Amalthea (, or as in Greek Αμάλθεια) is the third moon of Jupiter in order of distance from the planet. It was discovered on September 9, 1892, by Edward Emerson Barnard and named after Amalthea, a nymph in Greek mythology. 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 92 times larger than the full moon. Amalthea is the largest of the inner satellites of Jupiter. Irregularly shaped and reddish in colour, 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 spacecraft, and later, in more detail, by the Galileo orbiter 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 at Lick Observatory. 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's discovery of the Galilean satellites in 1610.

The satellite is named after the nymph Amalthea from Greek mythology who nursed the infant Zeus (the Greek equivalent of Jupiter) with goat's milk. Its Roman numeral designation is . The name "Amalthea" was not formally adopted by the IAU until 1975, although it had been in informal use for many decades. The name was initially suggested by Camille Flammarion Before 1975 Amalthea was most commonly known simply as . The adjectival form of the name is Amalthean.

Orbit

Amalthea circles Jupiter at a distance of 181 000 km (2.54 Jupiter radii). The orbit of Amalthea has an eccentricity of 0.003 and an inclination 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 and can be explained by the influence of the innermost Galilean satellite, Io: in the past Amalthea will have passed through several mean motion resonances 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 (that is, its reflectivity increases with the wavelength from the green to near-infrared). The reddish color may be due to sulfur originating from Io or some other non ice material. It is considered to be the reddest known object in the Solar System. Bright patches of green 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: 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 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 craters, 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, measures 80 km across and is probably twice as deep as Pan. Amalthea has two prominent and named mountains, Mons Lyctas and Mons Ida 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 shape by its own gravity. However, on November 5 2002, the Galileo orbiter 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" or, more probably, something in between. Recent measurements from the Subaru telescope 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 body.

Amalthea radiates slightly more heat than it receives from the Sun, 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, although for very different reasons.

Relationship with Jupiter's rings

Due to its low density and irregular shape, the escape velocity at the surface points of Amalthea closest to and furthest from Jupiter 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 moonlets near the orbit of Amalthea. Since they were sighted only from one location, their true distances couldn't be measured. The moonlets may be anywhere 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 impacts 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 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 of −4.7, similar to that of Venus 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 degrees across, it would appear roughly 92 times larger than the Full Moon. Because Amalthea is in synchronous rotation, 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 spacecraft made the first images of Amalthea, which resolved its surface. They also measured the visible and infrared spectra and measured the moon's surface temperature. Later, the Galileo orbiter completed the imaging of Amalthea's surface and a close flyby enabled it to constrain the moon's internal structure and composition. In 2006 Amalthea's orbit was refined by New Horizons spacecraft's instruments.

Named geological features

There are four named geological features on Amalthea: two craters and two faculae (bright spots), which are believed to be mountains.

Feature	Named after
Pan (crater)	Pan, Greek god
Gaea (crater)	Gaia, Greek goddess
Lyctos Facula	Lyctos, Crete
Ida Facula	Mount Ida, Crete

In fiction

Main article: Amalthea in fiction

Amalthea is the setting of several works of science fiction, including stories by Arthur C. Clarke and James Blish.

References

External links

• Amalthea Profile by NASA's Solar System Exploration
• Another find for Galileo – Galileo Millennium Mission status report (2003-04-09)
• Jupiter's Amalthea Surprisingly Jumbled – JPL press release (2002-12-09)
• Jupiter From Amalthea, a painting by Frank Hettick, 2002.