Amalthea is a moon of Jupiter. It has the third closest orbit around Jupiter among known moons and was the fifth moon of Jupiter to be discovered, so it is also known as . It is also the fifth largest moon of Jupiter, after the four Galilean Moons. Edward Emerson Barnard discovered the moon on 9 September 1892 and named it after Amalthea of Greek mythology. It was the last natural satellite to be discovered by direct visual observation; all later moons were discovered by photographic or digital imaging.
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. Jupiter would appear 46.5 degrees in diameter from its surface. Amalthea is the largest of the inner satellites of Jupiter and is 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 ridges.
Close range images of Amalthea were taken in 1979 by the Voyager 1 and 2 spacecraft, and in more detail by the Galileo orbiter in the 1990s.
History
Discovery
Amalthea was discovered on 9 September 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.
Name
Amalthea 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 1976, although it had been in informal use for many decades. The name was initially suggested by Camille Flammarion. Before 1976, Amalthea was most commonly known simply as .
Orbit
Amalthea orbits 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 has passed through several mean-motion resonances with Io that 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. This color may be due to sulfur originating from Io or some other non-ice material. Bright patches of less red tint 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 . From this, Amalthea's surface area is likely between 88,000 and 170,000 square kilometers, or somewhere near 130,000. 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 likely twice as deep as Pan. Amalthea has several prominent bright spots, two of which are named. They are Lyctos Facula and Ida Facula, with width reaching up to 25 km. They are located on the edge of ridges.
