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(David Morrison and Jane Samz, "Voyage to Jupiter," NASA SP-439, U.S. Government Printing Office, Washington, D.C., 1980, pp. 7-9.)

* All four Galilean satellites orbit within the magnetosphere of Jupiter; in contrast, our Moon lies well outside the terrestrial magnetosphere. Striking evidence of the interaction of the satellites and the magnetosphere was provided when it was found that the innermost large satellite, Io, actually affects the bursts of radio static produced by Jupiter. Only when Io is at certain places in its orbit are these strong bursts detected. Theorists suggested that electric currents flowing between the satellite and the planet might be responsible for this effect.

The Jovian Satellites

* For nearly three centuries after their discovery in 1610, the only known moons of Jupiter were the four large Galilean satellites. In 1892 E. E. Barnard, an American astronomer, found a much smaller fifth satellite orbiting very close to the planet, and between 1904 and 1974 eight additional satellites were found far outside the orbits of the Galilean satellites. The outer satellites are quite faint and presumably no more than a few tens of kilometers in diameter, and all have orbits that are much less regular than those of the five inner satellites. Four of them revolve in a retrograde direction, opposite to that of the inner satellites and Jupiter itself.

* In 1975 the International Astronomical Union assumed the responsibility for assigning names to the nonGalilean satellites of Jupiter. Following tradition, they named the inner satellite Amalthea for the she-goat that suckled the young god Jupiter. The outer eight were named for lovers of Jupiter: Leda, Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae, and Sinope. For the nonGalilean satellites, the "e" ending is reserved for satellites with retrograde orbits; those with normal orbits have names that end in "a."

* Because they are so large, the Galilean satellites have attracted the most attention from astronomers. More than fifty years ago large telescopes were used to estimate their sizes, and a careful series of measurements of their light variation showed that all four always keep the same face pointed toward Jupiter, just as our Moon always turns the same face toward Earth. Also, the subtle gravitational perturbations they exert on each other were used to determine the approximate mass of each.

* Callisto, the outermost Galilean satellite, is larger than the planet Mercury. It also has the lowest reflectivity, or albedo, of the four, suggesting that its surface may be composed of some rather dark, colorless rock. Callisto takes just over two weeks to orbit once around Jupiter.

* Ganymede, which requires only seven days for one orbit, is the largest satellite in the Jovian system, being only slightly smaller than the planet Mars. Its albedo is much higher than that of Callisto, or of the rocky planets such as Mercury, Mars, or the Moon. In 1971 astronomers first measured the infrared spectrum of reflected sunlight from Ganymede and found the characteristic absorptions of water ice, indicating that this satellite is partially covered with highly reflective snow or ice.

* Europa, which is slightly smaller than the Moon, circles Jupiter in half the time required by Ganymede. Its surface reflects about sixty percent of the incident sunlight, and the infrared spectrum shows prominent absorptions due to water ice; Europa appears to be almost entirely covered with ice. However, its color in the visible and ultraviolet part of the spectrum is not that of ice, so some other material must also be present.

* Io, innermost of the Galilean satellites, is the same size as our Moon. It orbits the planet in 42 hours, half the period of Europa. Like Europa, it has a very high reflectivity, but, unlike Europa, it has no spectral absorptions indicative of water ice. Before Voyager, identification of the surface material on Io presented a major problem to planetary astronomers.

* When the sizes and masses of these satellites were measured, astronomers could calculate their densities. The inner two, Io and Europa, both have densities about three times that of water, nearly the same as the density of the Moon, or of rocks in the crust of the Earth. Callisto and Ganymede have densities only half as large, far too low to be consistent with a rocky composition. The most plausible alternative to rock is a composition that includes ice as a major component. Calculations showed that if these satellites were composed of rock and ice, approximately equal quantities of each were required to account for the measured density. Thus the two outer Galilean satellites were thought to represent a new kind of solar system object, as large as one of the terrestrial planets, but composed in large part of ice.

* In 1973 the attention of astronomers was dramatically drawn to Io when Robert Brown of Harvard University detected the faint yellow glow of sodium from the region of space surrounding it. It seemed that this satellite had an atmosphere, composed of the metal sodium! Continued observations showed, however, that this was not an atmosphere in the usual sense of the word. The gas atoms were not bound gravitationally to Io, but continuously escaped from it to form a gigantic cloud enveloping the orbit of the satellite. Fraser Fanale and Dennis Matson of the Caltech Jet Propulsion Laboratory suggested that bombardment of Io by high energy particles from the jovian Van Allen belts was knocking off atoms of sodium by a process called sputtering, releasing these atoms and allowing them to expand outward to form the observed sodium cloud. No one anticipated then that powerful volcanic eruptions on Io might also be contributing to this remarkable gas cloud.


(NASA, Jet Propulsion Laboratory, "Our Solar System at a Glance," NASA Information Summaries, PMS 010-A (JPL), June 1991.)

Jupiter Satellites

In 1610, Galileo Galilei aimed his telescope at Jupiter and spotted four points of light orbiting the planet. For the first time, humans had seen the moons of another world. In honor of their discoverer, these four bodies would become known as the Galilean satellites or moons. But Galileo might have happily traded this honor for one look at the dazzling photographs returned by the Voyager spacecraft as they flew past these planet-sized satellites.

One of the most remarkable findings of the Voyager mission was the presence of active volcanoes on the Galilean moon Io. Volcanic eruptions had never before been observed on a world other than Earth. The Voyager cameras identified at least nine active volcanoes on Io, with plumes of ejected material extending as far as 280 kilometers (175 miles) above the moon's surface.

Io's pizza-colored terrain, marked by orange and yellow hues, is probably the result of sulfur-rich materials brought to the surface by volcanic activity. Volcanic activity on this satellite is the result of tidal flexing caused by the gravitational tug-of-war between Io, Jupiter, and the other three Galilean moons.

Europa, approximately the same size as our Moon, is the brightest Galilean satellite. The moon's surface displays a complex array of streaks, indicating the crust has been fractured. Caught in a gravitational tug-of-war like Io, Europa has been heated enough to cause its interior ice to melt‹apparently producing a liquid-water ocean. This ocean is covered by an ice crust that has formed where water is exposed to the cold of space. Europa's core is made of rock that sank to its center.

Like Europa, the other two Galilean moons, Ganymede and Callisto, are worlds of ice and rock. Ganymede is the largest satellite in the solar system, ‹larger than the planets Mercury and Pluto. The satellite is composed of about 50 percent water or ice and the rest rock. Ganymede's surface has areas of different brightness, indicating that in the past material oozed out of the moon's interior and was deposited at various locations on the surface.

Callisto, only slightly smaller than Ganymede, has the lowest density of any Galilean satellite, suggesting that large amounts of water are part of its composition. Callisto is the most heavily cratered object in the solar system; no activity during its history has erased old craters except more impacts.

Detailed studies of all the Galilean satellites will be performed by the Galileo Orbiter.