Amalthea (moon)

Amalthea (moon)

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Amalthea is the third moon
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 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,...

 in order of distance from the planet. It was discovered on September 9, 1892, by Edward Emerson Barnard and named after Amalthea
Amalthea (mythology)
In Greek mythology, Amalthea or Amaltheia is the most-frequently mentioned foster-mother of Zeus. Her name in Greek is clearly an epithet, signifying the presence of an earlier nurturing goddess, whom the Hellenes, whose myths we know, knew to be located in Crete, where Minoans may have called...

, a nymph
Nymph
A nymph in Greek mythology is a female minor nature deity typically associated with a particular location or landform. Different from gods, nymphs are generally regarded as divine spirits who animate nature, and are usually depicted as beautiful, young nubile maidens who love to dance and sing;...

 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 known as .

Amalthea is in a close orbit around Jupiter and is within the outer edge of the Amalthea Gossamer Ring, which is formed from dust ejected from its surface. From its surface, Jupiter would be an astonishing sight in its sky, appearing 46.5 degrees
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...

 in diameter. Amalthea is the largest of the inner satellites of Jupiter. Irregularly shaped and reddish in color, it is thought to consist of porous water ice with unknown amounts of other materials. Its surface features include large craters and high mountains.

Amalthea was photographed in 1979 and 1980 by the Voyager 1 and 2
Voyager program
The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...

spacecraft, and later, in more detail, by the Galileo orbiter
Galileo spacecraft
Galileo was an unmanned spacecraft sent by NASA to study the planet Jupiter and its moons. Named after the astronomer and Renaissance pioneer Galileo Galilei, it was launched on October 18, 1989 by the Space Shuttle Atlantis on the STS-34 mission...

 in the 1990s.

Discovery and naming



Amalthea was discovered on September 9, 1892, by Edward Emerson Barnard using the 36 inch (91 cm) refractor telescope
James Lick telescope
The James Lick Telescope is an antique refracting 36 inch telescope built in 1889 that can still be viewed through today...

 at Lick Observatory
Lick Observatory
The Lick Observatory is an astronomical observatory, owned and operated by the University of California. It is situated on the summit of Mount Hamilton, in the Diablo Range just east of San Jose, California, USA...

. It was the last planetary satellite to be discovered by direct visual observation (as opposed to photographically) and was the first new satellite of Jupiter since Galileo Galilei
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

's discovery of the Galilean satellites in 1610.

The satellite is named after the nymph
Nymph
A nymph in Greek mythology is a female minor nature deity typically associated with a particular location or landform. Different from gods, nymphs are generally regarded as divine spirits who animate nature, and are usually depicted as beautiful, young nubile maidens who love to dance and sing;...

 Amalthea
Amalthea (mythology)
In Greek mythology, Amalthea or Amaltheia is the most-frequently mentioned foster-mother of Zeus. Her name in Greek is clearly an epithet, signifying the presence of an earlier nurturing goddess, whom the Hellenes, whose myths we know, knew to be located in Crete, where Minoans may have called...

 from 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...

 who nursed the infant Zeus
Zeus
In the ancient Greek religion, Zeus was the "Father of Gods and men" who ruled the Olympians of Mount Olympus as a father ruled the family. He was the god of sky and thunder in Greek mythology. His Roman counterpart is Jupiter and his Etruscan counterpart is Tinia.Zeus was the child of Cronus...

 (the Greek equivalent of Jupiter) with goat's milk. Its Roman numeral designation is . The name "Amalthea" was not formally adopted by 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...

 until 1975, although it had been in informal use for many decades. The name was initially suggested by Camille Flammarion
Camille Flammarion
Nicolas Camille Flammarion was a French astronomer and author. He was a prolific author of more than fifty titles, including popular science works about astronomy, several notable early science fiction novels, and several works about Spiritism and related topics. He also published the magazine...

. Before 1975 Amalthea was most commonly known simply as . The adjectival form of the name is Amalthean.

Orbit


Amalthea circles 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,...

 at a distance of 181 000 km (2.54 Jupiter radii). The orbit of Amalthea has an eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 of 0.003 and an inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...

 of 0.37° relative to the equator of Jupiter. Such appreciably nonzero values of inclination and eccentricity, though still small, are unusual for an inner satellite
Inner satellite
In astronomy, an inner moon is a natural satellite following a prograde, low inclination orbit inwards of the large satellites of the parent planet. They are generally thought to have been formed in situ at the same time as the coalescence of the original planet...

 and can be explained by the influence of the innermost Galilean satellite, Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

: in the past Amalthea will have passed through several mean motion resonances
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

 with Io that will have excited its inclination and eccentricity (in a mean motion resonance the ratio of orbital periods of two bodies is a rational number like m:n).

Amalthea's orbit lies near the outer edge of the Amalthea Gossamer Ring, which is composed of the dust ejected from the satellite.

Physical characteristics


The surface of Amalthea is very red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...

 (that is, its reflectivity increases with the wavelength from the green to near-infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

). The reddish color may be due to sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 originating from Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

 or some other non-ice material. Bright patches of green
Green
Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...

 appear on the major slopes of Amalthea, but the nature of this color is currently unknown. The surface of Amalthea is slightly brighter than surfaces of other inner satellites of Jupiter. There is also a substantial asymmetry between leading and trailing hemispheres
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

: the leading hemisphere is 1.3 times brighter than the trailing one. The asymmetry is probably caused by the higher velocity and frequency of impacts
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

 on the leading hemisphere, which excavate a bright material—presumably ice—from the interior of the moon.

Amalthea is irregularly shaped, with the best ellipsoidal approximation being . Like all other inner moons of Jupiter it is tidally locked with the planet, the long axis pointing towards Jupiter at all times. Its surface is heavily scarred by crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

s, some of which are extremely large relative to the size of the moon: Pan, the largest crater, measures 100 km across and is at least 8 km deep. Another crater, Gaea
Gaea (crater)
Gaea is an impact crater on Amalthea, one of the small moons of Jupiter. The crater is 80 km wide and at least 15 km deep. Gaea is one of two named craters on Amalthea, the other being Pan. It is named after the Greek goddess Gaia....

, measures 80 km across and is likely twice as deep as Pan. Amalthea has two prominent and named mountains, Mons Lyctas
Mons Lyctas
Lyctos Facula is a mountain on one of Jupiter's smallest moons Amalthea. It is believed to have a height of 20 kilometers , almost two and a half times higher than Mt. Everest . It is one of two known mountains that appear on Amalthea, the other being Ida Facula.-References:*...

 and Mons Ida
Mons Ida
Ida Facula is a mountain on Amalthea, one of Jupiter's smallest moons. It is known to be twenty kilometers in height, like the neighboring mountain Lyctos Facula.-References:*...

, with local relief reaching up to 20 km.

Amalthea's irregular shape and large size led in the past to a conclusion that it is a fairly strong, rigid body, where it was argued that a body composed of ices or other weak materials would have been pulled into a more spherical
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

 shape by its own gravity. However, on November 5, 2002, the Galileo orbiter
Galileo spacecraft
Galileo was an unmanned spacecraft sent by NASA to study the planet Jupiter and its moons. Named after the astronomer and Renaissance pioneer Galileo Galilei, it was launched on October 18, 1989 by the Space Shuttle Atlantis on the STS-34 mission...

 made a targeted flyby that came within 160 km of Amalthea and the deflection of its orbit was used to compute the moon's mass (its volume had been calculated previously—to within 10% or so—from a careful analysis of all extant images). In the end, Amalthea's density was found to be as low as , so it must be either a relatively icy body or very porous "rubble pile
Rubble pile
In astronomy, rubble pile is the informal name for an object that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity...

" or, more likely, something in between. Recent measurements from the Subaru telescope
Subaru (telescope)
Subaru Telescope is the 8.2 metre flagship telescope of the National Astronomical Observatory of Japan, located at the Mauna Kea Observatory on Hawaii. It is named after the open star cluster known in English as the Pleiades...

 suggest that the moon is indeed icy, indicating that it cannot have formed in its current position, since the hot primordial Jupiter would have melted it. It is therefore likely to have formed farther from the planet or to be a captured Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 body. Unfortunately, NASA didn't publish any picture of Amalthea during this famous flyby on November 5, 2002, and the resolution (number of pixels) of the released Jovian satellite pictures is generally low.

Amalthea radiates slightly more heat than it receives from 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...

, which is probably due to the influence of Jovian heat flux (<9° K), sunlight reflected from the planet (<5° K), and charged particle bombardment (<2° K). This is a trait shared with Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

, although for very different reasons.

Relationship with Jupiter's rings


Due to its low density and irregular shape, the escape velocity
Escape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...

 at the surface points of Amalthea closest to and furthest from 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,...

 is no more than 1 m/s and dust can easily escape from it after, e.g. micrometeorite impacts; this dust forms the Amalthea Gossamer Ring.

During its flyby of Amalthea, the Galileo orbiter's star scanner detected nine flashes which appear to be small moonlet
Moonlet
Moonlet is an informal term for a particularly small natural satellite. In astronomical literature, it has been used in at least two situations:...

s near the orbit of Amalthea. Since they were sighted only from one location, their true distances could not be measured. The moonlets may be anywhere in size from gravel to stadium-sized. Their origins are unknown, but they may be gravitationally captured into current orbit or they may be ejecta from meteor impact
Impact event
An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...

s on the moon. On the next and final orbit, Galileo detected more of these moonlets. However, this time Amalthea was on the other side of the planet, so it is probable that the particles form a ring
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...

 around the planet near Amalthea's orbit.

Views to and from Amalthea




From Jupiter's surface—or rather, from just above its cloudtops—Amalthea would appear very bright, shining with a magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 of −4.7, similar to that of Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 from Earth. At only 5 arcminutes across, its disc would be barely discernible and it would thus appear starlike. Amalthea's orbital period is only slightly longer than its parent planet's day (about 20% in this case), which means it would cross Jupiter's sky very slowly. The time between moonrise and moonset would be over 29 hours.

From the surface of Amalthea, Jupiter would look enormous: 46 degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...

s across, it would appear roughly 92 times larger than the full moon
Full moon
Full moon lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.Lunar eclipses can only occur at...

. Because Amalthea is in synchronous rotation
Synchronous rotation
In astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...

, Jupiter would not appear to move, and would be invisible from one side of Amalthea. The Sun would disappear behind the planet's bulk for an hour and a half each revolution. Amalthea's short rotation period gives it just under six hours of daylight. Though Jupiter would appear 900 times brighter than the full moon, its light would be spread over an area some 8500 times greater and it would not look as bright per surface unit.

Exploration


In 1979–1980, the Voyager 1 and 2
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...

spacecraft made the first images of Amalthea, which resolved its surface. They also measured the visible and infrared spectra
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...

 and surface temperature. Later, the Galileo orbiter
Galileo spacecraft
Galileo was an unmanned spacecraft sent by NASA to study the planet Jupiter and its moons. Named after the astronomer and Renaissance pioneer Galileo Galilei, it was launched on October 18, 1989 by the Space Shuttle Atlantis on the STS-34 mission...

 completed the imaging of Amalthea's surface. Amalthea provided the final satellite fly-by for Galileo on November 5, 2002, at a height of approximately 163 km (101.3 mi), permitting the moon's mass to be accurately determined, while changing Galileo's trajectory so that it would plunge into Jupiter in September 2003, having finished its mission. In 2006 Amalthea's orbit was refined by New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...

 spacecraft's instruments.

Named geological features


There are four named geological features on Amalthea: two craters and two facula
Facula
A facula , Latin for "little torch", is literally a "bright spot." It is used in planetary nomenclature for naming certain surface features of planets and moons, and is also a type of surface phenomenon on the Sun....

e (bright spots), which are believed to be mountains.
Feature Named after
Pan (crater)
Pan (crater)
Pan is the largest crater on Jupiter's moon Amalthea. It is 100 kilometers across and at least 8 kilometers deep. It is named after Pan, the Greek god of shepherds and the countryside....

 
Pan
Pan (mythology)
Pan , in Greek religion and mythology, is the god of the wild, shepherds and flocks, nature, of mountain wilds, hunting and rustic music, as well as the companion of the nymphs. His name originates within the Greek language, from the word paein , meaning "to pasture." He has the hindquarters, legs,...

, Greek god
Gaea (crater)
Gaea (crater)
Gaea is an impact crater on Amalthea, one of the small moons of Jupiter. The crater is 80 km wide and at least 15 km deep. Gaea is one of two named craters on Amalthea, the other being Pan. It is named after the Greek goddess Gaia....

 
Gaia
Gaia (mythology)
Gaia was the primordial Earth-goddess in ancient Greek religion. Gaia was the great mother of all: the heavenly gods and Titans were descended from her union with Uranus , the sea-gods from her union with Pontus , the Giants from her mating with Tartarus and mortal creatures were sprung or born...

, Greek goddess
Lyctos Facula  Lyctos, Crete
Crete
Crete is the largest and most populous of the Greek islands, the fifth largest island in the Mediterranean Sea, and one of the thirteen administrative regions of Greece. It forms a significant part of the economy and cultural heritage of Greece while retaining its own local cultural traits...

Ida Facula  Mount Ida
Mount Ida
In Greek mythology, two sacred mountains are called Mount Ida, the "Mountain of the Goddess": Mount Ida in Crete; and Mount Ida in the ancient Troad region of western Anatolia which was also known as the Phrygian Ida in classical antiquity and is the mountain that is mentioned in the Iliad of...

, Crete
Crete
Crete is the largest and most populous of the Greek islands, the fifth largest island in the Mediterranean Sea, and one of the thirteen administrative regions of Greece. It forms a significant part of the economy and cultural heritage of Greece while retaining its own local cultural traits...


In fiction



Amalthea is the setting of several works of science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

, including stories by Arthur C. Clarke
Arthur C. Clarke
Sir Arthur Charles Clarke, CBE, FRAS was a British science fiction author, inventor, and futurist, famous for his short stories and novels, among them 2001: A Space Odyssey, and as a host and commentator in the British television series Mysterious World. For many years, Robert A. Heinlein,...

 and James Blish
James Blish
James Benjamin Blish was an American author of fantasy and science fiction. Blish also wrote literary criticism of science fiction using the pen-name William Atheling, Jr.-Biography:...

.

External links