Neptunes Rings and Moons

Each new ring and moon system discovered in the solar system has brought its own unexpected results, requiring planetary scientists to revise their theories of formation and maintenance of planetary systems. Neptune is no exception.While several of Neptune's rings are conventional dusty features, some of its rings contain dense arcs connected by tenuously thin segments. The theory of ring formation requires an even distribution of particles around the planet, and so some complex and not completely understood interactions with small moons may be required to maintain these strange ring arcs. Neptune's moons contain an outstanding member, as well: its huge moon Triton. Triton is a moon of extremes: It is one of the brightest objects in the solar system, with an albedo of about 0.7; it is one of the very few moons with an atmosphere and one of only four solar system objects to have an atmosphere consisting largely of nitrogen; it orbits Neptune in a retrograde sense, unheard of for a major moon; and it has the coldest surface of any solar system object. Despite its exceptional cold, it has active cryovol-canic geysers (geysers of ice and perhaps exotic liquids), joining only Io and the Earth as bodies known to be volcanically active currently.Triton, shown in the color insert on page C-5, with its seasonal heating, nitrogen atmosphere, and water content, is therefore a possible location for the development of life.

Rings

Neptune's rings were first observed from Earth-based instruments in July 1984.William Hubbard, a scientist at the University of Arizona, and

In Neptune's outermost ring, material mysteriously clumps into three dense arcs separated by thinner ring material. (nasa/jpl/Voyager2)

his colleagues André Brahic and Bruno Sicardy, L. Elicer, Françoise Roques, and Faith Vilas were observing the occultation of a bright star by Neptune.A star is said to be occulted by a planet if the planet passes directly in front of the star, such that the star's light passes through any atmosphere around the planet, or is briefly hidden by rings around the planet, before being completely obscured by the planet itself. Dips in the star's light intensity implied to the scientists that there was an object nine miles (15 km) in width and 60 miles (100 km) long next to Neptune. The star's light was observed to dip only on one side of the planet and not the other, and so the objects around Neptune were inferred to be the solar system's first arcs, or incomplete rings.

These discoveries were so important that NASA and the Jet Propulsion Laboratory reprogrammed the Voyager 2 mission specifically to look for the rings as it passed Neptune in 1989 (see figure below). Images from Voyager 2, such as those shown here, and more recently from the Hubble Space Telescope, show that most of the rings are complete and not arcs, but that they are narrow and dusty in areas and so will not reliably block starlight.They all seem to consist of ices with some silicates and carbonaceous materials.

In Neptune's outermost ring, material mysteriously clumps into three dense arcs separated by thinner ring material. (nasa/jpl/Voyager2)

NEPTUNE'S RINGS

Ring

Galle

Complete or arcs?

Arago unnamed

Le Verrier

Lassell complete complete complete complete complete

the bright outer edge of Lassell a ring of dust in the moon Galatea's orbit nine (15)

Adams five arcs

Five rings around Neptune have been named. In order, moving away from Neptune, are the rings Galle, Le Verrier, Lassell,Arago, an unnamed ring, and Adams, as listed in the table above. Adams and Le Verrier are the narrowest and brightest of the rings, and those most likely to show in images of Neptune. Lassell is a broad ring, 2,485 miles (4,000 km) in width. Arago is the narrow bright edge of the ring Lassell, though not as bright as Adams and Le Verrier. Galle is another broad ring, with a width of about 1,200 miles (2,000 km). Of these six rings, only three or four may be distinct, independent entities. The unnamed ring is simply a trail of dust in the orbit of the moon Galatea, and so may not be a proper ring in terms of its formation and maintenance. Arago is simply the bright outermost edge of the broad ring Lassell.

The outermost ring, Adams, does consist of five distinct arcs: Liberté, Equalité 1, Equalité 2, Fraternité, and Courage, with a thin ring of dust connecting them. Liberté, the two Equalité arcs, and Fraternité are the brightest of the arcs, and make up between them about 40 degrees of arc. They are thought to consist of clumps of material bigger than those making up the rest of the Adams ring. The dim and dusty portions of the Adams ring appear to be about 19 miles (30 km) (out of the plane of the ring), while the bright arc segments are 68 miles (110 km) in vertical extent.

The formation of rings almost requires that the particles become uniformly distributed along the entire ring. Particles at the inner edge of the ring orbit slightly faster than those in the outer portions of the ring because of their slightly closer proximity to Neptune. Ring particles are therefore moving past each other continually while orbiting. This action works to spread particles evenly around the ring.

Ring arcs, on the other hand, require some continuous outside influence to prevent their spreading out. Resonances (orbits in ratios of integers) may hold ring particles in specific portions of the ring. The moon Galatea has a 42:43 resonance with the Adams ring. The gravitational interactions between the moon and ring particles are magnified in specific positions when orbits are resonant, and so this resonance with Galatea may keep ring particles in arcs. More recent observations, however, indicate that the ring arcs are not in the correct position to be controlled by Galatea. In 1999 Christophe Dumas, from the Jet Propulsion Laboratory and the California Institute of Technology, and his colleagues looked at the rings using the Hubble Space Telescope, while another team led by Bruno Sicardy of the Observatory of Paris took images using the Canada-France-Hawaii telescope. Both teams concluded that Galatea's influence was insufficient to control the arcs. A second moon, perhaps as small as four miles (6 km) in diameter, could complete the effects required to maintain the arcs. Such a small moon would be very difficult to detect and so could be doing its work undiscovered.

Moons

Neptune has 13 known moons, six of which were discovered by Voyager 2, and five of which were discovered in the last two years using high-resolution observations. Only Triton and Nereid were known before the space age. Many of Neptune's moons are irregular and therefore are unlikely to have differentiated internally or to have experienced any geologic activity. Even Nereid is irregular and unlikely to have differentiated. The exception is Triton, a large moon with geologic activity and an atmosphere.

The four moons closest to Neptune orbit within the Adams ring. Galatea orbits very close to the Adams rings, creating its own faint ring of dust. Larissa orbits just outside the ring system, and Proteus beyond Larissa. Even Proteus is so close to Neptune that Earth-based observatories cannot resolve it. The five moons discovered in 2002 and 2003 are small, and little is known about them.

NEPTUNE'S MOONS

Moon

Orbital period (Earth days)

Moon's Radius (miles [km])

Year discovered

Orbital inclination

Orbital eccentricity

Orbital direction

1. Naiad

0.294

30 by 19 by 16 (48 by 30 by 26)

1989

4.74

0.000

prograde

2. Thalassa

0.311

34 by 31 by 16 (54 by 50 by 26)

1989

0.205

0.000

prograde

3. Despina

0.335

56 by 46 by 40 (90 by 74 by 64)

1989

0.065

0.000

prograde

4. Galatea

0.429

63 by 57 by 45 (102 by 92 by 72)

1989

0.054

0.000

prograde

5. Larissa

0.555

67 by 63 by 52 (108 by 102 by 84)

1981/1989 (see text)

0.201

0.001

prograde

6. Proteus

1.122

135 by 129 by 125 (218 by 208 by 201)

1989

0.039

0.000

prograde

7. Triton

5.88

841 (1,353)

1846

156.8

0.000

retrograde

8. Nereid

360.1

106 (170)

1949

7.23

0.751

prograde

9. S/2002 N1

1,874.8

19 (30)

2002

134.1

0.572

retrograde

10. S/2002 N2

2,925.6

12 (19)

2002

52.74

0.137

prograde

11. S/2002 N3

2,980.4

12 (19)

2002

39.56

0.416

prograde

12. S/2003 N1

9,136.1

12 (19)

2003

137.3

0.450

retrograde

13. S/2002 N4

9,007.1

19 (30)

2002

139.3

0.605

In Greek mythology, naiads are the water spirits who live in and govern springs, streams, and fountains.The moon Naiad is an irregular and relatively small satellite. Naiad's orbit is almost circular (eccentricity 0.0003) and lies close to Neptune's equator (orbital inclination 4.7 degrees). Naiad,Thalassa, Despina, Galatea, Larissa, and Proteus were all discovered by Voyager 2, with Naiad the last to be discovered.

2. Thalassa

Thalassa is the Greek word for "sea." In Greek mythology, Thalassa was the daughter of Ether, the god of the upper atmosphere, and his sister Hemera, the goddess of daylight, whose job it was to pull away the nighttime veils of the god Erebus. Thalassa, like Naiad, shows no sign of geological activity or alteration.Thalassa's orbit is even more perfectly circular than Naiad's (eccentricity 0.0002) and lies even closer to Neptune's equatorial plane (orbital inclination 0.21 degrees).

3. Despina

In Greek mythology, Despina is the daughter of Neptune and Demeter, the goddess of fertility. Despina is thought to be the inner shepherd moon to the Le Verrier ring. Like the moons interior to it, Despina shows no sign of geological activity or alteration. Its orbital eccentricity is 0.0001, and its orbital inclination is a mere 0.07 degrees.

4. Galatea

Galatea is named for a naiad in Greek mythology who lived on the island of Sicily and was the love interest of the cyclops Polyphemos, who imprisoned the hero Odysseus. Galatea is an inner shepherd to the Adams ring, with its curious arcs. Galatea was thought for a time to be responsible for the formation and maintenance of the ring arcs through gravitational interaction, but more thorough study has shown that one moon cannot be responsible alone.There may be a small, as-yet-unseen moon interacting with Galatea and the arcs. Galatea has a nearly circular orbit with very low inclination.

5. Larissa

In Greek mythology, Larissa was either the daughter or mother of Pelasgus, the founding citizen of the oldest group of inhabitants of Greece, the Pelasges, according to Herodotus. The existence of the moon Larissa was first implied by data from a stellar occultation experiment in 1981 led by Harold Reitsema, now a manager at Ball Aerospace. Larissa's existence was confirmed in images obtained in 1989 by Voyager 2. The slightly larger sizes of Larissa and Proteus (when compared to the inner moons) allows slightly better images to be taken. Like Proteus, Larissa is irregularly shaped and heavily cratered. Larissa orbits just outside the Adams ring, but it is not thought to influence the maintenance of Adams's arcs.

Larissa Moon

Neptune's moon Proteus was photographed on August 25, 1989, by the Voyager 2 spacecraft at a range of 90,000 miles (144,000 km). (nasa/Voyager 2/nssdc)

6. Proteus

Proteus is named after the mythological shape-shifting son of Poseidon, god of the sea, and Tethys, a titan and goddess of ocean fertility. (Proteus's family tree points out a fundamental inconsistency: Neptune is the god of the sea in Roman mythology, but all of Neptune's moons are named for figures in Greek mythology.) Proteus is Neptune's second-largest moon after Triton and is about as large as a solar system body can be without differentiating internally and obtaining a spherical shape. Its surface is exceptionally dark, reflecting only 6 percent of the sunlight that strikes it; Proteus is therefore one of the darkest solar system moons and difficult to see in images. Its surface is heavily cratered. The largest crater is Pharos, with a diameter of 155 miles (250 km), forming a nine-mile (15-km)-deep depression that covers most of the moon's southern hemisphere.This image of Proteus (above), Neptune's second-largest moon, was taken by Voyager 2 in 1989.

j. Triton

Triton is the largest of Neptune's moons, at 1,682 miles (2,706 km) in diameter, and is named for a Greek god of the sea, son of Poseidon

Neptune's moon Proteus was photographed on August 25, 1989, by the Voyager 2 spacecraft at a range of 90,000 miles (144,000 km). (nasa/Voyager 2/nssdc)

(Neptune in the Roman pantheon). William Lassell, a wealthy Liverpool brewer and owner of the largest telescope in Britain, discovered it in 1846, just a month after Neptune itself. Before the Voyager 2 mission, Triton was thought to be the largest moon in the solar system because it is so bright, with an albedo of 0.7. Triton's high albedo means that its surface reflects more than 10 times as much sunlight as does its dark neighbor Proteus. Its brightness was misleading, though, because Ganymede, a satellite of Jupiter, is actually the largest moon in the solar system.

Triton orbits in the opposite direction from the planet's direction, and as a result of tidal friction is slowly spiraling in toward the planet. Triton is the only large moon in the solar system to have a retrograde orbit. Triton's orbit is almost circular but highly inclined, at 157 degrees to the plane of Neptune's equator. Because of its retrograde, highly inclined orbit, the moon is not stable and will eventually crash into Neptune; it is almost certainly a captured asteroid. Its proposed compositional similarity to Pluto also suggests that Triton was once in independent orbit around the Sun in the Kuiper belt, as Pluto is. Some gravitational interaction altered its orbit and allowed it to pass close enough for capture by Neptune. Its capture would have created gravitational disturbances with Neptune's other moons, and may be responsible for Nereid's highly eccentric orbit. Modeling indicates that Triton will collide with Neptune in 10 million to 100 million years, forming rings that may be larger than Saturn's.

Triton's atmosphere is its most remarkable feature; only two other moons in the solar system have atmospheres (Jupiter's Io and Saturn's Titan). Triton's atmospheric pressure at its surface is about 0.000014 bars. Its atmosphere consists mainly of molecular nitrogen (N2) with about 1/100 of a percent of methane.Triton is one of only four solar system bodies that have a large nitrogen component in their atmos-pheres:The first is the Earth, the second is Saturn's moon Titan, and Pluto also has a sporadic, thin, nitrogen-based atmosphere.Though its atmosphere is exceptionally thin, it may still form thin clouds of nitrogen ices. Triton is the largest contributor of plasma to Neptune's magnetosphere, and nitrogen atoms from Triton are regularly added to Neptune's upper atmosphere.

Triton is also a moon of temperature extremes.Triton has the coldest known surface temperature of any planet or moon, about —390°F (—235°C). Only Triton's very low temperature allows it to retain its atmosphere.Triton's highly inclined orbit and Neptune's own obliquity cause Triton to experience complex and severe seasons, perhaps the most extreme of any solar system body.The moon also has bright polar caps, visible in this image (shown in the lower color insert on page C-5) of Triton's limb, consisting of condensed nitrogen and methane that wax and wane with the moon's seasons, much as do Mars's polar caps.

Triton is thought to consist of a mixture of ices and rocky material because its density is 131 pounds per cubic foot (2,066 kg/m3), midway between the densities of water at 63 pounds per cubic foot (1,000 kg/m3) and rock at about 190 pounds per cubic foot (~3,000 kg/m3). Voyager 2 spotted active geysers up to five miles (8 km) high and trailing away 60 miles (100 km). Triton therefore is volcanically active, spewing what is thought to be a mixture of liquid nitrogen, dust, and methane or ammonia from ice volcanoes. There are few fresh impact craters on Triton because its surface has been re-covered

This view of Neptune's moon Triton is about300 miles (500 km) across and encompasses two depressions, possibly old impact basins, that have been extensively modified by flooding, melting, faulting, and collapse. (nasa/jpl/Voyager2)

Canteloupe texture and faults on Triton are shown in this image taken by Voyager 2. The vertical linear feature in the image is probably a graben about 20 miles (35 km) wide. The ridge in the center of the graben may be ice that has welled up by plastic flow. The lack of craters in the surrounding terrain shows it is relatively young. (nasa/jpl/Voyager 2)

with fresh volcanic effusions (in a small part of the equatorial region the moon is heavily cratered).Triton's old craters have all been extensively modified by volcanic flooding and flow of the crust, until they have become just suggestions of craters, called palimpsests, as shown in the figure on page 91. Triton, Io, and the Earth are the only solar system bodies known to be volcanically active at the present time, though Venus and Mars may also be volcanically active.

Triton's surface has a unique "cantaloupe" texture, crosscut with ridges between its poles and equator (see figure above).This Voyager 2 image shows the cantaloupe texture and two large crosscutting ridges, thought to be formed by faults.The smallest detail that can be seen is about 1.5 miles (2.5 km) across. Features on Triton's surface can be used to infer conditions inside the planet. Some of the troughs and ridges on Triton are thought to be graben, a geologic feature created by extension of the crust. Pulling the moon's crust in extension caused faults to form that allow blocks of the crust to sink down rela-

tive to their surroundings, as shown in the figure below. This kind of feature is relatively common on Earth and earned its name, graben, from the German word for grave. The long linear feature extending vertically across the image of cantaloupe texture probably is a graben about 20 miles (35 km) across.The ridge in the center of the graben is probably ice that has welled up by plastic flow in the floor of the graben. The surrounding terrain is a relatively young icy surface with few impact craters.

Two nine-mile (15-km)-wide troughs on Triton, called the Raz Fossae, are also thought to form a graben (for more on words like fossae, see the sidebar "Fossa, Sulci, and Other Terms for Planetary Landforms" on page 96). Javier Ruiz, a scientist at the University of Madrid, has made an analysis ofTriton's lithosphere using the dynamics implied by the fossae. If the stiff outer shell of the moon, its lithosphere, consists mainly of water and ammonia ices, then the depth and width of the fossae imply that the moon's lithosphere is only about 12 miles (20 km) thick. Beneath this ice layer may lie a shallow liquid ocean, perhaps similar to the ocean on Europa that is thought to be a potential site for the development of life.

8. Nereid

Nereid is named for the nereids, a group of about 50 sea nymphs in Graben, long, lowareas

Greek mythology, the daughters of Nereus, a shape-changing sea god bounded by faults, are formed and oracle, and Doris, a titan. Nereid is, on average, about 15 times by crustal extension.

Fossa, Sulci, and Other Terms for Planetary Landforms

O n Earth the names for geological features often connote how they were formed and what they mean in terms of surface and planetary evolution. A caldera, for example, is a round depression formed by volcanic activity and generally encompassing volcanic vents. Though a round depression on another planet may remind a planetary geologist of a terrestrial caldera, it would be misleading to call that feature a caldera until its volcanic nature was proven. Images of other planets are not always clear and seldom include topography, so at times the details of the shape in question cannot be determined, making their definition even harder.

To avoid assigning causes to the shapes of landforms on other planets, scientists have resorted to creating a new series of names largely based on Latin, many of which are listed in the following table, that are used to describe planetary features. Some are used mainly on a single planet with unusual features, and others can be found throughout the solar system. Chaos terrain, for example, can be found on Mars, Mercury, and Jupiter's moon Europa. The Moon has a number of names for its exclusive use, including lacus, palus, rille, oceanus, and mare. New names for planetary objects must be submitted to and approved by the International Astronomical Union's (IAU) Working Group for Planetary System Nomenclature.

Feature astrum, astra catena, catenae chaos chasma, chasmata colles corona, coronae crater, craters dorsum, dorsa ffacula, faculae fluctus

Nomenclature for Planetary Features

Description radial-patterned features on Venus chains of craters distinctive area of broken terrain a deep, elongated, steep-sided valley or gorge small hills or knobs oval-shaped feature a circular depression not necessarily created by impact ridge bright spot flow terrain

Feature

Description

fossa, fossae

narrow, shallow, linear depression

labes

landslide

labyrinthus, labyrinthi

complex of intersecting valleys

lacus

small plain on the Moon; name means "lake"

lenticula, lenticulae

small dark spots on Europa (Latin for freckles); may

be domes or pits

linea, lineae

a dark or bright elongate marking, may be curved or straight

macula, maculae

dark spot, may be irregular

mare, maria

large circular plain on the Moon; name means "sea"

mensa, mensae

a flat-topped hill with cliff-like edges

mons, montes

mountain

oceanus

a very large dark plain on the Moon; name means "ocean"

palus, paludes

small plain on the Moon; name means "swamp"

patera, paterae

an irregular crater

planitia, planitiae

low plain

planum, plana

plateau or high plain

reticulum, reticula

reticular (netlike) pattern on Venus

rille

narrow valley

rima, rimae

fissure on the Moon

rupes

scarp

sinus

small rounded plain; name means "bay"

sulcus, sulci

subparallel furrows and ridges

terra, terrae

extensive land mass

tessera, tesserae

tile-like, polygonal terrain

tholus, tholi

small dome-shaped mountain or hill

undae

dunes

vallis, valles

valley

vastitas, vastitates

extensive plain

(continues)

Fossa, Sulci, and Other Terms for Planetary Landforms

(continued)

The IAU has designated categories of names from which to choose for each planetary body, and in some cases, for each type of feature on a given planetary body. On Mercury, craters are named for famous deceased artists of various stripes, while rupes are named for scientific expeditions. On Venus, craters larger than 12.4 miles (20 km) are named for famous women, and those smaller than 12.4 miles (20 km) are given common female first names. Colles are named for sea goddesses, dorsa are named for sky goddesses, fossae are named for goddesses of war, and fluctus are named for miscellaneous goddesses.

The gas giant planets do not have features permanent enough to merit a nomenclature of features, but some of their solid moons do. Io's features are named after characters from Dante's Inferno. Europa's features are named after characters from Celtic myth. Guidelines can become even more explicit: Features on the moon Mimas are named after people and places from Malory's LeMorte d'Arthur legends, Baines translation. A number of asteroids also have naming guidelines. Features on 253 Mathilde, for example, are named after the coalfields and basins of Earth.

farther from Neptune than is Triton. Nereid takes 360 Earth days to make an orbit of Neptune, similar to the length of Earth's year. By comparison, Triton takes less than six Earth days to orbit Neptune. Nereid's highly eccentric orbit takes the moon from 834,210 miles (1,342,530 km) to 6,006,870 miles (9,667,120 km) away from Neptune.This is the most eccentric orbit of any moon in the solar system, with an eccentricity of 0.753. Nereid may be a captured asteroid, with its irregular shape and a highly eccentric orbit, or its orbit may have been disturbed when Neptune captured Triton.

The outermost five of Neptune's moons are newly discovered, small, irregular bodies about which little is known. In coming years they will be better imaged, no doubt, and studied, and still more remote and smaller moons will be discovered. The outermost moons now known orbit at three to 10 times the distance of Nereid to Neptune. Some orbit in a prograde sense, and some in a retrograde sense.They are likely to be captured asteroids, as are the outer moons of Jupiter and Saturn.

Part Two: Pluto and the Kuiper Belt

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