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.
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.
Amalthea's orbit lies near the outer edge of the Amalthea Gossamer Ring, which is composed of the dust ejected from the satellite.
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.
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.
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.
|Pan (crater)||Pan, Greek god|
|Gaea (crater)||Gaia, Greek goddess|
|Lyctos Facula||Lyctos, Crete|
|Ida Facula||Mount Ida, Crete|