Amalthea is one of Jupiter’s moons. It was discovered in 1892 by American astronomer Edward Emerson Barnard.
Mass: 7.17 x 1018 kg
Composition: Water-ice and rock
Orbit: 1.81 x 108 m from Jupiter
Amalthea is Jupiter’s fifth-largest moon and its third nearest in orbit. It has an irregular ellipsoid shape resembling a potato. It was the first of Jupiter’s moons to be indentified from Earth observations after Italian astronomer Galileo Galilei discovered Europa, Io, Callisto and Ganymede in 1610. It is the moon to be identified by direct visual observation rather than photography. Jupiter has 67 moons, 49 of which have been named by the International Astronomical Union (IAU). 16 carry numbers and 2 are uncertain. 13 of the moons were discovered by early observations from Earth. Voyager spacecraft missions in the 1970s discovered 3 and observations from Earth using sensitive devices found 41 moons, 2 of which remain unconfirmed.
The names of Jupiter’s moons derive from characters from Greek mythology who were the lovers and descendants of Jupiter (Zeus), the king of the gods. Amalthea was Jupiter’s foster mother, who nursed him with goat’s milk. French astronomer Nicholas Camille Flammarion first named Amalthea in 1893 but the name was not adopted formally by the IAU until 1975. It was also known as Jupiter V.
The composition of Amalthea is water-ice and rock. Its interior is porous, undifferentiated into a crust, mantle and core and resembles a loosely packed mass of rubble. The surface is red, probably due to sulphur from volcanism on Io that spiralled towards Jupiter and impacted on Amalthea’s surface. Amalthea has no known atmosphere.
The surface is heavily cratered. The largest crater, Pan, measures 100 km in diameter and is 8 km deep. Two mountains, Mons Lyctas and Mons Ida, are over 20 km in height. Their slopes display bright green patches of unknown origin. Amalthea radiates more heat than it receives from the sun. The heat could be due to tidal stresses or to electrical currents that have been induced by Jupiter’s powerful magnetic field.
Amalthea orbits Jupiter with a period of 0.498 Earth days, or 11 hr 57 min. It rotates synchronously with its long axis pointed towards Jupiter. This means that the same hemisphere of Amalthea always faces Jupiter. Its spin period, Amalthea’s day, is the same period as its orbit.
The Voyager 1 and 2 spacecraft missions in 1979 and 1980 took the first photographs of Amalthea from a distance of 420,200 km. The Galileo missions to Jupiter between 1995 and 2003 passed within 163 km of Amalthea’s surface and measured its temperature and infrared spectra. The Galileo mission also detected a number of moonlets around the satellite, ranging in size between that of a piece of gravel and that of a sports stadium.