National Geographic : 1980 Jan
provoked the jealousy of his wife, Juno. Juno destroyed Callisto's allure by turning her into a bear. A similar fate befell Io, the inspiration for the closest Galilean moon. After her romance with Jupiter, she ended up as a heifer, pursued by Juno's gadfly. The two middle moons inherited their names from more fortunate friends of Jove. To elude the watchful Juno, he approached beautiful Europa as a bull. His noble gen tleness seduced Europa. She climbed upon his back, and the two flew off to Crete, where she became an object of worship. Ganymede, a handsome youth, also caught Jupiter's eye, and he whisked the boy away to become the cupbearer to the gods. tant, glancing look at Europa, as Voyag er 1's trajectory sped it in for its swing around Jupiter. Unfortunately, from more than a million kilometers out the craft saw only enough of Europa to tantalize. "It looks like the earth in a deep ice age with all its oceans frozen," said imaging team member Larry Soderblom. Enormous and mysterious stripes criss crossed Europa, raising questions only Voy ager 2's much closer flyby four months later could clarify. For the time being, everyone's attention was easily distracted by Io. White patches on its surface radiated brilliantly amid mot tled shades of red and orange. When he saw one of the first close-ups, team leader Brad Smith said that Io looked like a pizza. A heart-shaped region a thousand kilome ters across, which in a few days proved to be the deposits laid down by an active volcano, dominated early Io images. Then, as the spacecraft drew close, it found strange black hot spots-later speculated to be crusted over lakes oflavalike sulfur. Here and there rose mountains as high as ten kilometers perhaps remnants of an earlier crust. Disturbingly, Voyager 1 did not see a sin gle meteorite crater on Io. On earth such processes as weathering and volcanism have erased most craters. But the surfaces of every solid body we have studied in the solar system-the moon, Mercury, Mars and its moons-are peppered with them. Crater counting has become a cornerstone of plan etary sciences, the way that scientists date What Voyager Saw: Jupiter'sDazzling Realm features on other planets. Basically, the more craters a surface has, the older it is. "We were terribly upset to see no impact craters on Io," recalled Smith. "If our theories were right, the absence of craters meant that some processes had to have re worked Io's surface within the past mil lion years. That's just a blink of an eye geologically." The team anxiously awaited close-ups of the next moon, Ganymede. If Ganymede also lacked craters, then the idea that mete orites bombarded the entire solar system would be suspect. Or perhaps Jupiter had somehow protected its moons from the me teorites that struck the inner planets. In either case a vital method of dating bodies in the solar system would be lost. The weary team sequestered themselves in their conference room to greet the first Ganymede close-ups. It took only a few sec onds for Larry Soderblom to cry "Crater!" His colleagues cheered with relief. They quickly found several more, and cham pagne corks blew. Ganymede had more than craters to dis play. Its crust appeared broken into frag ments, like the pieces of a mosaic. These pieces seemed to be made of two distinct types of terrain. One had shoulder-to shoulder craters; the other looked heavily grooved, as if someone had raked it. There was scarcely time to puzzle over these features, however, before Callisto close-ups began coming in. Mercury-size Callisto was darker, but its surface was pricked by innumerable bright craters. Callisto apparently had been severely battered during its youth. One giant impact region has concentric ridges that look like frozen ripples and extend for 3,000 kilo meters. This was surely one of the most cataclysmic meteorite strikes to ever occur in the solar system. The crater counters quickly realized that all of Callisto and much of Ganymede are completely saturat ed with craters. That means their surfaces are extremely old. These moons, we can safely say, have been dead a long time. How ever, Io's surface, as the discovery of volcanoes would reveal, is being reworked continuously.