Fig. 6.46. A diagram depicting the orbits of the known Jovian regular satellites. These satellites have small, circular orbits and low inclinations. (Credit: Scott C. Sheppard)

a diameter ~150 km. This little moon was discovered by Perrine in 1904. There appear to be two distinct prograde groups of satellites and at least three retrograde groups. The groups suggest that these satellites are the result of the breakup from collision of multiple parent bodies. Breakup could be the result of impact with interplanetary bodies, primarily comets, or by collision with other satellites. Each retrograde group contains one large object (with a radius > 14 km) along with several smaller ones (with a radius < 4 km). Outside of these distinct groups, there are many satellites with diameters of just 2 km. Recent surveys predict that Jupiter should be surrounded by ~100 rocky satellites with diameters larger than ~1 km [469].

Evidence from physical observations of these satellites is limited, but we know that their colors range from neutral, the color of the Sun, to moderately red. The satellites lack the ultra-red material found on the Centaurs and Kuiper Belt Objects. The retrograde satellites are systematically redder than the prograde satellites, suggesting that the retrograde satellites are fragments of a D-type body while the prograde satellites result from the breakup of a C-type body [470]. However, while

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Fig. 6.47. A diagram depicting the orbits of the Jovian irregular satellites. Irregular satellites have large orbits, inclinations, and eccentricities. These are probably objects captured during the early formation of Jupiter. (Credit: Scott C. Sheppard)

the colors of irregular satellites are very similar to C, P, and D-type carbonaceous outer main belt asteroids, the spectra of Jupiter's irregular satellites are consistent with C-type asteroids [471]. Evidence for oxidized iron in phyllosilicates has been found in an absorption feature in the spectra of the moon Himalia [472].

While the regular satellites, with their circular, non-eccentric orbits are expected to exist in permanent orbits around Jupiter, the irregular satellites must be considered temporarily captured. In fact there are several examples of comets that are in temporary capture orbits around Jupiter, and others that scientists expect to be captured in the next 100 years. The most famous example of a temporarily captured satellite is comet Shoemaker-Levy 9 (SL9), which dramatically crashed into Jupiter in 1994. SL9 was probably in orbit around Jupiter for decades to a century. Had it not collided with Jupiter it would eventually have been ejected by the planet, either to orbit the Sun as a short period comet or to leave the solar system completely [473].

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