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Neptune
The eighth planet from the Sun, Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky.
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Neptune
Sub-Giant Introduction
The eighth planet from the Sun, Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky. When Uranus didn't travel exactly as astronomers expected it to, two mathematicians, working independently of each other, proposed the position and mass of another, as yet unknown planet that could account for Uranus' orbit. Although "the establishment" ignored the predictions, a young astronomer decided to look for the predicted planet. Thus, Neptune was discovered in 1846. Seventeen days later, its largest moon, Triton, was also discovered.
Nearly 4.5 billion kilometers from the Sun, Neptune orbits the Sun once every 165 years, and therefore it has not quite made a full circle around the Sun since it was discovered. It is invisible to the naked eye because of its extreme distance from Earth. Interestingly, due to Pluto's unusual elliptical orbit, Neptune is actually the farthest planet from the Sun for a 20-year period out of every 248 Earth years.
Neptune's Statistics
Star System : Meridian Solar System
Discovered by : Johann Gotfried Galle
Date of discovery : September 23, 1846
Mass (kg) : 1.024e+26
Mass (Earth = 1) : 1.7135e+01
Equatorial radius (km) : 24,746
Equatorial radius (Earth = 1) : 3.8799e+00
Mean density (gm/cm^3) : 1.64
Mean distance from the Sun (km) : 4,504,300,000
Mean distance from the Sun (Earth = 1) : 30.0611
Equatorial surface gravity (m/sec^2) : 11.0
Equatorial escape velocity (km/sec) : 23.50
Visual geometric albedo : 0.41
Magnitude (Vo) : 7.84
Mean cloud temperature : -193 to -153°C
Atmospheric pressure (bars) : 1-3
Planetary Orbits
Neptune's Orbital Statistics
Rotational period (hours) 16.11
Orbital period (years) 164.79
Mean orbital velocity (km/sec) 5.45
Orbital eccentricity 0.0097
Tilt of axis (degrees) 29.56
Orbital inclination (degrees) 1.774
Neptune's Interior
The atmosphere of Neptune, similar to Uranus, consists of mainly hydrogen, methane, and helium. Below it is a liquid hydrogen layer including helium and methane. The lower layer is liquid hydrogen compounds, oxygen, and nitrogen. It is believed that the planet core comprises rock and ice. Average density, as well as the greatest proportion of core per planet size, is the greatest among the gaseous planets.
Interstellar Card
Neptune's Atmospheric Composition
Hydrogen : 85%
Helium : 13%
Methane : 2%
Neptune's blue-green atmosphere is shown in greater detail than ever before by the Voyager 2 spacecraft as it rapidly approaches its encounter with the giant planet. This colorimage, produced from a distance of about l6 million kilometers,shows several complex and puzzling atmospheric features. TheGreat Dark Spot (GDS) seen at the center is about l3,000 km by6,600 km in size -- as large along its longer dimension as theEarth. The bright, wispy "cirrus-type" clouds seen hovering inthe vicinity of the GDS are higher in altitude than the darkmaterial of unknown origin which defines its boundaries. A thinveil often fills part of the GDS interior, as seen on the image. The bright cloud at the southern (lower) edge of the GDS measuresabout l,000 km in its north-south extent. The small, brightcloud below the GDS, dubbed the "scooter," rotates faster thanthe GDS, gaining about 30 degrees eastward (toward the right) inlongitude every rotation. Bright streaks of cloud at thelatitude of the GDS, the small clouds overlying it, and a dimlyvisible dark protrusion at its western end are examples ofdynamic weather patterns on Neptune, which can changesignificantly on timescales of one rotation (about l8 hours).
Neptune's Great Dark Spot and rings
This is how Neptune's Great Dark Spot and rings may have looked in 1989 from a position just beneath Neptune's ring plane. The outermost Adams ring is near the top of the frame, and beneath that is the much broader and diffuse Lassell ring. Further in toward Neptune and abutting the Lassell ring is the thin LeVerrier ring, and beyond that is the diffuse Galle ring.
The Great Dark Spot is believed to be a storm similar to, but only half the size of, Jupiter's Great Red Spot. While Jupiter's Great Red Spot has been raging for at least 400 years, subsequent observations of Neptune's Great Dark Spot in 1994 by the Hubble Space Telescope revealed that this storm has since disappeared.
The Great Dark Spot was a very dynamic weather system, generating massive, white clouds similar to high-altitude cirrus clouds on Earth. Unlike cirrus clouds on Earth however, which are composed of crystals of water ice, Neptune's cirrus clouds are made up of crystals of frozen methane. Neptune's clouds are driven by winds of 1,200 mph, the fastest winds of any planet in the Solar System. How such high-velocity winds come to be on a planet so far from the Sun is still a mystery.
Neptune's Moons
We don't know with what beverage William Lassell may have celebrated his discovery of Neptune's moon, Triton, but beer made it possible.
Lassell was one of 19th century England's grand amateur astronomers, using the fortune he made in the brewery business to finance his telescopes. He spotted Triton on October 10, 1846 - just 17 days after a Berlin observatory discovered Neptune.
Curiously, a week before he found the satellite, Lassell thought he saw a ring around the planet. That turned out to be a distortion caused by his telescope. But when NASA's Voyager 2 visited Neptune in 1989, it revealed that the gas giant does have rings, though they're far too faint for Lassell to have seen.
Since Neptune was named for the Roman god of the sea, its moons were named for various lesser sea gods and nymphs in Greek mythology.
Triton (not to be confused with Saturn's moon, Titan), is far and away the largest of Neptune's satellites. American astronomer Gerard Kuiper (for whom the Kuiper Belt was named) found Neptune's third-largest moon, Neried, in 1949. He missed Proteus, the second-largest, because it's too dark and too close to Neptune for telescopes of that era. That slightly non-spherical moon is thought to be right at the limit of how massive an object can be before its gravity pulls it into a sphere.
Proteus and five other moons had to wait for Voyager 2 to make themselves known. All six are among the darker objects found in the solar system. Astronomers using improved ground-based telescopes found five more satellites in 2002 and 2003, bringing the known total to 13.
Voyager 2 revealed fascinating details about Triton. Part of its surface resembles the rind of a cantaloupe. Ice volcanoes spout what is probably a mixture of liquid nitrogen, methane, and dust, which instantly freezes and then snows back down to the surface. One Voyager 2 image shows a frosty plume shooting 8 km (5 miles) into the sky and drifting 140 km (87 miles) downwind.
Triton's icy surface reflects so much of what little sunlight reaches it that the moon is one of the coldest objects in the solar system, about -240°C (-400°F).
It's the only large moon in the solar system that circles its planet in a direction opposite to the planet's rotation (a retrograde orbit), which suggests that it may once have been an independent object that Neptune captured. The disruptive effect this would have had on other satellites could help to explain why Nereid has the most eccentric orbit of any known moon - it's almost seven times as far from Neptune at one end of its orbit as at the other end.
Neptune's gravity acts as a drag on the counter-orbiting Triton, slowing it down and making it drop closer and closer to the planet. Millions of years from now, Triton will come close enough for gravitational forces to break it apart - possibly forming a ring around Neptune bright enough for Lassell to have seen.
Moon : Triton
Triton [TRY-tun] 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.
With a radius of 1,350 (839 mi), about 22% smaller than Earth's moon, Triton is by far the largest satellite of Neptune. It is one of only three objects in the Solar System known to have a nitrogen-dominated atmosphere (the others are Earth and Saturn's giant moon, Titan). Triton has the coldest surface known anywhere in the Solar System (38 K, about -391° Farenheit); it is so cold that most of Triton's nitrogen is condensed as frost, making it the only satellite in the Solar System known to have a surface made mainly of nitrogen ice. The pinkish deposits constitute a vast south polar cap believed to contain methane ice, which would have reacted under sunlight to form pink or red compounds. The dark streaks overlying these pink ices are believed to be an icy and perhaps carbonaceous dust deposited from huge geyser-like plumes, some of which were found to be active during the Voyager 2 flyby. The bluish-green band visible in this image extends all the way around Triton near the equator; it may consist of relatively fresh nitrogen frost deposits. The greenish areas includes what is called the cataloupe terrain, whose origin is unknown, and a set of 'cryovolcanic' landscapes apparently produced by icy-cold liquids (now frozen) erupted from Triton's interior.
Moon : Proteus
Proteus [PROH-tee-us], like all six of Neptune's newly discovered small satellites, is one of the darkest objects in the solar system - "as dark as soot" is not too strong of a description. Like Saturn's satellite, Phoebe, it reflects only 6 percent of the sunlight that strikes it. Proteus is about 400 kilometers (250 miles) in diameter, larger than Nereid. It wasn't discovered from Earth because it is so close to Neptune that it is lost in the glare of reflected sunlight. Proteus circles Neptune at a distance of about 92,800 kilometers (57,700 miles) above the cloud tops, and completes one orbit in 26 hours, 54 minutes. Scientists say it is about as large as a satellite can be without being pulled into a spherical shape by its own gravity. Proteus is irregularly shaped and shows no sign of any geological modification. It circles the planet in the same direction as Neptune rotates, and remains close to Neptune's equatorial plane.
Moon : Galatea
Galatea [gal-eh-TEE-eh] lies 37,200 kilometers (23,100 miles) from Neptune. It has a diameter of 180 kilometers (110 miles) and completes an orbit in 10 hours, 18 minutes. It is irregularly shaped and shows no sign of any geological modification. Galatea circles the planet in the same direction as Neptune rotates, and remains close to Neptune's equatorial plane. The above image is smeared so that Galatea appears highly elongated.
Moon : Despina
Despina, only 27,700 kilometers (17,200 miles) from Neptune's clouds, orbits every 8 hours. Its diameter is about 150 kilometers (90 miles). It is irregularly shaped and shows no sign of any geological modification. Despina circles the planet in the same direction as Neptune rotates, and remains close to Neptune's equatorial plane.
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