Jupiter possesses 63 moons or satellites as of this writing, including a large number of smaller ones (Scott S. Sheppard, personal communication). Some of these small satellites reside in prograde orbits, and others reside in retrograde orbits (Table 6.1). Satellites that orbit in the same direction as the planet's rotation are prograde, and those that orbit opposite the direction of the planet's rotation are retrograde. Some
Fig. 6.43. An artist's concept of the interior of Callisto, depicting a salty ocean beneath an icy ^ tfl crust. Prior to Galileo, scientists thought Callisto was relatively inactive. Callisto's cratered surface lies at the top of an ice layer. Immediately beneath the ice, the blue band represents the possible ocean whose depth must exceed 10 km. The mottled interior is composed of ice and rock. (Credit:
satellites have small, circular orbits with low inclinations, and the others have very large, very elongated orbits with high inclinations (Fig. 6.44).
The category of regular satellites includes the Galilean moons previously discussed. The other regular satellites are very small bodies that have small circular orbits and low inclinations. Besides the four Galilean moons there are four inner regular satellites or moons. These are Metis 44 km, Adrastea 16 km, Amalthea 168 km, and Thebe 98 km (Fig. 6.45). All of the regular satellites probably formed in the early circumjovian disk of gas and dust around Jupiter during Jupiter's formation. All regular satellites have prograde orbits (Fig. 6.46).
There are a large number of irregular satellites orbiting Jupiter. Irregular satellites orbit Jupiter in very large orbits, with large inclinations and eccentricities, especially compared to the regular satellites  (Fig. 6.47). Some irregular satellites have prograde orbits and others have retrograde orbits. These bodies
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