Home      Discussion      Topics      Dictionary      Almanac
Signup       Login
Phoebe (moon)

Phoebe (moon)

Overview
Phoebe is an irregular
Irregular satellite
In astronomy, an irregular moon is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They are believed to have been captured by their parent planet, unlike regular satellites, which form in situ....

 satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

. It was discovered by William Henry Pickering
William Henry Pickering
William Henry Pickering was an American astronomer, brother of Edward Charles Pickering. He was elected a Fellow of the American Academy of Arts and Sciences in 1883.-Work:...

 on 17 March 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Observatory
Boyden Observatory
Boyden Observatory is a South African astronomical research observatory and science education centre.-History:The observatory was originally founded in 1889 by the Harvard University at Mount Hardvard near Lima, Peru, but relocated to Arequipa, Peru in October 1890.Significant work done at Arequipa...

 near Arequipa
Arequipa
Arequipa is the capital city of the Arequipa Region in southern Peru. With a population of 836,859 it is the second most populous city of the country...

, Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....

, by DeLisle Stewart
DeLisle Stewart
DeLisle Stewart was an American astronomer.In 1896 he became a staff member of Harvard College Observatory, and from 1898 to 1901 he worked at that observatory's station at Arequipa, Peru, where he took the photographic plates that William Henry Pickering used to discover Saturn's moon Phoebe...

. It was the first satellite to be discovered photographically.

Phoebe was the first target encountered upon the arrival of Cassini–Huygens to the Saturn system in 2004, and is thus unusually well-studied for a natural satellite of its size.
Discussion
Ask a question about 'Phoebe (moon)'
Start a new discussion about 'Phoebe (moon)'
Answer questions from other users
Full Discussion Forum
 
Unanswered Questions
Encyclopedia
Phoebe is an irregular
Irregular satellite
In astronomy, an irregular moon is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They are believed to have been captured by their parent planet, unlike regular satellites, which form in situ....

 satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

. It was discovered by William Henry Pickering
William Henry Pickering
William Henry Pickering was an American astronomer, brother of Edward Charles Pickering. He was elected a Fellow of the American Academy of Arts and Sciences in 1883.-Work:...

 on 17 March 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Observatory
Boyden Observatory
Boyden Observatory is a South African astronomical research observatory and science education centre.-History:The observatory was originally founded in 1889 by the Harvard University at Mount Hardvard near Lima, Peru, but relocated to Arequipa, Peru in October 1890.Significant work done at Arequipa...

 near Arequipa
Arequipa
Arequipa is the capital city of the Arequipa Region in southern Peru. With a population of 836,859 it is the second most populous city of the country...

, Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....

, by DeLisle Stewart
DeLisle Stewart
DeLisle Stewart was an American astronomer.In 1896 he became a staff member of Harvard College Observatory, and from 1898 to 1901 he worked at that observatory's station at Arequipa, Peru, where he took the photographic plates that William Henry Pickering used to discover Saturn's moon Phoebe...

. It was the first satellite to be discovered photographically.

Phoebe was the first target encountered upon the arrival of Cassini–Huygens to the Saturn system in 2004, and is thus unusually well-studied for a natural satellite of its size. Cassini's trajectory to Saturn and time of arrival were specifically chosen to permit this flyby. After the encounter and its insertion into orbit, Cassini would not go much beyond the orbit of Iapetus
Iapetus (moon)
Iapetus ), occasionally Japetus , is the third-largest moon of Saturn, and eleventh in the Solar System. It was discovered by Giovanni Domenico Cassini in 1671...

.

Name


The moon is named after Phoebe
Phoebe (mythology)
In Greek mythology "radiant" Phoebe , was one of the original Titans, who were one set of sons and daughters of Uranus and Gaia. She was traditionally associated with the moon , as in Michael Drayton's Endimion and Phœbe, , the first extended treatment of the Endymion myth in English...

, a Titan
Titan (mythology)
In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age....

 in Greek mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

. It is also designated Saturn IX. The IAU
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 nomenclature standards
Planetary nomenclature
Planetary nomenclature, like terrestrial nomenclature, is a system of uniquely identifying features on the surface of a planet or natural satellite so that the features can be easily located, described, and discussed. The task of assigning official names to features is taken up by the International...

 have stated that features on Phoebe are to be named after characters in the Greek myth of Jason and the Argonauts
Jason
Jason was a late ancient Greek mythological hero from the late 10th Century BC, famous as the leader of the Argonauts and their quest for the Golden Fleece. He was the son of Aeson, the rightful king of Iolcus...

. In 2005
2005 in science
The year 2005 in science and technology involved some significant events.-Astronomy:* April 8 – Total solar eclipse*February 23 – Astronomers announce the discovery of a galaxy, VIRGOHI21, that consists almost entirely of dark matter...

, the IAU officially named 24 craters (Acastus, Admetus, Amphion, Butes, Calais, Canthus, Clytius, Erginus, Euphemus, Eurydamas, Eurytion, Eurytus, Hylas, Idmon, Iphitus, Jason, Mopsus, Nauplius, Oileus, Peleus, Phlias, Talaus, Telamon, and Zetes).

Dr. Toby Owen of the University of Hawaii
University of Hawaii
The University of Hawaii System, formally the University of Hawaii and popularly known as UH, is a public, co-educational college and university system that confers associate, bachelor, master, and doctoral degrees through three university campuses, seven community college campuses, an employment...

 at Manoa, chairman of the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 Outer Solar System Task Group said
"We picked the legend of the Argonauts for Phoebe as it has some resonance with the exploration of the Saturn system by Cassini–Huygens. We can't say that our participating scientists include heroes like Hercules and Atalanta, but they do represent a wide, international spectrum of outstanding people who were willing to take the risk of joining this voyage to a distant realm in hopes of bringing back a grand prize."

Orbital characteristics


For more than 100 years, Phoebe was Saturn's outermost known moon, until the discovery of several smaller moons in 2000. Phoebe is almost 4 times more distant from Saturn than its nearest major neighbor (Iapetus
Iapetus (moon)
Iapetus ), occasionally Japetus , is the third-largest moon of Saturn, and eleventh in the Solar System. It was discovered by Giovanni Domenico Cassini in 1671...

), and is substantially larger than any of the other moons orbiting planets at comparable distances.

All of Saturn's moons up to Iapetus orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

 very nearly in the plane of Saturn's equator. The outer irregular satellite
Irregular satellite
In astronomy, an irregular moon is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They are believed to have been captured by their parent planet, unlike regular satellites, which form in situ....

s follow fairly to highly eccentric orbits, and none is expected to rotate synchronously as all the inner moons of Saturn do (except for Hyperion
Hyperion (moon)
Hyperion , also known as Saturn VII, is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. It is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance...

). See Saturn's satellites families.

Physical characteristics



Phoebe is roughly spherical and has a diameter of 220 kilometres (136.7 mi), which is equal to about one-fifteenth of the diameter of Earth's Moon. Phoebe rotates on its axis every nine hours and it completes a full orbit around Saturn in about 18 months. Its surface temperature is 75 K (-198°C).

Most of Saturn's inner moons have very bright surfaces, but Phoebe's albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

 is very low (0.06), as dark as lampblack. The Phoebean surface is extremely heavily scarred, with craters up to 80 kilometres across, one of which has walls 16 kilometres high.

Phoebe's dark coloring initially led to scientists surmising that it was a captured asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

, as it resembled the common class of dark carbonaceous asteroids
C-type asteroid
C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids, and an even higher percentage in the outer part of the asteroid belt beyond 2.7 AU, which is dominated by this asteroid type...

. These are chemically very primitive and are thought to be composed of original solids that condensed out of the solar nebula
Solar nebula
In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation...

 with little modification since then.

However, images from the Cassini–Huygens space probe indicate that Phoebe's craters show a considerable variation in brightness, which indicate the presence of large quantities of ice below a relatively thin blanket of dark surface deposits some 300 to 500 m (984.3 to 1,640.4 ft) thick. In addition, quantities of carbon dioxide have been detected on the surface, a finding which has never been replicated on an asteroid. It is estimated that Phoebe is about 50% rock, as opposed to the 35% or so that typifies Saturn's inner moons. For these reasons, scientists are coming to believe that Phoebe is in fact a captured centaur
Centaur (planetoid)
Centaurs are an unstable orbital class of minor planets that behave with characteristics of both asteroids and comets. They are named after the mythological race of beings, centaurs, which were a mixture of horse and human...

, one of a number of icy planetoids from the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

 that orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

 the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 between Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, 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 times...

. Phoebe is the first such object to be imaged as anything other than a dot.

Material displaced from Phoebe's surface by microscopic meteor impacts may be responsible for the dark surfaces of Hyperion
Hyperion (moon)
Hyperion , also known as Saturn VII, is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. It is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance...

. Debris from the biggest impacts may have been the building blocks of the other moons of Phoebe's group—all of which are less than 10 km in diameter.

Map





Spacecraft flybys


The Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

 spacecraft passed by Phoebe in September 1981, although the 2.2 Gm
Gigametre
A gigametre is a unit of length in the metric system, equal to one billion meters, the SI base unit of length, hence to 1,000,000 km or approximately 621,370 miles....

 (2.2 million kilometres) distance and low resolution meant that relatively little could be learned from the resulting images.

The Cassini spacecraft flew within 2068 kilometres (1,285 mi) of Phoebe on 11 June 2004, returning many high-resolution images of the moon and its scarred surface. Due to the mission plan, Phoebe happened to be in the best part of its orbit to be photographed by the incoming Cassini probe, which, due to Phoebe's distance from Saturn, otherwise would probably not have returned pictures much better than those that Voyager returned. In addition, due to its rapid rotation period of approximately 9 hours, 56 minutes, Cassini was able to map virtually the entire surface of Phoebe.

Phoebe ring


The Phoebe ring is one of the rings of Saturn
Rings of Saturn
The rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about Saturn...

. This ring is tilted 27 degrees from Saturn's equatorial plane (and the other rings). It extends from at least 128 to 207 times the radius of Saturn; Phoebe orbits the planet at an average distance of 215 Saturn radii. The ring is about 20 times as thick as the diameter of the planet. Since the ring's particles are presumed to have originated from micrometeoroid
Micrometeoroid
A micrometeoroid is a tiny meteoroid; a small particle of rock in space, usually weighing less than a gram. A micrometeor or micrometeorite is such a particle that enters the Earth's atmosphere or falls to Earth.-Scientific interest:...

 impacts on Phoebe, they should share its retrograde orbit, which is opposite to the orbital motion of the next inner moon, Iapetus
Iapetus (moon)
Iapetus ), occasionally Japetus , is the third-largest moon of Saturn, and eleventh in the Solar System. It was discovered by Giovanni Domenico Cassini in 1671...

. Inwardly migrating ring material would thus strike Iapetus's leading hemisphere, possibly causing the two-tone coloration of that moon. Although very large, the ring is virtually invisible—it was discovered using NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's infra-red Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...

.

External links