Phoebe ( ) is an
irregular satellite of Saturn with a mean diameter of . It was discovered by
William Henry Pickering on March 18, 1899
from photographic plates that had been taken starting on 16 August 1898 at the
Boyden Station
Boyden Observatory is an astronomical research observatory and science education centre located in Maselspoort, north-east of the city of Bloemfontein in Free State, South Africa. The observatory is managed by the Physics Department of the U ...
of the Carmen Alto Observatory near
Arequipa
Arequipa (; Aymara and qu, Ariqipa) is a city and capital of province and the eponymous department of Peru. It is the seat of the Constitutional Court of Peru and often dubbed the "legal capital of Peru". It is the second most populated city ...
,
Peru
, image_flag = Flag of Peru.svg
, image_coat = Escudo nacional del Perú.svg
, other_symbol = Great Seal of the State
, other_symbol_type = National seal
, national_motto = "Firm and Happy f ...
, by
DeLisle Stewart
DeLisle Stewart (March 16, 1870 – February 2, 1941) 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 th ...
. It was the first satellite to be discovered photographically.
Phoebe was the first target encountered upon the arrival of the ''
Cassini'' spacecraft in the Saturn system in 2004, and is thus unusually well-studied for an
irregular satellite of its size. ''Cassinis trajectory to Saturn and time of arrival were specifically chosen to permit this flyby.
After the encounter and its insertion into orbit, ''Cassini'' did not go much beyond the orbit of
Iapetus
In Greek mythology, Iapetus (; ; grc, Ἰαπετός, Iapetós), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas (mythology), Atlas, Prometheus, Epimetheus (mythology), Epimetheus, and Menoetius (mythology), Menoetius. ...
.
Phoebe is roughly spherical and has a differentiated interior. It was spherical and hot early in its history and was battered out of roundness by repeated impacts. It is believed to be a captured
centaur
A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse.
Centaurs are thought of in many Greek myths as bein ...
that originated in the
Kuiper belt
The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 tim ...
.
Phoebe is the second largest retrograde satellite in the Solar System after
Triton
Triton commonly refers to:
* Triton (mythology), a Greek god
* Triton (moon), a satellite of Neptune
Triton may also refer to:
Biology
* Triton cockatoo, a parrot
* Triton (gastropod), a group of sea snails
* ''Triton'', a synonym of ''Triturus'' ...
.
History
Discovery
Phoebe was discovered by
William Henry Pickering on 17 March 1899
from photographic plates that had been taken starting on 16 August 1898 at the
Boyden Observatory
Boyden Observatory is an astronomical research observatory and science education centre located in Maselspoort, north-east of the city of Bloemfontein in Free State, South Africa. The observatory is managed by the Physics Department of the U ...
near
Arequipa
Arequipa (; Aymara and qu, Ariqipa) is a city and capital of province and the eponymous department of Peru. It is the seat of the Constitutional Court of Peru and often dubbed the "legal capital of Peru". It is the second most populated city ...
,
Peru
, image_flag = Flag of Peru.svg
, image_coat = Escudo nacional del Perú.svg
, other_symbol = Great Seal of the State
, other_symbol_type = National seal
, national_motto = "Firm and Happy f ...
, by
DeLisle Stewart
DeLisle Stewart (March 16, 1870 – February 2, 1941) 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 th ...
.
It was the first satellite to be discovered photographically.
Naming
Phoebe was named after
Phoebe, a
Titaness
In Greek mythology, the Titans ( grc, οἱ Τῑτᾶνες, ''hoi Tītânes'', , ''ho Tītân'') were the pre-Olympian gods. According to the ''Theogony'' of Hesiod, they were the twelve children of the primordial parents Uranus (mythology), ...
in
Greek mythology
A major branch of classical mythology, Greek mythology is the body of myths originally told by the ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the origin and nature of the world, the lives and activities o ...
that was associated with the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
.
It is also designated Saturn IX in some scientific literature. The
IAU nomenclature standards have stated that features on Phoebe are to be named after characters in the Greek myth of
Jason and the Argonauts. In
2005
File:2005 Events Collage V2.png, From top left, clockwise: Hurricane Katrina in the Gulf of Mexico; the Funeral of Pope John Paul II is held in Vatican City; " Me at the zoo", the first video ever to be uploaded to YouTube; Eris was discover ...
, 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).
Toby Owen of the
University of Hawaii at Manoa, chairman of the
International Astronomical Union
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
Outer Solar System Task Group said:
Orbit
Phoebe's orbit is
retrograde; that is, it orbits Saturn opposite to Saturn's rotation. 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
In Greek mythology, Iapetus (; ; grc, Ἰαπετός, Iapetós), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas (mythology), Atlas, Prometheus, Epimetheus (mythology), Epimetheus, and Menoetius (mythology), Menoetius. ...
), and is substantially larger than any of the other moons orbiting planets at comparable distances.
All of Saturn's
regular moons except Iapetus
orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...
very nearly in the plane of Saturn's equator. The outer
irregular satellites follow moderately to highly
eccentric
Eccentricity or eccentric may refer to:
* Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal"
Mathematics, science and technology Mathematics
* Off-center, in geometry
* Eccentricity (graph theory) of a v ...
orbits, and none are expected to rotate synchronously as all the inner moons of Saturn do (except for
Hyperion). See
Saturn's satellites' families.
Phoebe ring
The Phoebe ring is one of the
rings of 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 40 times as thick as the diameter of the planet. Because the ring's particles are presumed to have originated from
micrometeoroid
A micrometeoroid is a tiny meteoroid: a small particle of rock in space, usually weighing less than a gram. A micrometeorite is such a particle that survives passage through Earth's atmosphere and reaches Earth's surface.
The term "micrometeor ...
impacts on Phoebe, they should share its
retrograde orbit,
which is opposite to the orbital motion of the next inner moon,
Iapetus
In Greek mythology, Iapetus (; ; grc, Ἰαπετός, Iapetós), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas (mythology), Atlas, Prometheus, Epimetheus (mythology), Epimetheus, and Menoetius (mythology), Menoetius. ...
. Inwardly migrating ring material would thus strike Iapetus's leading hemisphere, contributing to its
two-tone coloration.
Although very large, the ring is virtually invisible—it was discovered using
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeedin ...
's
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...
Spitzer Space Telescope
The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003. Operations ended on 30 January 2020. Spitzer was the third space telescope dedicated to infrared astronomy, ...
.
Material displaced from Phoebe's surface by microscopic meteor impacts may be responsible for the dark areas on the surface of
Hyperion.
[The composition implied by spectra does not seem to support the earlier suggestion that Phoebe could be the source of the dark material deposited on ]Iapetus
In Greek mythology, Iapetus (; ; grc, Ἰαπετός, Iapetós), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas (mythology), Atlas, Prometheus, Epimetheus (mythology), Epimetheus, and Menoetius (mythology), Menoetius. ...
. Debris from the biggest impacts may be the origin of the other moons of Phoebe's group (the
Norse group)—all of which are less than 10 km in diameter.
Physical characteristics
Phoebe is roughly spherical and has a diameter of
[ (), approximately one-sixteenth that of the ]Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
. It is Saturn's ninth-largest moon, but it might be the eighth-most massive. Hyperion, another one of Saturn's moons, has a larger radius, but is less massive than Phoebe. Phoebe rotates every nine hours and 16 minutes, and completes a full orbit around Saturn in about 18 months. Its surface temperature is on average .
Most of Saturn's inner moons have very bright surfaces, but Phoebe's albedo
Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...
is very low (0.06), as dark as lampblack. The Phoebean surface is 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
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...
, as it resembled the common class of dark carbonaceous asteroids. These are chemically very primitive and are thought to be composed of original solids that condensed out of the solar nebula
The formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened int ...
with little modification since then.
However, images from '' Cassini'' 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 thick. In addition, quantities of carbon dioxide have been detected on the surface, a finding that has never been replicated for 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 think that Phoebe is in fact a captured centaur
A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse.
Centaurs are thought of in many Greek myths as bein ...
, one of a number of icy planetoid
According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
s from the Kuiper belt
The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 tim ...
that orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...
the Sun between Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousand ...
and Neptune
Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 time ...
. Phoebe is the first such object to be imaged as anything other than a dot.
Despite its small size, Phoebe is thought to have been a spherical body early in its history, with a differentiated interior, before solidifying and being battered into its current, slightly non-equilibrium shape.
Named features
Apart from one regio named after Phoebe's daughter, Leto, all named features are craters named after characters from the Greek legend of Jason and the Argonauts
The Argonauts (; Ancient Greek: ) were a band of heroes in Greek mythology, who in the years before the Trojan War (around 1300 BC) accompanied Jason to Colchis in his quest to find the Golden Fleece. Their name comes from their ship, ''Argo ...
.
Maps
Phoebe 2005 Mercator PIA07795.jpg, Map of Phoebe's middle latitudes. The higher latitudes have been clipped from the main map, but can be seen in the polar projections.
Phoebe 2005 south polar projection PIA07797.jpg, Map of Phoebe's south polar region
Phoebe north polar region PIA 07796.png, Map of Phoebe's north polar region
Phoebe contour map PIA15507.jpg, 3D map showing Phoebe's once spherical shape
Formation
Phoebe formed in the Kuiper belt
The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 tim ...
within three million years after the origin of the Solar System
The formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a ...
. This was early enough that sufficient radioactive material was available to melt it into a sphere and stay warm enough to have liquid water for tens of millions of years.[
]
Observation and exploration
Unlike Saturn's other moons, Phoebe was not favorably placed for the ''Voyager'' probes. ''Voyager 2
''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, '' Voyager 1'', ...
'' observed Phoebe for a few hours in September 1981. In the images, taken from a distance of 2.2 million kilometres at low phase angle, the size of Phoebe was approximately 11 pixels and showed bright spots on the otherwise dark surface.
'' Cassini'' passed from Phoebe on 11 June 2004, returning many high-resolution images, which revealed a scarred surface. Because ''Voyager 2'' had not been able to produce any high quality images of Phoebe, obtaining them was a priority for the ''Cassini'' mission and its flight path was deliberately designed to take it close by; otherwise, ''Cassini'' would likely not have returned images much better than ''Voyagers. Because of Phoebe's short rotation period of approximately 9 hours, 17 minutes, ''Cassini'' was able to map virtually the entire surface of Phoebe. The close fly-by enabled the mass of Phoebe to be determined with an uncertainty of only 1 in 500.[Roth et al., AAS Paper 05-311]
See also
* Moons of Saturn
Notes
References
External links
Phoebe Profile
a
NASA's Solar System Exploration site
Cassini–Huygens Multimedia: Images: Moons: Small Moons
''Cassini Pass Reveals Moon Secrets''
BBC News, June 14, 2004
* Asaravala, A.
Wired, (May 4, 2005)
NASA: Natural Satellite Physical Parameters
Cassini images of Phoebe
Images of Phoebe at JPL's Planetary Photojournal
* Movie o
Phoebe's rotation
from the National Oceanic and Atmospheric Administration
Phoebe basemap
(December 2005) from Cassini images
Phoebe atlas (March 2006) from Cassini images
Phoebe nomenclature
from th
USGS planetary nomenclature page
Phoebe on T. Denk's Outer Saturnian moons websites
{{DEFAULTSORT:Phoebe (Moon)
Norse group
Moons of Saturn
Irregular satellites
18990317
Former dwarf planets
Moons with a retrograde orbit