earth is 3.9 times the diameter, 30 times the distance from
the sun, 17 times as massive, and 0.3 times the density.
Neptune travels around the Sun in an elliptical orbit at an average
distance of 4.504 billion km (2.799 billion miles). Neptune consists
largely of hydrogen and helium, and it has no apparent solid surface.
The first two thirds of Neptune is composed of a mixture of molten rock,
water, liquid ammonia and methane. The outer third is a mixture of
heated gases comprised of hydrogen, helium, water and methane. The
atmospheric composition is 85% Hydrogen, 13% Helium, and 2% methane. The
planet's atmosphere, particularly the outer layers, contains substantial
amounts of methane gas. Absorption of red light by the atmospheric
methane is responsible for Neptune's deep blue color.
Neptune is a dynamic planet with several large, dark spots reminiscent
of Jupiter's hurricane-like storms. The largest spot, known as the Great
Dark Spot, is about the size of the earth and is similar to the Great
Red Spot on Jupiter.
Neptune receives less than half as much
sunlight as Uranus, but heat escaping from its interior makes Neptune
slightly warmer than Uranus. The heat liberated may also be responsible
for Neptune's stormier atmosphere, which exhibits the fastest winds seen
on any planet in the solar system. Most of the winds there blow
westward, opposite to the rotation of the planet. Near the Great Dark
Spot, winds blow up to 2,000 kilometers (1,200 miles) an hour. Voyager 2
found that the winds averaged about 300 meters per second (700
miles/hour) in the planet's atmosphere.
Long bright clouds, similar to cirrus clouds on Earth, were seen high in
Neptune's atmosphere. At low northern latitudes, Voyager captured images
of cloud streaks casting their shadows on cloud decks below.
Feathery white clouds fill the boundary between the dark and light blue
regions on the Great Dark Spot. The pinwheel shape of both the dark
boundary and the white cirrus suggests that the storm system rotates
counterclockwise. Periodic small scale patterns in the white cloud,
possibly waves, are short lived and do not persist from one Neptunian
rotation to the next. (Courtesy NASA/JPL)
Until the Voyager 2 encounter in 1989, the rings surrounding Neptune
were thought to be arcs. We now know that the rings completely circle
the planet, but the thickness of each ring varies along its length.
Neptune has a set of four rings which are narrow and very faint. The
rings are made up of dust particles thought to have been made by tiny
meteorites smashing into Neptune's moons. From ground based telescopes
the rings appear to be arcs but from Voyager 2 the arcs turned out to be
bright spots or clumps in the ring system. The exact cause of the bright
clumps is unknown.
The magnetic field of Neptune, like that
of Uranus, is highly tilted at 47 degrees from the rotation axis and
offset at least 13,500 kilometers or 8,500 miles from the physical
center. Comparing the magnetic fields of the two planets, scientists
think the extreme orientation may be characteristic of flows in the
interior of the planet and not the result of that planet's sideways
orientation or of any possible field reversals within the planet.
Neptune also has eight known satellites.
Only two of these, Triton and Nereid, had been observed prior to the
Voyager 2 flyby. Triton is the largest of the eight satellites and is
almost as big as the Earth's Moon. The other Neptunian satellites range
in diameter from 58 to 416 km (36 to 258 miles). Apart from Triton, the
moons of Neptune are irregularly shaped and have very dark surfaces.
Triton is the largest moon of Neptune, with a diameter of 2,700
kilometers (1,680 miles). It was discovered by William Lassell, a
British astronomer, in 1846 scarcely a month after Neptune was
discovered. Triton is colder than any other measured object in the Solar
System with a surface temperature of -235° C (-391° F). It has an
extremely thin atmosphere. Nitrogen ice particles might form thin clouds
a few kilometers above the surface. The atmospheric pressure at Triton's
surface is about 14 microbars, 1/70,000th the surface pressure on Earth.
Triton is the only large satellite in the
solar system to circle a planet in a retrograde direction -- in a
direction opposite to the rotation of the planet. It also has a density
of about 2.066 grams per cubic centimeter (the density of water is 1.0
gram per cubic centimeter). This means Triton contains more rock in its
interior than the icy satellites of Saturn and Uranus do. The relatively
high density and the retrograde orbit has led some scientists to suggest
that Triton may have been captured by Neptune as it traveled through
space several billion years ago. If that is the case, tidal heating
could have melted Triton in its originally eccentric orbit, and the
satellite might even have been liquid for as long as one billion years
after its capture by Neptune.
Triton is scarred by enormous cracks.
Voyager 2 images showed active geyser-like eruptions spewing nitrogen
gas and dark dust particles several kilometers into the atmosphere.
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