The Adrastea

The minor Jovian satellites fall naturally into three families. One family, orbiting well inside the realm of the Galilean satellites, consists of Amalthea (JV, read “Jupiter five”) and its smaller sisters Metis (JXVI), Thebe (JXIV), and Adrastea (JXV). Of these, only Amalthea is widely enough known so that it is usually called by its name instead of its number. (Amalthea also has the double distinction of having been named by the great French popularizer of astronomy, Camille Flammarion, and having served as the site of a classic science fiction story by Arthur C. Clarke, entitled “Jupiter Five.”) These satellites have very small orbital eccentricities (e < 0.02) and inclinations (i < 1°). Two of them (Metis and Adrastea) orbit so close to Jupiter that they have orbital periods of less than a Jovian day, so that, as seen from Jupiter, they rise in the west and set in the east, opposite to the apparent motions of the Galilean satellites. Metis and Adrastea have almost identical orbital periods and orbit just outside Jupiter's tenuous, narrow ring.

The particles comprising the diffuse tenuous rings of Jupiter almost certainly have their origin in the release of dust from each of the four moonlets—Adrastea, Metis, Amalthea, and Thebe—embedded in the rings. These small rocky objects are continually pummeled by bits of space debris that are accelerated to high relative speeds by Jupiter's intense gravity. When struck by this flotsam, puffs of dust are ejected from the moonlet surfaces.

The main ring has a relatively sharp outer edge suspiciously coincident with the orbit of Adrastea; just interior to this, the satellite Metis creates a depression in ring brightness. The fact that the main ring extends only inward from the small source satellites strongly suggests that ring particles drift inward