The Orion Group Presents

Gerald Fitzgerald November 4, 2002Physics 1411, Section 8250Jeff

The Orion Group presents

NEPTUNE, PLUTO and QUAOAR

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NEPTUNE

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NEPTUNE, THE PERSONALITY

The Greeks called him Poseidon, and he was the great god of the sea and of earthquakes, and in that part of the world, they had plenty of both. But when the Romans took over, they would not be outdone – they renamed every god they could find, and Poseidon became Neptune.

He was the brother of Jupiter (Zeus) and of Pluto (Hades). After the defeat of their father Saturn (Cronos), who had a nasty habit of eating his children, the three brothers divided the world into three parts -- Jupiter took the sky, Neptune the sea and Pluto the underworld.

Neptune had a violent temper. He was a powerful, bearded man holding a trident, seated in a magnificent seashell drawn by sea-horses, commanding terrifying forces of nature, able to unleash devastating storms and earthquakes if he was in a foul mood. When he wanted something, by God he got it! One day, for example, he happened to see a water nymph named Amphitrite dancing naked on an island somewhere, and never one to do anything halfway, instantly fell in love with her. He asked her to marry him but she refused.

Neptune sent one of his servants, a dolphin, to find her and persuade her otherwise, which he did. She changed her mind and married the sea god. As a reward for helping him win the heart of the fair Amphitrite, Neptune named the constellation “Delphinus” in honor of the dolphin. Neptune and Amphitrite had several children. Among them was Triton, whose name was given in 1846 by William Lassell to the principal moon of the planet Neptune.

NEPTUNE, THE CONTROVERSY

The discovery of Neptune represents one of the most controversial episodes in the history of British science. It concerns the failure of British astronomers to discover the planet despite the existence of calculations that predicted its existence.

In 1781, Sir William Herschel used a telescope to discover the planet Uranus. It was the first “modern” planet, that is, the first planet not known since ancient times, because it is too faint to be noticed with the naked eye. Its discovery doubled the size of the known Solar System practically overnight. Herschel received Royal patronage for his sensational discovery, and with it, built the biggest and best telescopes in the world, with which he and his son John surveyed the northern and southern skies.

Isaac Newton's theory of gravitation, and Edmond Halley's use of it to predict the return of the famous comet that now bears his name, had explained the physics of the heavens. Against this background, it seemed as if Britain had established almost “proprietorial” rights to celestial discoveries.

In 1845, a Cambridge mathematician, John Couch Adams, predicted the existence of an unseen planet to account for the fact that Uranus was being pulled slightly out of position in its orbit. Adams attributed this pull to the gravitational effect of an unknown body, and calculated its position.

Adams visited the Royal Observatory at Greenwich to present his findings to the Astronomer Royal, Sir George Airy, the top scientific civil servant of his day. But Airy was unavailable, and although Adams left a note of his calculations, Airy took little interest, believing that the Royal Observatory should not be diverted from its publicly funded work on timekeeping and navigation, in which it was world leader, to search for new planets.

It wasn’t until a French mathematician, Urbain Leverrier, published a similar prediction that Airy decided to take action. Not to be outdone by those “French rivals,” he asked Professor James Challis at Cambridge University to begin a search. Challis began in July 1846 and actually sighted the new planet four times without recognizing it! Meanwhile, Leverrier had enlisted the help of astronomers at Berlin Observatory, who found the new planet on the night of September 23, 1846.

When it became known that Airy and Challis had let the French and Germans beat them to the discovery, they found themselves at the center of a national scandal.

NEPTUNE, THE PLANET

Neptune is the outermost planet of the gas giants, with an equatorial diameter of 30,760 miles. If it were hollow, it would contain nearly 60 Earths. It orbits the Sun every 165 years. It has eight moons, six of which were found by Voyager in 1989. A Neptunian day lasts for 16 hours and 6.7 minutes.

Even though Neptune is four times larger than Earth, its surface gravity is nearly identical. This is probably due to the fact that it is composed mostly of gas. 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. Methane gas is what gives Neptune its characteristic blue color.

The cloud patterns of Neptune's methane atmosphere are the same as all the gas giants, with several giant storms, including the largest, the Great Dark Spot, similar to Jupiter's Great Red Spot, which at the time of its discovery was the size of Earth (it has since dissipated). The structure of the atmosphere, as on Earth, consists of a troposphere, stratosphere, mesosphere, and thermosphere. Voyager detected a small, oddly-shaped, eastward-moving cloud orbiting Neptune every 16 hours or so. This “scooter” as it was named was thought to be a plume rising above a deeper cloud deck.

This image shows bands of sunlit cirrus-like clouds in Neptune's northern hemisphere. These clouds cast shadows on the blue cloud deck 35 miles below. The white streaky clouds are from 30 to 100 miles wide and extend for thousands of miles.

Neptune has the strongest winds of any planet, most of them blowing westward, opposite to Neptune’s rotation. Near the Great Dark Spot, winds have been estimated at up to 1,200 miles an hour.

Neptune’s magnetic field, like that of Uranus, is highly tilted at 47 degrees from the planet’s rotation axis and offset at least 8,500 miles from the physical center. The planet actually appears to roll rather than spin in its orbit around the Sun.

The Hubble Space Telescope is allowing astronomers to study Neptune's dynamic atmosphere with a level of detail not possible since the 1989 flyby of the Voyager 2 space probe. Building on Voyager's initial discoveries, Hubble is revealing that Neptune has a remarkably dynamic atmosphere that changes over just a few days.

The temperature difference between Neptune's strong internal heat source and its frigid cloud tops (-260° Fahrenheit) might trigger instabilities in the atmosphere that drive large-scale weather changes. In addition to hydrogen and helium, the main constituents, Neptune's atmosphere is composed of methane and hydrocarbons, like ethane and acetylene.

Neptune’s Vital Statistics

Atmospheric composition Hydrogen 85%

Helium 13%

Methane 2%

Atmospheric pressure (bars) 1-3

Average Cloud Top Temperature (K): 63 o K

Density: 1,638 kg/m3

Diameter: 49,493 kilometers (30,760 miles)

Equatorial escape velocity (km/sec) 23.50

Equatorial radius (Earth = 1) 3.8799

Equatorial radius (km) 24,746kilometers

Equatorial surface gravity (m/sec^2) 11.0

Magnitude (Vo) 7.84

Mass: 102.4 x 1024 kilograms (17.2 x Earth’s)

Maximum Distance from Sun: 4.54 billion kilometers (2.82 billion miles)

Mean cloud temperature -193°C to -153°C(-315.4°F to –243.4°F)

Mean density (gm/cm3) 1.64

Mean distance from the Sun (Earth = 1) 30.0611AU

Mean distance from the Sun 4,504,300,000kilometers

Mean orbital velocity (km/sec) 5.45

Minimum Distance from Earth: 4.3 billion km (2.68 billion miles)

Minimum Distance from Sun: 2.446 billion km (2.77 billion miles)

Orbital eccentricity 0.0097

Orbital inclination 1.774degrees

Orbital period 164.79years

Orbital Semimajor Axis: 30.07 AU (Earth’s = 1 AU)

Revolution Period about the Sun: 165 years

Rotation Period about Axis: 17.24 hours

Rotational period 16.11hours

Satellites/Rings: 8 known moons, faint rings

Surface Gravity: 11 m/s2 (1.12 x Earth's)

Temperature at Cloud Tops: -210°C (-346°F)

Tilt of Axis 29° 36”

This picture of Neptune was taken by Voyager 2 on August 20, 1989. It shows the Great Dark Spot near the center, at 22 degrees south and circuits Neptune every 18.3 hours. The bright clouds to the south and east of the Great Dark Spot constantly change their appearances in periods as short as four hours.
Scientists deduce Neptune’s internal structure by observing its radius, mass, period of rotation, the shape of its gravitational field and the behavior of hydrogen, helium, and water at high pressure. This cutaway view visualizes Neptune composed of an outer envelope of molecular hydrogen, helium and methane. Below this region, Neptune appears to be composed of a mantle rich in water, methane and ammonia. These elements are under high temperature and pressure.
An almost true-color picture of Neptune and Triton taken with the Hubble Space Telescope. The south pole is to the lower left. The picture reveals a bright cloud feature at the south pole, near the bottom of the image, with bright cloud bands at 30° South and 60° South latitude and a bright cloud band near 30° N latitude.
These images were taken 9 hours apart to show the differences in cloud formation in each hemisphere. The colors are almost spot-on.
These Hubble Space Telescope views of Neptune provide three views of changing weather conditions. The images were taken in 1994 on October 10 (upper left), October 18 (upper right), and November 2 (lower center), when Neptune was 2.8 billion miles from Earth.
In 1994, the HST discovered a new Great Dark Spot, located in the northern hemisphere. The spot is nearly identical to the one in the southern hemisphere that was discovered in 1989 by Voyager 2. This image was taken on November 2, 1994 with Hubble's Wide Field Planetary Camera 2, when Neptune was 2.8 billion miles from Earth. Hubble can resolve features as small as 625 miles (1,000 kilometers) across in Neptune's cloud tops.

Also in 1994, Hubble showed that the southern dark spot had disappeared.

Like its predecessor, the new spot has high altitude clouds along its edge, caused by gasses that have been pushed to higher altitudes where they cool to form methane ice crystal clouds. The dark spot may be a zone of clear gas that is a window to a cloud deck lower in the atmosphere.

Planetary scientists can’t say how long this new spot will last. But the high resolution of the Hubble telescope will allow astronomers to follow the spot's evolution and continued development.

This is one of the final images from Voyager 2. Triton appears at the bottom right. The image was taken on August 31, 1989.

NEPTUNE’S RINGS

Neptune’s four rings are narrow and faint, made of dust particles thought to have been made by tiny meteorites smashing into Neptune's moons.

The following table is a summary of the rings of Neptune.

Name / Distance* / Width / Thickness / Mass / Albedo
1989N3R / 41,900 km / 15 km / Not yet known / NYK / low
1989N2R / 53,200 km / 15 km / Not yet known / NYK / low
1989N4R / 53,200 km / 5,800 km / Not yet known / NYK / low
1989N1R / 62,930 km / Less than 50 km / Not yet known / NYK / low

*The distance is measured from the planet center to the start of the ring.

These two 591-second exposures of the rings of Neptune were taken by Voyager 2 on August 26, 1989 from a distance of 174,000 miles. The two main rings are fully visible. You can also see a faint inner ring at about 25,000 miles from the planet. Neptune itself is over-exposed in the center of the picture, which was necessary in order to capture the delicate ring images.

NEPTUNE’S MOONS

This table summarizes the radius, mass, distance from the planet center, discoverer and the date of discovery of each of the moons of Neptune:

Moon / # / Radius
(km) / Mass
(kg) / Distance
(km) / Discoverer / Date
Naiad / III / 29 / unknown / 48,000 / Voyager 2 / 1989
Thalassa / IV / 40 / unknown / 50,000 / Voyager 2 / 1989
Despina / V / 74 / unknown / 52,500 / Voyager 2 / 1989
Galatea / VI / 79 / unknown / 62,000 / Voyager 2 / 1989
Larissa / VII / 104x89 / unknown / 73,600 / Voyager 2 / 1989
Proteus / VIII / 200 / unknown / 117,600 / Voyager 2 / 1989
Triton / I / 1,350 / 2.1422 / 354,800 / W. Lassell / 1846
Nereid / II / 170 / unknown / 5,513,400 / G. Kuiper / 1949

TRITON

Triton (“TRY-ton”) is the seventh and largest of Neptune's satellites.

In Greek mythology, Triton was a god of the sea, the son of Poseidon (Neptune). He is usually portrayed as having the head and trunk of a man and the tail of a fish.

Triton was discovered by Lassell in 1846, only a few weeks after the discovery of Neptune itself.

Triton has been visited by only one spacecraft, Voyager 2, on August 25, 1989. Almost everything we know about it comes from this encounter.

Triton's orbit is retrograde, which means that it orbits Neptune in a direction opposite to the rotation of the planet. It is the only large moon in the Solar System which orbits “backwards.” The only other moons with retrograde orbits are Jupiter's moons Ananke, Carme, Pasiphae and Sinope and Saturn's Phoebe, all of which are less than 1/10th the diameter of Triton.

Triton could not have condensed from the primordial Solar Nebula in its current configuration. It must have formed elsewhere (perhaps in the Kuiper Belt?) and later been captured by Neptune (perhaps involving a collision with another Neptunian moon, now shattered). A capture scenario could account not only for Triton's orbit, but also for the unusual orbit of Nereid and provide the energy needed to melt and differentiate Triton's interior.