Image of Amalthea acquired by the Galileo spacecraft.
AMALTHEA - MOON OF JUPITER
Amalthea is an inner moon of Jupiter, with only Adrastea and Metis closer to Jupiter. Amalthea is named after the nymph who raised Jupiter with goat's milk.
Amalthea was discovered on the 9th September 1892 by Barnard using the 91- centimetre refractor at Lick Observatory. This was the last time a moon was to be discovered by direct visual observation through a telescope. All moon discoveries since then have been through photography or space-probe imagery. In photographs where the telescope tracks the stars, moons sometimes show up as streaks of light.
Orbit
The average distance between Amalthea and Jupiter is only 181,300 kilometres. Travelling at an average speed of 26.47 kilometres per second it takes Amalthea only half a day to complete one orbit of Jupiter. Amalthea also rotates on its axis in half a day, and so the same part of Amalthea (the blunt end) always points to Jupiter.
Map of Jupiter's ring system and inner moons.	The orbits of Jupiter's inner moons.	Voyager view of Amalthea against a backdrop of the Jovian atmosphere.
Physical properties
Like many of the small moons, Amalthea has an irregular shape, with the longest diameter being only 270 kilometres. Its low density (1800 kg m-3) is similar to the density of asteroids.
Interior
There is no information concerning the interior of Amalthea.
Magnetic field
No magnetic field has been detected.
Atmosphere
Amaltheas's surface gravity (0.5% that of Earth) is too low for it to retain an atmosphere.
Surface
Amalthea has a cratered surface, with two craters being quite large compared to the moon's small size. The largest crater, Pan, is 90 kilometres long and is located in Amalthea's "northern hemisphere". The other large crater, Gaeo, is 75 kilometres long and lies across Amalthea's south pole. These two craters are between 8 and 16 kilometres deep. Amalthea also has two known mountains - Mons Ida and Mons Lyctas. Amalthea is covered with craters because there are no processes at work to cause erosion.
The topography of Amalthea is quite varied, and fluctuates over short distances by as much as 20 kilometres. There are bright patches on some of the main slopes and these have a greenish tint. It is not known what causes the green colouration.
Amalthea seems to be very similar to Phobos, one of the moons of Mars. It is possible that, like Phobos, Amalthea too is a captured asteroid.The surface of Amalthea is very dark and red - in fact, Amalthea is the second reddest object in the Solar System. (The reddest is the asteroid Pholus). Amalthea's surface only reflects about 5% of the received sunlight. The most likely cause of this is a covering of sulphur, with the sulphur coming from the volcanoes of Io. The sulphur escapes from Io and spirals towards Jupiter, impacting not only on Amalthea but also on the other dark, inner moons of Jupiter.
Voyager 1 view of Amalthea, the reddest moon in the Solar System.
Amalthea is exposed to the intense radiation field of Jupiter, and is exposed to high dosages of energetic ions, protons and electrons in the Jovian magnetosphere. These particles impact Amalthea at between 40 and 60 kilometres per second. Amalthea is also bombarded with high-velocity micrometeorites, as well as sulphur, oxygen and sodium escaping from Io. Amalthea emits more energy than it receives. The additional heating is probably caused by electrical currents generated by Jupiter's magnetic field.
Amalthea and Jupiter's rings
The constant bombardment of Amalthea causes dust to be ejected from the surface. This dust then enters into orbit around Jupiter, as Amalthea's gravity is too weak to pull it back. The dust is responsible for forming Jupiter's ring system.
More about Jupiter's rings
Four views of Amalthea acquired by the Galileo spacecraft.