Other Satellites

The only other Jovian moon that was close enough to the trajectories of the Voyager spacecraft to allow surface features to be seen was Amalthea, a small, potato-shaped moon that is named for a figure in Greek mythology associated with the infant Jupiter.

Amalthea circles Jupiter once every 11 hours 57 minutes (0.498 Earth day) at a distance of 181,000 km (112,500 miles) in a nearly circular orbit that lies within half a degree of Jupiter's equatorial plane. Photographs transmitted by the Voyager 1 and 2 spacecraft in 1979 and confirmed by the Galileo orbiter in the late 1990s show that Amalthea is an irregular rocky body measuring 262 * 146 * 134 km (163 * 91 * 83 miles). Like the Moon, which always keeps the same face toward Earth, Amalthea rotates at the same rate that it revolves around Jupiter and thus keeps the same face toward the planet. Amalthea's long axis always points toward Jupiter.

By measuring the gravitational influence of Amalthea on the Galileo spacecraft, scientists determined that the moon has such a remarkably low den-sity—0.9 gram per cubic cm—that it could float in water. Evidently, Amalthea is highly porous, perhaps as a result of collisions that repeatedly shattered its rocky interior. Low densities ascribed to this same cause also have been observed for some of the inner moons of Saturn.

Amalthea has a dark, reddish surface marked by impact craters. The leading hemisphere (that facing the direction of motion) is some 30 percent brighter than the trailing one, presumably as a result of bombardment by small meteoroids that have entered the Jovian system. The red colour probably results from contamination by particles of sulfur and sulfur compounds that are continually shed by the nearby volcanically active satellite Io. The largest impact crater on Amalthea is Pan, which has a diameter of about 90 km (55 miles).

In addition to providing new images of Amalthea, the Galileo orbiter was able to view the effect of impacts on Thebe and Metis. These two moons are also tidally locked, keeping the same face oriented toward Jupiter and, like Amalthea, are some 30 percent brighter on their leading sides.

Before the turn of the 21st century, eight outer moons were known, comprising two distinct orbital families. The more distant group—made up of Ananke, Carme, Pasiphae, and Sinope— has retrograde orbits around Jupiter. The closer group— Leda, Himalia, Lysithea, and Elara—has prograde orbits. (In the case of these moons, retrograde motion is in the direction opposite to Jupiter's spin and motion around the Sun, which are counterclockwise as viewed from above Jupiter's north pole, whereas prograde, or direct, motion is in the same direction.) In 1999 astronomers began a concerted effort to find new Jovian satellites using highly sensitive electronic detectors that allowed them to detect fainter—and hence smaller—objects. When in the next few years they discovered a host of additional outer moons, they recognized that the two-family division was an oversimplification. There must be well more than 100 small fragments orbiting Jupiter that can be classified into several different groups according to their orbits. Each group apparently originated from an individual body that was captured by Jupiter and then broke up. The captures could have occurred near the time of Jupiter's formation when the planet was itself surrounded by a nebula that could slow down objects that entered it. These small moons may be related to the Trojan asteroids.

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