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Amalthea is the third innermost of Jupiter's known moons. Named after the nymph who nursed the infant Jupiter with goat's milk, it was discovered in 1892 by Edward Emerson Barnard. This event was both the first discovery of a Jovian moon since the four Galilean moons in 1610, and the last discovery of any moon in the solar system by direct visual observation.
Amalthlea is the largest of Jupiter's small moons (if that makes any sense), its irregular shape measuring 270 km by 170 km by 150 km. It orbits its primary at a distance of 181,300 km with a period of 0.498179 (Earth) days and rotates synchronously, its longer axis always pointed toward the planet. Its location makes it the target of all sorts of bombardments - heavy radiation from Jupiter, high-velocity metorites accelerated by Jupiter's gravity well, and sulfur from the many volcanoes of Io. The last is the source of Amalthea's deep red color; it is, in fact, the reddest known object in the solar system.
Its surface is highly irregular as well, containing two large craters: Pan (90 km diameter) and Gaea ( 75 km). Two mountains, named Mons Ida and Mons Lyctos, have been observed. Their slopes display bright green patches of unknown origin. Gaea, which is the deeper of the two main craters, contains a large spot of some bright material as well. Amalthea's composition is unknown, though probably similar to an asteroid. Its mass is 7.17e18 kg, with a density of 1.8 g/cm3. Like Io, Amalthea radiates more energy than it receives from the Sun; this is probably caused by electric currents induced by Jupiter's strong magnetic field.