Amalthea is one of Jupiter's inner moons, along with Thebe, Metis, and Adrastea. It was the first of the inner moons to be discovered, in 1892, and was named after a foster-mother of Zeus in Greek mythology. In the Roman pantheon, the name for Zeus was Jupiter; it was this name which has been used to refer to the solar system's largest planet since the time of the Roman empire.
All of the inner moons are known to contribute to Jupiter's tenuous ring system through dust blown off their surfaces. However, Amalthea is the main contributor to one particular ring, the Amalthea gossamer ring. The Galileo space probe passed through the Amalthea ring in 2002 and 2003, shortly before it was deliberately crashed into Jupiter's atmosphere at the end of its multi-year mission.
Given its mass, Amalthea is probably made up mostly of water ice. However, its surface is covered by a reddish rock, similar to the surfaces of Thebe, Metis, and possibly Adrastea. It is still not known for certain whether the red surfaces of these moons is the result of the material present during their formation or whether it was accreted afterward when the moons passed through the plume of material left by the massive volcanoes on one of Jupiter's largest moons, Io. Its surface is home to a large number of impact craters, two of which span a large proportion of its surface. The largest of these, Pan, is 60 miles long and several miles deep. Two large moons are also prominent on the surface. Both of these, Lyctos Facula and Ida Facula, are over twice as high as Mount Everest.
Like all small inner moons, Amalthea is tidally locked. This means that its day lasts the same length of time as the period of its orbit around Jupiter - about 12 hours. The same process affects Earth's own Moon, although the Moon's orbit and day are much longer, lasting about one month.
Amalthea is much larger than the other inner moons, at 250 kilometres across (about 150 miles) in its largest dimension. Although this is still not quite large enough for its own gravity to compress the moon into a sphere, it does mean that it is much easier to study than its relatively tiny counterparts. It was discovered by telescope (the last moon to be discovered in this way, before telescope photography replaced direct observation) in 1892, and was also imaged by Voyager 1, Voyager 2, and Galileo. Images of the moon will probably also be generated in several years' time, when NASA's next Jovian space probe, Juno, reaches the planet. However, Amalthea and the inner moons are less significant targets of interest than Jupiter itself or its large moons, especially Europa, which is believed to harbour a massive underground water ocean capable of supporting primitive life.