Jupiter | .. * . o o O O      . . . .      . . . .
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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.


The name of the nurse who, on Mount Ida in Crete, fed the infant Zeus and brought him up in secrecy to keep him safe from Cronus, who was searching for him and wanting to eat him. In some sources, Amalthea is the she-goat who suckled the child, and in others she is a Nymph, the most usual form of the story. It was said that Amalthea had hung the baby in a tree to prevent his father from finding him 'in heaven, or on earth, or in the sea', and that she had gathered the Curetes round him so that their songs and noisy dances should drown his cries. The goat that gave its milk was simply called Aix (a she-goat). She was a terrifying beast, descended from Helios (the Sun) and the Titans were so frightened of her mere appearance that the Earth, at their request, had hidden her in a cave in the Cretan mountains. Later, when Zeus was fighting the Titans, he made himself armour from her skin. This armour was called the aegis.

There is also a story that one day, while at play, Zeus took one of the goat's horns and gave it as a present to Amalthea, promising her that this horn would be miraculously filled with all the kinds of fruit she wanted. This is the Horn of Almathea or the Horn of Plenty (see Achelous).


Table of Sources:
- Hyg. Fab. 139; 182
- Ovid, Fast. 5, 115
- Callim. Hymn 1, 46ff.
- Strabo 8, 7, 5, p. 387
- Pseudo-Eratosth. Catast. 13
- Diod. Sic. 5, 70, 2

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