Adrastea is one of Jupiter’s small inner moons, similar in position and composition to Thebe and Metis. After its discovery in 1979, it was given the name of Zeus’s foster mother in Greek mythology. In the Roman pantheon, the Greek god Zeus was known as Jupiter; it is this name which was given to the solar system’s largest planet. The Voyager probes were responsible for the discovery of these small inner moons; in Adrastea’s case, it first turned up in an image taken by Voyager 2.
Adrastea is just twenty kilometres (about 12 miles) across in its largest dimension, making it far too small to have compressed into a sphere under the force of its own gravity. It is smaller than its own inner-moon companions, but still seems relatively light for its size. This suggests that Adrastea, like Amalthea and Metis, is made up largely of water ice. Even Galileo never approached close enough to take a detailed picture of the moon’s surface, so it is not yet known what the surface features might be, or whether the moon has a red rocky surface like Metis and Thebe.
Like all of Jupiter’s inner moons, Adrastea’s close orbit (130,000 kilometres, about one-third of the distance from Earth to the Moon) means that it is tidally locked, so that the length of its day and the length of its orbital period around Jupiter are the same – in its case, about seven hours. (The same process affects Earth’s Moon, but the Moon’s day and orbit last about one month.)
None of Jupiter’s inner moons are well understood: they are too small and distant to be studied in depth from Earth, leaving only data gleaned by the Voyager probes and (more recently) by the Galileo spacecraft. NASA’s next Jupiter space probe, Juno, will probably produce new images of Adrastea after it reaches Jupiter in several years’ time, but none of the inner moons are major scientific research targets on par with Jupiter itself or with the Galilean moons, one of which (Europa) is believed to have an underground water ocean capable of supporting primitive life.
In the meantime, there is one possible defining feature of Adrastea relative to its closest neighbours. Dust escaping from the surface of the inner moons is known to contribute the bulk of the mass to Jupiter’s tenuous ring system. However, Adrastea may be the most important contributor to the rings. Its orbit brings it at least very close to the main ring, but the total mass of dust in Jupiter’s entire ring system is still much less than that of Adrastea or of any other moon.
