Neptune

Saturday 31 December 2016 - 14:20:45 Posted by  Bobby

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neptune
 Neptune is similar in structure to Uranus. It has a core of rock and possibly ice, surrounded by a mix of water, methane, and ammonia ices, and topped off with a hydrogen-rich atmosphere. The planet undergoes seasonal change, and its atmosphere is unexpectedly dynamic. It experiences ferocious equatorial winds, fast-moving bright clouds, and short-lived gigantic storms. The planet is surrounded by a sparse system of five complete rings and one partial ring. Only one of its 13 moons, Triton, is of notable size, and four are within the planet's ring system.

Ring System

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 Neptune has five rings: Galle, Le Verrier, Lassell, Arago, and Adams. Its rings were named after the astronomers who made an important discovery regarding the planet. The rings are composed of at least 20% dust with some of the rings containing as much as 70% dust; the rest of the material comprising the rings is small rocks. The planet’s rings are difficult to see because they are dark and vary in density and size. Astronomers think Neptune’s rings are young compared to the age of the planet, and that they were probably formed when one of Neptune’s moons was destroyed.

Atmosphere

neptuneatmosphere
 Neptune's atmosphere is made up of hydrogen, helium, and methane. Like Earth's atmosphere, Neptune's has clouds and storm systems that revolve around the planet, but with wind speeds of 300 m/sec (700 mi/hr), and clouds of frozen methane.

Great Dark Spot

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The "Great Dark Spot" discovered by Voyager 2 is a storm system that resembles Jupiter's Red Spot, and is surrounded by cirrus clouds of methane.

Rings of Darkness

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 Like the Solar System's other giant planets, Neptune has a system of rings, though they are nowhere near as spectacular as those of Saturn. The first hint of their existence came from ground-based observations in the 1980's, when astronomers watched as a background star winked in and out as Neptune passed in front.

 As well as the starlight being blocked when the main bulk of the planet got in the way, the background stars dipped slightly several times even when Neptune wasn't quite in front of them. The interpretation was that the stars could have been blocked by unseen rings. Not only that, the data seemed to indicate that perhaps the rings were broken and incomplete.

 It took Voyager 2's arrival at its destination in 1989 for confirmation of this theory to come. Voyager's images revealed a series of rings. One of them, the thin Adams ring, indeed had several ring arcs, which were given the French names Liberte, Egalite, and Fraternite (Liberty, Equality, and Brotherhood).

 Recent observations indicate that Neptune's ring system is not stable, and one or more of the rings may break up sometime in the next 100 years or so. The separate arcs and the Adams ring, along with regular ring "ripples," are thought to be kept in check by the gravitational tug of the tiny moon Galatea.

 Neptune's rings are composed of a fairly dark material, and contain a much higher concentration of tiny dust particles than do Saturn's rings, for instance.

Magnetic Field

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 Neptune's magnetic field is tilted 47 degrees with respect to the planet's axis of rotation, and offset from the planets center by about half its radius. Neptune doesn't have an iron core like Earth; it's magnetic field is probably generated by electrical currents in the water layer or somewhere in the middle strata of the planet's atmosphere.

 From the surface of the Earth, Neptune is too faint to be seen with the unaided eye. Powerful binoculars or a telescope, plus a chart showing the planet's exact position, are needed to reveal its small bluish-green disk.

 It's great distance from the Sun means it has a very slow orbital speed - it takes around 165 Earth years to complete one orbit - and therefore it moves only very slowly through the night sky.

 Neptune is so far from the Sun that the sunlight is about 900 times dimmer than it is on Earth.

 With Pluto's status now changed to that of "dwarf planet," Neptune has become the most distant "full" planet in our Solar System.

 But even without Pluto's recent demotion, Neptune's near-circular orbit - and Pluto's elliptical one - means that for a small part of its orbit, Pluto is actually nearer to the Sun than Neptune, making Neptune on those occasions the farthest planet away.

Cold Moons

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 Neptune has 13 moons, the largest of which is Triton. Triton is notable for several reasons - it's icy surface is very cold and very reflective, and it has a retrograde orbit, that is, it goes around Neptune the "wrong" way.

Triton

triton

 Prior to Voyager 2's flyby of Neptune, not much was known about Triton. Using an assumption about how reflective its surface was likely to be, astronomers calculated a diameter for it based on how bright it looks from the distance of Earth.

 As Voyager got closer, however, it was apparent that Triton was far more reflective than previously believed - this meant the calculation had to be redone, and a smaller diameter figure was arrived at. Triton is, in fact, one of the shiniest bodies in the Solar System. It is also the coldest, at a very chilly  -391 degrees farenheit. Voyager 2 revealed Triton to be a world with a geologically young surface, with few craters visible. It is thought to comprise around 25% water ice, with the remainder being mainly rock material. It also has a very thin atmosphere of nitrogen.

 Most amazingly of all, though, it has active volcanoes, or perhaps geysers, shooting what is thought to be liquid nitrogen high above the surface. Seasonal heating by the sun is believed to be the energy source driving this very powerful volcanic activity.

 Triton's retrograde orbit leads astronomers to believe that it was not formed along with its parent planet, but was captured as it drifted by. It's even possible that Triton was part of a double-planetoid system, with its companion being flung away at the same time as Triton was captured. Neptune's third largest moon, Nered, was the second to be found, in 1949. It wasn't until Voyager 2's encounter in 1989 that six more moons were also discovered. Another five were added by 2003, all found using earth-based telescopes - three of these, like Triton, have retrograde orbits. Little is yet known about these small moons. A number of them are "irregulars" believed to have been captured rather than formed along with the planet.

 Neptune Statistics

Discovered September 23rd, 1846, by Johann Galle
Average distance from the Sun 2,793.1 million miles (4,495.1 million km) 30 AU
Equatorial diameter 30,775 miles (49,528 km)
Axial tilt 29.58°
Axial rotation period 16.11 hours
Orbital period (in Earth years) 164.79 years
Mass (Earth=1) 17.1
Volume (Earth=1) 57.7
Gravity at cloud tops (Earth=1) 1.2
Average density (water=1) 1.6
Escape velocity 14.6 miles/s (23.5 km/s)
Sunlight strength 0.1% of Earth's
Albedo (reflectivity) 41%
Number of satellites 13

 Neptune Timeline

1612
Galileo's astronomical drawings show that he had first observed Neptune on two occasions, both times making it for a fixed star, as the planet was just beginning its yearly retrograde cycle.
1613
In January, Galileo again produces drawings showing he has observed Neptune. However, once again he believes it to be a fixed star.
1821
Alexis Bouvard publishes astronomical tables of the orbit of Uranus. Subsequent observations reveal there are substantial deviations from the tables. This leads Bouvard to hypothesize that perhaps there is some other body or planet that is disturbing Uranus.
1843
John couch Adams calculates the orbit of an eighth planet. He sends his calculations to Sir George Airy, the Astronomer Royal, for a clarification. Airy begins to draft a reply but never sends it.
1846
John Herschel begins to champion the mathematical approach to a new planet and persuades James Challis to search for it.
1846
After much procrastination, chalice begins his reluctant search in July.
1846
Urbain Le Verrier produces his own calculations that a planet must exist beyond Uranus, causing it's unusual behavior.
1846
Using calculations of Le Verrier, Neptune is discovered by German astronomer Johann Gottfried Galle and Heinrich Louis d'Arrest on September 23rd. Adams and Leverrier are jointly honoured with the discovery. The planet does not get its current name for some time.
1846
Challis later realizes that he had observed the planet twice in August, failing to identify it owing to his casual approach to the work.
1846
Le Verrier - through the Board of Longitude - suggests Neptune as the name for the new planet. Leverrier Struve comes out in favor of that name on December 29th, 1846, to the St. Petersburg Academy of Sciences. From then on Neptune became the internationally accepted name.
1846
Only days after Neptune is discovered, amateur astronomer William Lassell thinks he sees a ring around the planet. It turns out to be a distortion caused by his telescope. (The Rings we now know of were found by the Voyager 2 flyby in 1989 and could not have been seen by Lassell.)
1846
Seventeen days after thinking he has discovered a ring, William Lassell discovers Neptune's largest moon, Triton.
1949
Dutch-American astronomer Gerard Kuiper (for whom the Kuiper Belt was named) finds Neptune's third largest moon, Nereid.
1977
Voyager 2 is launched with a mission to fly by Neptune and other planets on its way to Interstellar space.
Mid 1980s
Evidence for incomplete arcs around Neptune first arise when stellar occulation experiments are found to occasionally show an extra "blink" just before or after the planet occulted the star.
1989
Voyager 2 does a flyby of Neptune on August 24 and 25, skimming the North pole by a mere 3,000 miles.
1989
Voyager 2 finds six new Neptunian moons and three new rings, plus a broad sheet of ring material.
2002 - 2003
Astronomers using improved ground-based telescopes found 5 more moons, bringing the known total to 13 moons.

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