Jupiter and Its Moons

The first probe to venture to the outer planets was Pioneer 10, which flew past Jupiter in 1973. Jupiter, 318 times the mass of the Earth, is essentially a huge ball of gas, though it has a hot, dense—perhaps solid—core made up of heavy elements. Jupiter's powerful magnetic field traps high concentrations of charged subatomic particles (such as protons and electrons), generating lethal levels of radiation. In 1974, Pioneer 11 encountered Jupiter on its way to Saturn. These missions proved that spacecraft could safely traverse the asteroid belt (a band of rocky debris orbiting the sun between Mars and Jupiter) and survive the onslaught of radiation at Jupiter to return data to Earth. Following in their path, the more advanced Voyager 1 and Voyager 2 probes flew past Jupiter in 1979 on their way to Saturn.

The Voyagers showed that Jupiter has a roiling, turbulent atmosphere, with gigantic storms appearing and then being swallowed up again. They identified hydrogen, helium, and methane in Jupiter's atmosphere and measured their abundance. The probes also found that the planet is circled by a thin ring of dust. The probes also discovered three small moons that were previously unknown—Adrastea, Metis, and Thebe.

But the most startling findings by the Voyager mission at Jupiter centered on the planet's four largest satellites: Io, Europa, Ganymede, and Callisto. The probes showed that the inner two moons, Io and Europa, are composed mostly of rock and are about the size of Earth's moon. The outer two, Ganymede and Callisto, are larger—about 5,000 kilometers (3,000 miles) in diameter—but they are less dense than the inner moons and appear to be composed of roughly equal parts of rock and ice. Although they are similar in size and mass, Ganymede and Callisto seem to have had vastly different geologic histories. Callisto is covered by craters, with nothing to break the monotony of its battered surface. In contrast, Ganymede—the solar system's largest moon—has vast regions where craters have been “erased” by geologic processes and replaced by complex networks of grooves.

Europa's rocky surface, Voyager data reported, is covered by a bright, smooth layer of water ice, which is cracked in places like an eggshell. Even more startling was Io, revealed by Voyager's cameras to be covered by active volcanoes that spew tremendous plumes of smoke and lava flows consisting of both molten sulfur and molten rock.

Scientists wondered what the source of energy was that powered all this activity. The answer, they concluded, was gravity. Jupiter's tremendous gravity exerts alternately pulling and squeezing forces on the larger moons as they travel around the planet in their slightly elliptical (oval) orbits. Close-in Io is most affected by this gravitational “kneading”—the constant stresses have heated Io to the melting point of rock, sealing its fate as a world of explosive volcanism. Farther out, Europa is also stressed, but only enough internal warming has occurred to melt underground ice to a slush, which then rises to the surface and refreezes. Most intriguing is the possibility that an ocean of liquid water, warmed by gravitational stresses, still exists beneath the icy surface of Europa.

To study Jupiter further, NASA in 1989 launched the Galileo spacecraft on a voyage to the giant planet. Galileo, which consisted of an orbiter and an atmospheric probe, arrived at Jupiter in December 1995. The probe, released from the orbiter several months earlier, plunged into Jupiter's atmosphere, surviving acceleration, pressure, and heating for 57 minutes until it melted and then vaporized. During its descent the probe found no evidence of the water clouds that scientists had expected to find. Concentrations of water vapor in the atmosphere were also lower than expected. One possible explanation for these findings is that the probe fell into an unusually dry, cloudless patch of Jupiter's atmosphere. The probe also reported that wind speeds increased at lower elevations. This indicated that, unlike Earth, where the winds are faster in the upper atmosphere because they are driven by heat from the sun, Jupiter's winds are driven by heat from within the planet.

After the probe mission was completed, the main part of the spacecraft rocketed into orbit around Jupiter to begin a survey of the planet and its moons. Information from Galileo was still being collected in mid-1997 and would be the subject of study for many years. But the spacecraft had already made several significant discoveries about Jupiter's satellites. A close approach to Ganymede revealed evidence that, unlike other moons encountered by probes, it possesses a magnetic field. And a survey of Io, with careful measurement of how the moon's gravity deflected the spacecraft's path, enabled scientists to deduce that this volcanic moon has a distinct, metallic inner core.

Galileo also revealed that the fractured sheath of ice enveloping Europa is divided into an incredibly complex network of cracks. The cracks crisscross one another at many angles, with many cracks obliterated by fresh flows of ice. The newest cracks might be places where liquid water lies only a few kilometers beneath the surface. In April 1997, a scientific panel declared that evidence gathered by the Galileo probe had confirmed this theory. In fact, observations indicate that there is probably more water beneath the surface of Europa than in all the oceans on Earth combined. This subsurface ocean might even support forms of life that rely on heat from within the moon or small amounts of sunlight penetrating the cracks in the surface.