Download Neptune 1

Neptune 1
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Name – In Roman mythology, Neptune (Greek: Poseidon) is the god of the sea.
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Discovery – After the discovery of Uranus, it was noticed that its orbit was not in
accordance with Newton’s laws. In the early 1840s, Englishman John Couch Adams
suggested that this inconsistency could be explained by a more distant planet
perturbing the orbit of Uranus. Perturbation is defined as the gravitational effect of
one object on the orbit of another. Adams calculated the location of the planet and
urged a telescopic search. Unfortunately, he was a student at the time and was unable
to convince British astronomers to carry out the search. In the summer of 1846,
French astronomer Urbain Jean Leverrier made similar calculations independently.
He sent a letter to Johann Galle, of the Berlin Observatory, suggesting a search for the
eighth planet. On the night of September 23, 1846, Galle pointed his telescope to the
position suggested by Leverrier. There, within 1° of its predicted position, he saw the
planet Neptune. Neptune was truly discovered utilizing mathematics and physics,
and merely confirmed with a telescope.
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Galileo’s astronomical drawings show that he had first observed Neptune on
December 27, 1612, and again on January 27, 1613. On both occasions, Galileo
had mistaken Neptune for a fixed star when it appeared very close to Jupiter in the
night sky. Believing it to be a fixed star, he cannot be credited with its discovery.
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Spacecraft exploration – Neptune has been visited by only one spacecraft, Voyager 2
in 1989 (12 years after its launch). Much of what we know about Neptune comes
from this single encounter. However, ground-based and Hubble Space Telescope
observations have added a great deal.
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Location – Neptune is the eighth planet from the Sun, with an average distance of
approximately 2,792,000,000 miles.
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Size – Neptune measures 30,710 miles in diameter.
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Orbital period – 164.79 years.
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Rotational period – 16.11 hours.
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Number of Satellites – 14 known moons.
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Atmosphere
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Composition – Neptune’s atmosphere is composed of hydrogen (80%), helium
(18%), and methane (2%). The remainder is comprised of trace amounts of
hydrogen deuteride, ethane, water, and hydrocarbons. These hydrocarbons formed
through photochemical reactions.
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Ammonia – Like Uranus, the low temperature (nearly -400º F) associated with
Neptune’s upper atmosphere, does not allow ammonia to exist in the gaseous
state.
Structure 2 – Like Uranus, the color of Neptune’s atmosphere is due to the absorption
of red light by methane gas. The subtle color differences between Uranus (bluegreen) and Neptune (deeper blue) may have to do with the depths of the clouds and
the color of the cloud particles themselves. During the Voyager encounter in 1989,
Neptune revealed a cloudy, banded atmosphere. Like Jupiter, Saturn, and Uranus, the
banded pattern lies parallel to latitude lines and is caused by Neptune’s rapid rotation
and lack of a solid (obstructive) surface.
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Dark Spots – Voyager 2 images revealed two major dark features, dubbed the
Great Dark Spot 3 (about the size of the Earth) and the Small Dark Spot 4 (also
known as the Wizard’s Eye) located in Neptune’s southern hemisphere. Unlike
Jupiter’s Great Red Spot, these dark spots are thought to be holes in the methane
cloud deck of Neptune. Towering above the dark spots are white, cirrus-like
clouds composed of methane ice crystals, which most likely form by rising
convection currents. Around the Great Dark Spot, winds were measured blowing
up to 1,300 miles an hour, the fastest in our Solar System.
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Hubble space telescope observations of Neptune in 1994 show that the Great
Dark Spot has disappeared. The reason for the spot’s disappearance is
unknown. Many scientists believe heat transfer from the planet’s core
disrupted existing circulation patterns. A few months later a new dark spot in
Neptune’s northern hemisphere was discovered 5.
Scooter 6 – Scooter is a small white cloud that initially appeared to move faster
around Neptune than any other white clouds observed (hence, the name).
Subsequent images have revealed even faster clouds. As Voyager watched, it
changed shape from round to square to triangular. Scooter may be a plume rising
from lower in the atmosphere, but its true nature remains a mystery.
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Interior
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Structure 7 – Neptune’s internal structure resembles that of Uranus. (Refer to notes
on Uranus regarding interior.) However, Neptune’s higher density (1.64 g/cm3
compare to Uranus’ density of 1.27 g/cm3) suggests it possesses a larger proportion of
rocky materials.
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Magnetic Field 8 – Neptune has a magnetic field, but it is the weakest of the Jovian
planets, and is only about one-fourth that of Earth’s. Like Uranus, Neptune’s
magnetic axis is offset and tilted (47) to the axis of rotation. The source of
Neptune’s magnetic field and the reasons it is offset and tilted are thought to be the
same as Uranus’. (Refer to notes on Uranus regarding magnetic field.)
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Internal Heat – Neptune has an internal heat source, radiating more than 2.5 times as
much energy as it receives from the Sun. Like Jupiter and Saturn, the source of this
heat is generated from the planet’s continuing contraction from a protoplanetary
phase. In other words, Neptune’s mass is moving inward under the influence of
gravity, and the resulting energy of motion (kinetic energy) is transformed into heat.
Rings
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Discovery – The rings of Neptune (like Uranus’ ring system) were first detected in
stellar occultation observations from Earth. However, evidence suggested that the
rings were incomplete arcs that extended only part of the way around the planet. The
true character of the rings was revealed through images taken by Voyager 2.
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Structure 9 – Voyager 2 detected five dark, narrow rings surrounding Neptune. In
order of increasing distance from the planet, they are named: Galle, Leverrier,
Lassell, Arago, and Adams. Moreover, Voyager 2 revealed that Neptune’s ring arcs
are actually five prominent concentrations of material (now named Courage, Liberty,
Equality 1, Equality 2, and Fraternity) within the “continuous” Adams Ring 10. The
gravitational effects of Neptune’s minor moon, Galatea, may be responsible for their
formation. Recent Earth-based observations of Neptune’s outer rings show that some
parts of them have dramatically deteriorated and one section is close to disappearing
altogether. In particular, Liberty arc has faded considerably since the Voyager 2
observations, and might disappear in as little as one century.
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Composition – Neptune’s ring particles may consist of methane-rich ice darkened by
radiation. For the most part, the rings of Neptune (like Jupiter) are made up of dustsized particles.
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Origin – Like Uranus, the rings of Neptune are thought to be relatively young, and
probably resulted from the collisional fragmentation of onetime inner moons.
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Moons
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Triton 11 – Neptune has one major moon, Triton. Its characteristics do not match
other icy satellites. However, they do match the characteristics observed for Pluto
and other Kuiper Belt Objects (discussed later).
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Orbit – Triton has unusual orbital properties. The moon orbits Neptune in the
direction opposite to the planet’s rotation (a retrograde orbit). However, its
rotation is still synchronous with its period of revolution; meaning the same side
of Triton always faces Neptune. In addition, its orbital plane is tipped at an
unusually steep 21º away from the planet’s equator. Triton’s strange orbit
suggests that it was gravitationally captured by Neptune after the satellite formed
elsewhere. In fact, Triton may well be a captured Kuiper Belt Object.
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Atmosphere – Triton has a very tenuous and transparent atmosphere, with a
pressure sixteen-millionths the sea level pressure on Earth. Like Titan, the
moon’s large size and low temperature (about -400º F) hinder atmospheric gases
from reaching escape velocity. Triton’s atmosphere is primarily composed of
nitrogen (N2), with small amounts of methane (CH4) and possibly carbon
monoxide (CO). These gases are derived from the sublimation (evaporation) of
ice from the moon’s surface. Voyager images of the limb of Triton revealed a
haze layer nearly 10 miles above the surface 12. This haze layer is probably
produced as ultraviolet sunlight and charged particles from Neptune’s
magnetosphere dissociate N2 and CH4, which then recombine into more complex
organic compounds. These compounds most likely precipitate onto the surface,
adding color to the regions of Triton’s surface where they accumulate.
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Surface – Five molecules on Triton’s surface have been identified: nitrogen,
methane, carbon monoxide, carbon dioxide, and water. Because of the moon’s
extremely low temperature, all five are frozen. Water-ice serves as the hard
bedrock of Triton’s crust and mantle. Overall, Triton’s surface has a slightly
pinkish color 13. Chemical reactions are evidently occurring in the icy nitrogen
and methane, producing organic compounds with a reddish coloration. Triton is
relatively flat; its observed topography never varies beyond a half mile. Only a
few impact craters are recognized on Triton, indicating the surface is relatively
young. Nearly all other landforms appear to have been created by eruption of
liquid or slushy ices from the interior (cryovolcanism).
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Terrains – Triton exhibits a vast, rugged surface, with a texture that bears a
striking resemblance to the skin of a cantaloupe 14. Most likely shaped by the
upwelling movement of fluid materials from the interior, the “cantaloupe
terrain” is characterized by large, roughly circular depressions (called cavi)
more than 15 miles across, with the entire region crisscrossed by long,
interconnected ridges. This terrain is considered the oldest landform on
Triton. Triton also exhibits large smooth terrain. In particular, the floors of
walled plains called planitia are rather flat and surrounded by terraces roughly
500 feet high 15. These plains apparently underwent episodes of flooding by
cyrovolcanic fluids, freezing, remelting, and collapse.
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Polar Ice Caps – Triton’s south pole is covered by a highly reflective cap of
frozen nitrogen and methane 16. It is thought that Triton also has a north polar
cap, but because the north pole was on the night side during the Voyager 2
encounter, little is known.
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Dark Streaks – Voyager 2 images revealed dark streaks on Triton’s south
polar ice cap, which most likely represent deposits by geyser-like plumes of
nitrogen gas mixed with dust (possibly silicate and carbonaceous material) 17.
Apparently, these plumes rise about 5 miles above the surface, and at this
altitude, smoothly flowing winds carry the lofted material downwind for 50 to
100 miles, while maintaining it in streamers that are about 5 miles wide 18.
The dust eventually falls to the surface and creates the dark streaks on the ice.
All the geysers observed were located close to where the Sun is perceived to
be directly overhead (the zenith point). This indicates that solar heating,
although very weak at Triton’s great distance from the Sun, plays a crucial
role. It is thought that the surface of Triton may consist of a translucent layer
of frozen nitrogen overlying a darker layer, which creates a kind of “solid
greenhouse effect”. Solar radiation passes through the surface ice, slowly
heating and vaporizing subsurface nitrogen until enough gas pressure
accumulates for it to erupt (along with some of the dark substrate material)
through the crust. This plume activity is distinct from Triton’s larger scale
cryovolcanic eruptions, which are powered by internal heat.
Interior – Triton measures about 1,675 miles in diameter, with an average density
of 2.06 g/cm3. This points to a composition of about 25% water-ice and 75%
rock. Because Triton’s surface indicates a long history of melting, its interior is
most likely differentiated into a solid core, a mantle and a crust. Water comprises
the moon’s mantle, which lies over a core of rock and metal. There is enough
rock in Triton’s interior for convection to be occurring within its mantle, powered
by radioactive decay. The heat may even be sufficient to maintain a subterranean
ocean, which suggests the possibility of life.
Minor Moons – To date, Neptune has 13 minor moons. Like Uranus, these moons
may have been initially asteroids captured by the planet’s gravity and/or fragments
from intersatellite collisions. They are irregular in shape and appear quite dark. The
dark coloration suggests that either icy materials at their surfaces darkened by some
sort of aging process involving methane ice, or the material is an undifferentiated
mixture of ice, silicate, and carbonaceous material.
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Proteus 19 – Proteus was not discovered prior to Voyager’s flyby because it is very
dark and so close to Neptune that it is difficult to see in the glare of the much
brighter planet. Proteus is little less than 260 miles in diameter; about as big as an
irregular body can be before its gravity pulls it into a more spherical shape. The
heavily cratered surface shows no signs of geologic activity.
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