New Horizons Detects Neptune’s Moon Triton

The top frame is a composite, full-frame (0.29° by 0.29°) LORRI image of Neptune taken Oct. 16, 2008, using an exposure time of 10 seconds and 4-by-4 pixel re-binning to achieve its highest possible sensitivity. The bottom frame is a twice-magnified view that more clearly shows the detection of Triton, Neptune’s largest moon. Neptune is the brightest object in the field and is saturated (on purpose) in this long exposure. Triton, which is about 16 arcsec east (celestial north is up, east is to the left) of Neptune, is approximately 180 times fainter.Scientists consider Triton to be one of the best analogs of Pluto in the solar system. All the other objects in the image are background field stars. The dark “tails” on the brightest objects are artifacts of the LORRI charge-coupled device (CCD); the effect is small but easily seen in this logarithmic intensity stretch.The original image was taken to test New Horizons’ optical navigation capabilities.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Friday, March 13, 2009

Add another moon to the New Horizons photo gallery: the spacecraft’s Long Range Reconnaissance Imager detected Triton, the largest of Neptune’s 13 known moons, during the annual spacecraft checkout last fall.

New Horizons was 2.33 billion miles (3.75 billion kilometers) from Neptune on Oct. 16, when LORRI, following a programmed sequence of commands, locked onto the planet and snapped away.

“We wanted to test LORRI’s ability to measure a faint object near a much brighter one using a special tracking mode,” says New Horizons Project Scientist Hal Weaver, of the Johns Hopkins University Applied Physics Laboratory, “and the Neptune-Triton pair perfectly fit the bill.” LORRI was operated in 4-by-4 format (the original pixels are binned in groups of 16), and the spacecraft was put into a special tracking mode to allow for longer exposure times. “We needed to achieve the highest possible sensitivity,” Weaver adds.

Mission scientists also wanted to measure Triton itself. “Among the objects visited by spacecraft so far, Triton is by far the best analog of Pluto,” says New Horizons Principal Investigator Alan Stern. The Voyager 2 spacecraft took spectacular images of Triton during its flyby of Neptune in 1989, showing evidence of cryovolcanic activity and cantaloupe-like terrain.

Triton is only slightly larger than Pluto (1,700 miles or 2,700 kilometers) in diameter compared to Pluto’s 1,500 miles (2,400 kilometers). Both objects have atmospheres primarily composed of nitrogen gas with a surface pressure only 1/70,000th of Earth’s, and comparably cold surface temperatures (-390° F on Triton and -370° F on Pluto). Triton is widely believed to have once been a member of the Kuiper Belt (as Pluto still is) that was captured into orbit around Neptune, probably during a collision early in the solar system’s history.

New Horizons can observe Neptune and Triton at solar phase angles (the Sun-object-spacecraft angle) that are not possible to achieve from Earth-based facilities, and this unique perspective can provide insight into the properties of Titan’s surface and Neptune’s atmosphere.

LORRI will continue to observe the Neptune-Triton pair during annual checkouts until the Pluto encounter in 2015.

New Horizons is currently in electronic hibernation, 1.2 billion miles (1.93 billion kilometers) from home, speeding away from the Sun at 38,520 miles (61,991 kilometers) per hour.

Rings of Neptune

The scheme of Neptune's ring-moon system. Solid lines denote rings; dashed lines denote orbits of moons.

The rings of Neptune were discovered in 1989 by the Voyager 2 spacecraft and are tenuous, faint and dusty, and resemble the rings of Jupiter more closely than those of Saturn or Uranus. Neptune possesses five known rings, each named for an astronomer who contributed important work on the planet:the Galle, LeVerrier, Lassell, Arago and Adams rings.Neptune also has a faint unnamed ring coincident with the orbit of Neptunian moon Galatea. Three other moons orbit between the rings: Naiad, Thalassa and Despina.

The rings of Neptune are made of extremely dark material, likely organic compounds processed by radiation similar to that found in the rings of Uranus.The proportion of dust in the rings (between 20 and 70%) is high, while their optical depth is low, at less than 0.1.Uniquely, the Adams ring is divided into five discrete arcs, named Fraternité, Egalite 1 and 2, Liberté, and Courage. The arcs occupy a narrow range of orbital longitudes and are remarkably stable, having changed only slightly since their initial detection in 1980.How the arcs maintain stability is still under debate. However, their stability is probably related to the resonant interaction between the Adams ring and its inner shepherd moon, Galatea.

Discovery and observations:

The first mention of rings around Neptune dates back to 1846 when William Lassell, the discoverer of Neptune's largest moon Triton, thought he had seen a ring around the planet.However, his claim was never confirmed and was likely an observational artifact. The first reliable detection of a ring was made in 1968 via stellar occultation, although that result had gone unnoticed until 1977 when the rings of Uranus were discovered.Soon after the Uranus discovery, a team from Villanova University led by Harold J. Reitsema began searching for rings around Neptune. On 24 May 1981, they detected a dip in a star's brightness during one occultation; however, the manner in which the star dimmed did not suggest a ring. Later, after the Voyager fly-by, it was found that the occultation was due to the small Neptunian moon Larissa, a highly unlikely event.

A pair of Voyager 2 images of Neptune's ring system

In the 1980s, occultations were much rarer for Neptune than for Uranus, which lay near the Milky Way at the time and was thus moving against a denser field of stars. Neptune's next occultation, on 12 September 1983, resulted in a possible detection of a ring.However, ground-based results were inconclusive. Over the next six years, approximately 50 other occultations were observed with only about one third of them yielding positive results. Something (probably incomplete arcs) definitely existed around Neptune, but the parameters of the ring system remained a mystery.The Voyager 2 spacecraft made the definitive discovery of the Neptunian rings during its historic fly-by of Neptune in 1989. It confirmed that occasional occultation events observed before were indeed caused by the arcs within the Adams ring (see below).After the Voyager fly-by the previous terrestrial occultation observations were reanalyzed yielding parameters of the ring's arcs as they were in 1980s, which matched those found by Voyager almost perfectly.

Recently, the brightest rings (Adams and LeVerrier) have been imaged with the Hubble Space Telescope and Earth-based telescopes, owing to advances in resolution and light-gathering power. They are visible, slightly above background noise levels, at methane-absorbing wavelengths in which the glare from Neptune is significantly reduced. The fainter rings are still far below the visibility threshold.

General properties:

Neptune possesses five distinct rings named, in order of increasing distance from the planet, Galle, LeVerrier, Lassell, Arago and Adams.In addition to these well-defined rings, Neptune may also possess an extremely faint sheet of material stretching inward from the LeVerrier to the Galle ring, and possibly farther in toward Neptune.Three of the Neptunian rings are narrow, with widths of about 100 km or less;in contrast, the Galle and Lassell rings are broad—their widths are between 2,000 and 5,000 km.The Adams ring consists of five bright arcs embedded into a fainter continuous ring.Counting counterclockwise, the arcs are: Fraternité , Egalite 1 and 2, Liberté, and Courage.The first three names mean liberty, equality, and fraternity, after the famous motto of the French Revolution. The terminology was suggested by their original discoverers, who had found them during stellar occultations in 1984 and 1985.Four small Neptunian moons have orbits inside the ring system: Naiad and Thalassa orbit in the gap between Galle and LeVerrier rings, Despina is just inward of LeVerrier ring, and Galatea lies slightly inward of the Adams ring.Galatea is embedded in an unnamed narrow and faint ringlet.

A Voyager ring image shown at increased brightness to bring out fainter features

The Neptunian rings contain a large quantity of micrometer-sized dust: the dust fraction by cross-section area is between 20% and 70%.In this respect they are similar to the rings of Jupiter, in which the dust fraction is 50–100%, and are very different from the rings of Saturn and Uranus, which contain little dust (less than 0.1%).The particles in Neptune's rings are made from a dark material; probably a mixture of ice with radiation-processed organics.The rings are a red in color, and their geometrical (0.05) and bond (0.01–0.02) albedos are similar to those of the Uranian rings' particles and the inner Neptunian moons.The rings are generally optically thin (transparent); their normal optical depths do not exceed 0.1.As a whole, the Neptunian rings resemble those of Jupiter; both systems consist of faint narrow dusty ringlets and even more faint broad dusty rings.

The rings of Neptune, like those of Uranus, are thought to be relatively young; their age is probably significantly less than that of the Solar System.Also, like those of Uranus, Neptune's rings likely resulted from the collisional fragmentation of onetime inner moons.Such events create moonlet belts, which act as the sources of dust for the rings. In this respect rings of Neptune are similar to faint dusty bands observed by Voyager 2 between the main rings of Uranus.

Inner rings:

The innermost ring of Neptune is called the Galle ring after Johann Gottfried Galle, the first man to see Neptune through a telescope.It is about 2,000 km wide and orbits 41,000–43,000 km from the planet.It is a faint ring with an average normal optical depth of around 10⁻⁴,and with an equivalent depth of 0.15 km.The fraction of dust in this ring is estimated from 40 to 70%.

The next ring is the LeVerrier ring after Urbain Le Verrier, who predicted Neptune's position in 1846.Its orbital radius is about 53,200 km, but it is narrow, with a width of about 113 km.Its normal optical depth is 0.0062 ± 0.0015, which corresponds to an equivalent depth of 0.7 ± 0.2 km.The dust fraction in the LeVerrier ring ranges from 40 to 70%.The small Neptunian moon Despina, which orbits just inside of it at 52,526 km, may play a role in the ring's confinement by acting as a shepherd.

The Lassell ring, also known as the plateau, is the broadest ring in the Neptunian system.It is named after William Lassell, the English astronomer who discovered Neptune's largest moon, Triton.This ring is a faint sheet of material occupying the space between the LeVerrier ring at about 53,200 km and the Arago ring at 57,200 km.Its average normal optical depth is around 10⁻⁴, which corresponds to an equivalent depth of 0.4 km.The ring's dust fraction is in the range from 20 to 40%.

There is a small peak of brightness near the outer edge of the Lassell ring located at 57,200 km from Neptune and less than 100 km wide,which some planetary scientists call the Arago ring after François Arago, a French mathematician, physicist, astronomer and politician.However, many publications do not mention Arago ring at all.

Adams ring:

The outer Adams ring, with an orbital radius of about 63,930 km,is the most famous and best studied of the Neptune's rings.It is named after John Couch Adams, who predicted the position of the Neptune independently of LeVerrier.This ring is narrow, slightly eccentric and inclined, with total width of about 35 km (15–50 km),and its normal optical depth is around 0.011 ± 0.003 outside the arcs, which corresponds to the equivalent depth of about 0.4 km.The fraction of dust in this ring is from 20 to 40%—lower than in other narrow rings.Neptune's small moon Galatea, which orbits just inside of the Adams ring at 61,953 km, acts like a shepherd, keeping ring particles inside a narrow range of orbital radii through a 42:43 outer Lindbland resonance.Galatea's gravitational influence creates 42 radial wiggles in the Adams ring with an amplitude of about 30 km, which have been used to infer Galatea's mass.

Ring arcs in the Adams ring (left to right: Fraternité, Egalité, Liberté), plus the LeVerrier ring on the inside

Arcs:

The brightest parts of the Adams ring, the ring arcs, were the first elements of Neptune's ring system to be discovered.The arcs are discrete regions within the ring in which the particles that comprise it are mysteriously clustered together. The Adams ring is known to comprise five short arcs, which occupy a relatively narrow range of longitudes from 247° to 294°.In 1986 they were located between 247–257° (Fraternité ), 261–264° (Égalité 1), 265–266° (Égalité 2), 276–280° (Liberté), and 284.5–285.5° (Courage) longitudes.The brightest and longest arc was Fraternité; the faintest was Courage. The normal optical depths of the arcs are estimated to lie in the range 0.03–0.09(0.034 ± 0.005 for the leading edge of Liberté arc as measured by stellar occultation), the radial widths are approximately the same as those of the continuous ring—about 30 km.The fraction of dust in the arcs is from 40 to 70%. The arcs in the Adams ring are somewhat similar to the arc in Saturn's G ring.

The highest resolution Voyager 2 images revealed a pronounced clumpiness in the arcs, with a typical separation between visible clumps of 0.1° to 0.2°, which corresponds to 100–200 km along the ring. Because the clumps were not resolved, they may or may not include larger bodies, but are certainly associated with concentrations of microscopic dust as evidenced by their enhanced brightness when backlit by the Sun.

The arcs are quite stable structures. They were detected by ground based stellar occultations in the 1980s, by Voyager 2 in 1989 and by Hubble Space Telescope and ground based telescopes in 1997–2005 and remained at approximately the same orbital longitudes.However some changes have been noticed. The overall brightness of arcs decreased since 1986.The Courage arc jumped forward by 8° to 294° (it probably jumped over to the next stable co-rotation resonance position) while the Liberté arc had almost disappeared by 2003.The Fraternité and Égalité (1 and 2) arcs have demonstrated irregular variations in their relative brightness. Their observed dynamics is probably related to the exchange of dust between them.Courage, a very faint arc found during the Voyager flyby, was seen to flare in brightness in 1998, while more recently it was back to its usual dimness. However, visible light observations show that the total amount of material in the arcs has remained approximately constant, but they are dimmer in the infra-red light where previous observations were taken.

Confinement:

The arcs in the Adams ring remain unexplained.Their existence is a puzzle because basic orbital dynamics imply that they should spread out into a uniform ring over a matter of years. Several theories about the arcs' confinement have been suggested, the most widely publicized of which holds that Galatea confines the arcs via its 42:43 co-rotational inclination resonance (CIR).The resonance creates 84 stable sites along the ring’s orbit, each 4° long, with arcs residing in the adjacent sites.However measurements of the rings' mean motion with Hubble and Keck telescopes in 1998 led to the conclusion that the rings are not in CIR with Galatea.

A later model suggested that confinement resulted from a co-rotational eccentricity resonance (CER).The model takes into account the finite mass of the Adams ring, which is necessary to move the resonance closer to the ring. A byproduct of this theory is a mass estimate for the Adams ring—about 0.002 of the mass of Galatea. A third theory proposed in 1986 requires an additional moon orbiting inside the ring; the arcs in this case are trapped in its Lagrangian points. However Voyager 2 placed strict constraints on the size and mass of any undiscovered moons, making such a theory unlikely.Some other more complicated theories hold that a number of moonlets are trapped in co-rotational resonances with Galatea, providing confinement of the arcs and simultaneously serving as sources of the dust.

Exploration:

The rings were investigated in detail during the Voyager 2 spacecraft's flyby of Neptune in August 1989.They were studied with optical imaging, and through observations of occultations in ultraviolet and visible light.Voyager 2 observed the rings in different geometries relative to the Sun, producing images of back-scattered, forward-scattered and side-scattered light.Analysis of these images allowed derivation of the phase function (dependence of the ring's reflectivity on the angle between the observer and Sun), and geometrical and bond albedo of ring particles.Analysis of Voyager's images also led to discovery of six inner moons of Neptune, including Galatea that is a shepherd of the Adams ring.

Neptune and its Internal heat

A size comparison of Neptune and Earth.

Neptune's more varied weather when compared to Uranus is believed to be due in part to its higher internal heat. Although Neptune lies half again as far from the Sun as Uranus, and receives only 40% its amount of sunlight, the two planets' surface temperatures are roughly equal.The upper regions of Neptune's troposphere reach a low temperature of −221.4 °C (51.7 K). At a depth where the atmospheric pressure equals 1 bar, the temperature is −201.15 °C (72.0 K).Deeper inside the layers of gas, however, the temperature rises steadily. As with Uranus, the source of this heating is unknown, but the discrepancy is larger: Uranus only radiates 1.1 times as much energy as it receives from the Sun;Neptune radiates about 2.61 times as much, which means the internal heat source generates 161% of the solar input.Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. Several possible explanations have been suggested, including radiogenic heating from the planet's core,dissociation of methane into hydrocarbon chains under atmospheric pressure,and convection in the lower atmosphere that causes gravity waves to break above the tropopause.

The Great Dark Spot of Neptune

The Great Dark Spot as seen from Voyager 2

The Great Dark Spot (GDS-89) was a dark spot on Neptune similar in appearance to Jupiter's Great Red Spot. It was detected in 1989 by NASA's Voyager 2 probe. Although it appeared similar to Jupiter's spot, which is an anticyclonic storm, it is believed that the Great Dark Spot was a relatively cloud-free region.

Characteristics:

The spot was about the same size as Earth, and was very similar in appearance to Jupiter's Great Red Spot. At first it was thought to be a storm, like the Great Red Spot, but closer observation revealed it to be a dark, elliptically-shaped depression in Neptune. Around the Great Dark Spot, winds were measured blowing up to 2,400 kilometers (1,500 miles) an hour, the fastest in our Solar System. The Great Dark Spot is thought to be a hole in the methane cloud deck of Neptune, similar to the holes in the ozone layer on Earth. In many images of Neptune, the spot has been observed at different sizes and shapes.The Great Dark Spot generated large white clouds similar to high-altitude cirrus clouds found on Earth. Unlike cirrus clouds, however, which are composed of crystals of ice, Neptune's cirrus clouds are made up of crystals of frozen methane. And while cirrus clouds usually form and then dissolve within a period of a few hours, the clouds in the Great Dark Spot had not dissolved after 36 hours, or two rotations of the planet.

Disappearance:

When the spot was to be photographed again in 1994 by the Hubble Space Telescope, the spot had disappeared completely, leaving astronomers to believe that it had either been covered up or vanished. However, an almost identical spot emerged in Neptune's northern hemisphere. This new spot, called the Northern Great Dark Spot (NGDS), has remained visible for several years.

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Special Note:

An Anticyclonic storm is a weather storm where winds around the storm flow contrary to the direction dictated by the Coriolis effect about a region of low pressure. In the northern hemisphere, anticyclonic storms involve clockwise wind flow; in the southern hemisphere, they involve anticlockwise (also called counterclockwise) wind flow.Anticyclonic storms usually form around high-pressure systems. These do not "contradict" the Coriolis effect; it predicts such anticyclonic flow about high-pressure regions. Anticyclonic storms, as high-pressure systems, usually accompany cold weather and are frequently a factor in large snowstorms. Jupiter's Great Red Spot is a well-known extraterrestrial example of an anticyclonic system.Anticyclonic tornadoes often occur; while tornadoes' vortices are low-pressure regions, this occurs because tornadoes occur on a small enough scale such that the Coriolis effect is negligible.

Atmosphere of Neptune

Photo Info:This Voyager 2 high resolution color image, taken 2 hours before closest approach, provides obvious evidence of vertical relief in Neptune's bright cloud streaks.These clouds were observed at a latitude of 29 degrees north near Neptune's east terminator. The linear cloud forms are stretched approximately along lines of constant latitude and the Sun is toward the lower left. The bright sides of the clouds which face the Sun are brighter than the surrounding cloud deck because they are more directly exposed to the sun. Shadows can be seen on the side opposite the sun. These shadows are less distinct at short wavelengths (violet filter) and more distinct at long wavelengths (orange filter). This can be understood if the underlying cloud deck on which the shadow is cast is at a relatively great depth, in which case scattering by molecules in the overlying atmosphere will diffuse light into the shadow.Because molecules scatter blue light much more efficiently than red light, the shadows will be darkest at the longest (reddest) wavelengths, and will appear blue under white light illumination.The resolution of this image is 11 kilometers (6.8 miles per pixel) and the range is only 157,000 kilometers (98,000 miles). The width of the cloud streaks range from 50 to 200 kilometers (31 to 124 miles), and their shadow widths range from 30 to 50 kilometers (18 to 31 miles). Cloud heights appear to be of the order of 50 kilometers (31 miles).

At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium.A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue,although Neptune's vivid azure differs from Uranus's milder aquamarine. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.

Neptune's atmosphere is sub-divided into two main regions; the lower troposphere, where temperature decreases with altitude, and the stratosphere, where temperature increases with altitude. The boundary between the two, the tropopause, occurs at a pressure of 0.1 bars.The stratosphere then gives way to the thermosphere at a pressure lower than 10⁻⁴ to 10⁻⁵ microbars.The thermosphere gradually transitions to the exosphere.Models suggest that Neptune's troposphere is banded by clouds of varying compositions depending on altitude. The upper level clouds occur at pressures below one bar, where the temperature is suitable for methane to condense. For pressures between one and five bars, clouds of ammonia and hydrogen sulfide are believed to form. Above a pressure of five bars, the clouds may consist of ammonia, ammonium sulfide, hydrogen sulfide and water. Deeper clouds of water ice should be found at pressures of about 50 bars, where the temperature reaches 0°C. Underneath, clouds of ammonia and hydrogen sulfide may be found.

High altitude clouds on Neptune have been observed casting shadows on the opaque cloud deck below. There are also high altitude cloud bands that wrap around the planet at constant latitude. These circumferential bands have widths of 50–150 km, and lie about 50–110 km above the cloud deck.

Neptune's spectra suggest that its lower stratosphere is hazy due to condensation of products of ultraviolet photolysis of methane, such as ethane and acetylene.The stratosphere is also home to trace amounts of carbon monoxide and hydrogen cyanide.The stratosphere of Neptune is warmer than that of Uranus due to elevated concentration of hydrocarbons.

For reasons that remain obscure, the planet's thermosphere is at an anomalously high temperature of about 750 K.The planet is too far from the Sun for this heat to be generated by ultraviolet radiation. One candidate for a heating mechanism is atmospheric interaction with ions in the planet's magnetic field. Other candidates are gravity waves from the interior that dissipate in the atmosphere. The thermosphere contains traces of carbon dioxide and water, which may have been deposited from external sources such as meteorites and dust.

Composition and structure of Neptune

The internal structure of Neptune:
1. Upper atmosphere, top clouds.
2. Atmosphere consisting of hydrogen, helium, and methane gas.
3. Mantle consisting of water, ammonia, and methane ices.
4. Core consisting of rock and ice.

With a mass of 1.0243×10²⁶ kg,Neptune is an intermediate body between Earth and the larger gas giants: its mass is seventeen times that of the Earth but just 1/19th that of Jupiter.Neptune's equatorial radius of 24,764 km is nearly four times that of the Earth. Neptune and Uranus are often considered a sub-class of gas giant termed "ice giants", due to their smaller size and higher concentrations of volatiles relative to Jupiter and Saturn.In the search for extrasolar planets Neptune has been used as a metonym: discovered bodies of similar mass are often referred to as "Neptunes",just as astronomers refer to various extra-solar "Jupiters."

Internal structure of Neptune:

Neptune's internal structure resembles that of Uranus. Its atmosphere forms about 5 to 10 percent of its mass and extends perhaps 10 to 20 percent of the way towards the core, where it reaches pressures of about 10 GPa. Increasing concentrations of methane, ammonia, and water are found in the lower regions of the atmosphere.Gradually this darker and hotter region condenses into a superheated liquid mantle, where temperatures reach 2,000 K to 5,000 K. The mantle is equivalent to 10 to 15 Earth masses, and is rich in water, ammonia, methane, and other compounds.As is customary in planetary science, this mixture is referred to as icy even though it is a hot, highly dense fluid. This fluid, which has a high electrical conductivity, is sometimes called a water-ammonia ocean. At a depth of 7,000 km, the conditions may be such that methane decomposes into diamond crystals that then precipitate toward the core.

The core of Neptune is composed of iron, nickel and silicates, with an interior model giving a mass about 1.2 times that of the Earth. The pressure at the centre is 7 Mbar, millions of times more than that on the surface of the Earth, and the temperature may be 5,400 K.

Neptune:An Introduction

Neptune from Voyager 2

Neptune is the eighth and farthest planet from the Sun in the Solar System. It is the fourth largest planet by diameter, and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 Earth masses and less dense.The planet is named after the Roman god of the sea. Its astronomical symbol is Astronomical symbol for Neptune., a stylized version of the god Neptune's trident.

Discovered on September 23, 1846,Neptune was the first planet found by mathematical prediction rather than regular observation. Unexpected changes in the orbit of Uranus led astronomers to deduce the gravitational perturbation of an unknown planet. Neptune was found within a degree of the predicted position. The moon Triton was found shortly thereafter, but none of the planet's other 12 moons were discovered before the 20th century. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.

Neptune is similar in composition to Uranus, and both have different compositions from those of the larger gas giants Jupiter and Saturn. As such, astronomers sometimes place them in a separate category, the "ice giants". Neptune's atmosphere, while similar to Jupiter's and Saturn's in being composed primarily of hydrogen and helium, contains a higher proportion of "ices" such as water, ammonia and methane, along with the usual traces of hydrocarbons and possibly nitrogen.In contrast the interior of Neptune is mainly composed of ices and rocks like that of Uranus.Traces of methane in the outermost regions, in part, account for the planet's blue appearance.

Neptune has the strongest winds of any planet in the solar system, measured as high as 2100 km/h.At the time of the 1989 Voyager 2 flyby, its southern hemisphere possessed a Great Dark Spot comparable to the Great Red Spot on Jupiter. Neptune's temperature at its cloud tops is usually close to −218 °C (55.1 K), one of the coldest in the solar system, due to its great distance from the Sun. The temperature in Neptune's centre is about 7000 °C, which is comparable to the Sun's surface and similar to most other known planets. Neptune has a faint and fragmented ring system, which may have been detected during the 1960s but was only indisputably confirmed by Voyager 2.

Neptune Linked to Potential Swarm of Asteroids

Neptune trojans with plutinos for reference.

15 June 2006

Astronomers have detected three new rocky bodies which share the same orbit as Neptune as it travels around the Sun.

The finding, detailed in the June 16 issue of the journal Science, brings the total number of the gas giant's asteroid companions, or "Trojans," up to four.

The highly inclined orbit of one of them supports the hypothesis that the Neptune Trojans were captured from a much larger asteroid "cloud" that surrounds the planet, and that they are not the broken remains of some larger object as some scientists have speculated.

The Trojans gather around one of Neptune's two so-called "Lagrangian" points. In these regions--located 60 degrees in front of and behind the planet in its orbit--the Sun and Neptune's gravity combine to ensnare passing objects.

The orbit of one of the new Trojans is tilted about 25 degrees relative to the plane that Neptune orbits the Sun, compared to only about 5 degrees for the other three Trojans.

The way the survey was set up, it was very unlikely that such a highly inclined object would be detected. The fact that it was indicates that there are at least as many--and possibly more--highly inclined Trojans existing far from the solar system plane compared to low inclination ones, said study team member Chadwick Trujillo of the Gemini Observatory Hawaii.

"The Neptune Trojans are a thick 'swarm,' not a thin population confined to the plane," Trujillo told.

Neptune May Have Thousands of Escorts

A schematic of the outer solar system in which the "Trojan" asteroids can be seen sharing the orbits of Jupiter and Neptune. At either of two points 60 degrees away from each planet, the gravitational forces of the planet and the Sun combine to lock the asteroids into a stable, synchronized orbit. Credit: Scott Sheppard

30 January 2007

Neptune may be escorted in its orbit by thousands of asteroid-like objects, perhaps more than exist in the entire asteroid belt.

So far, five of these enigmatic bodies, known as Trojans, have been found at one of Neptune's Lagrange points. These are places where the gravity of a planet and that of the Sun interact to create an area of gravitationally stability.

Jupiter's Lagrange regions are home to legions of Trojans, and around 2,000 cluster at these gravity graves along Jupiter's orbit 60 degrees ahead and 60 degrees behind the gas giant.The first Neptune Trojan was discovered in 2001 as part of the NASA funded Deep Ecliptic Survey at the Lagrange region 60 degrees and 3.1 billion miles (5 billion kilometers) ahead of Neptune.


Lagrange points exist in any two-body system.

A further three Neptune Trojans between 37 and 87 miles (60 and 140 kilometers) in diameter and shaded a pale red color have since been identified by Scott Sheppard of the Carnegie Institution of Washington and Chadwick Trujillo of the Gemini Observatory in Hawaiiusing the 6.5-meter Magellan telescope in Chile.

Despite their diminutive size and brightness, the Neptune Trojans quickly betrayed their existence by their distinct motion against background stars. The most recent Trojan discovered by Sheppard and Trujillo is moving at an unusual inclination of 25 degrees relative to the plane of the solar system (the ecliptic).

"The sky we covered searching for Neptune Trojans was all within 1.5 degrees of the ecliptic," Sheppard said. "High inclination objects will spend the majority of their time off the ecliptic. Thus detecting a high inclination Trojan in our survey suggests there is a large population of such objects. In fact, the high inclination objects appear to outnumber the low inclination objects by a ratio of four to one."

If so, there would be swarms of Trojans accompanying Neptune, perhaps up to twenty times more than at Jupiter. The sheer number of Trojans Neptune is thought to harbor reveal that these objects are an established part of Neptune's entourage, dating back to shortly after the planet's formation.

"Neptune cannot currently efficiently capture Trojans for long periods of time," Sheppard said. "Just after the planet formation epoch Neptune's orbit was likely much more eccentric due to its interactions with the other planets. Neptune's interactions with the myriad small bodies around its orbit which included comets, Kuiper Belt objects and other debris which formed nearby would have slowly circularized Neptune's orbit."

This process would have trapped many diverse objects at the Neptune Lagrange points irrespective of their inclination. This diversity is exciting as in 2014 we may get the opportunity to see a Trojan up close courtesy of the New Horizons spacecraft currently en-route to Pluto.

"If a Neptune Trojan could be found which the New Horizons spacecraft could image it would be one of the highlights of the mission," Sheppard said. "The Neptune Trojans are very faint and thus hard to observe from our location on Earth. Thus little is known about their surface properties or composition. The Neptune Trojans may be a unique type of solar system object of which no other stable reservoir currently exists."

At present this flyby of a Neptune Trojan is far from a certainty due to the fact that New Horizons will pass 60 degrees behind Neptune through the trailing Lagrange region known as "L5" where no Trojans have yet been identified.

"We are attempting to discover possible Neptune L5 Trojans, but because of the high background star confusion it will be a tough task for the next several years," Sheppard said. "The constraints on the New Horizons spacecraft are also severe since it will not be able to maneuver too far from its current trajectory. Thus, there is only a very low probability that a Neptune Trojan happens to be in a favorable location for the spacecraft to encounter."

Neptune Might Have Captured Triton

An artist's conception of Triton and its binary companion as they approach Neptune. This encounter facilitated Triton’s capture to an inclined retrograde orbit around Neptune, an event that catastrophically altered the Neptune satellite system. Credit: Craig Agnor

10 May 2006

Neptune's largest moon, Triton, was originally a member of a duo orbiting the Sun but was kidnapped during a close encounter with Neptune, a new model suggests.

Triton is unique among large moons in that it orbits Neptune in a direction opposite to the planet's rotation, which long ago led scientists to speculate that the moon originally orbited the Sun. But until now, no convincing theory for how Triton paired with Neptune existed.

Gravity might have pulled Triton away from its companion to make it an orbiting satellite of Neptune, researchers report in a new study published in the May 11 issue of journal Nature.

"We've found a likely solution to the long-standing problem of how Triton arrived in its peculiar orbit," said Craig Agnor, a researcher from the University of California, Santa Cruz. "In addition, this mechanism introduces a new pathway for the capture of satellites by planets that may be relevant to other objects in the solar system."

The new model predicts that Triton came from a binary setup much like Pluto and its moon, Charon.

"It's not so much that Charon orbits Pluto, but rather both move around their mutual center of mass, which lies between the two objects," Agnor said.

Binary systems can be pulled apart by gravitation when they encounter large planets like Neptune. The orbital motion of the binary system causes one member to move slower than the other, which can disrupt the system and permanently change the orbital companion.

This mechanism, known as an exchange reaction, could have delivered Triton to any of a variety of different orbits around Neptune, Agnor said.

The Neptune effect

photo:This picture of Neptune was produced from the last whole-planet images taken through the green and orange filters on the Voyager 2 narrow-angle camera. The images were taken at a range of 4.4 million miles from the planet, 4 days and 20 hours before closest approach. The picture shows the Great Dark Spot and its companion bright smudge near the center of the image. On the west limb the fast moving bright feature called Scooter and the little dark spot are visible. These clouds persisted as long as Voyager's cameras could resolve them. NASA / JPL

December 8, 2003

Lying far beyond Neptune's orbit and harboring billions of icy objects, the Kuiper Belt is the final frontier of our solar system. Believed to be the frozen, 3.5-billion-year-old remains of the early protoplanetary disk surrounding the Sun, this far-flung planetoid population continually puzzles astronomers regarding its true origin. Now, more than ten years after its initial discovery, the Kuiper Belt finally is revealing tantalizing clues to its ancient heritage. A new study indicates this ring of icy debris actually formed much closer to the Sun and was steadily pushed outward by the planet Neptune.

So far, astronomers have cataloged more than 900 bodies in the Kuiper Belt, some more than 620 miles (1,000 kilometers) in diameter. They believe larger Kuiper Belt objects, like planets, formed from smaller, colliding bits and pieces that gravitationally stuck together.

photo:The Edgeworth-Kuiper Belt is named for British astronomer Kenneth Edgeworth and Dutch-American astronomer Gerard Kuiper. (Many refer to it as simply the Kuiper Belt.)

According to this model, however, for the Kuiper Belt to have formed in its present position, it would have to hold more than ten times the amount of material that makes up Earth. Curiously, all telescope surveys indicate there is a hundred times less material currently contained in the Kuiper Belt. This has left scientists scratching their heads, trying to come up with theories to explain this "missing mass."Using computer models and simulations, Harold Levison of Colorado's Southwest Research Institute and Alessandro Morbidelli of France's Observatoire de la Côte d'Azur examined possible evolutionary scenarios for the Kuiper Belt and suggested a promising answer that could account for the observed lack of material there.

Instead of figuring out how the Kuiper Belt lost more than 99% of its original material, the two astronomers argue that none of it is missing — the Kuiper Belt simply spread out. Originally, they suggest, there were no objects beyond Neptune's present position around 30 astronomical units (AU). But material now in the Kuiper belt migrated out to 50 AU during the final stages of planet formation.

"We really didn't solve the mass depletion problem, we circumvented it," explains Levison. "According to our work, the void beyond Neptune was probably devoid of objects."Astronomers believe Neptune originally formed about 20 AU from the Sun and drifted outward to its stable orbit, where it resides today, by transferring energies from gravitational interactions during close encounters with large objects. Levison and Morbidelli's model proposes that many of these objects ended up being ejected to the outer solar system, forming the Kuiper Belt.

photo:The orbits of three newly discovered Kuiper Belt

"One of the puzzling aspects of Neptune's migration is why it stopped where it did," says Morbidelli. "Our new model explains this well. Neptune migrated until it hit the edge of the protoplanetary disk, at which point it abruptly stopped."

Reporting their findings in the November 27 publication of Nature, the authors state that while their theory explains many of the observable characteristics of the outer solar system, it also presents profound implications for understanding how the solar system developed as a whole.