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Voyager got only a far-off passing glimpse of the second Galilean moon, Europa. Scientists will get a better look in July, when a twin spacecraft, Voyager 2, also veers by Jupiter. And any disappointment about Europa was quickly offset by Voyager's dramatic encounter with lo, innermost of the Galilean moons.
A brilliant orange-red, lo (rhymes with My-Oh!) is almost as phenomenal as its mother planet. It is scarred with plateaus, dry plains, highlands and fault lines. It has at least one large, possibly still active volcano with a diameter of about 50 km (30 miles). All of which has prompted scientists to dub lo the pizza in the sky.
lo's surface, however, is surprisingly smooth, indicating that it is extremely young (10 million to 100 million years). It appears to have few impact (as opposed to volcanic) craters—the only rocky body discovered so far bereft of such markings. Scientists speculate that some unusual erosional process must be at work; possibly lo is scoured by the strong bombardment of charged particles from Jupiter. No wonder: lo lies inside a doughnut-shaped radiation belt, Jupiter's so-called flux tube, where Voyager measured fields that crackled with 400,000 watts of electricity.
Voyager also passed near Amalthea, Jupiter's innermost moon, until now only a pinpoint of light discernible to the most exacting astronomer. This tiny non-Galilean moon emerged as a strangely elongated object about 130 km (80 miles) high by 220 km (136 miles) long.
The most unexpected phenomenon, however, occurred when Voyager began detecting a stream of matter inside the orbit of Amalthea. Fortunately, mission controllers had preprogrammed the camera shutter to remain open for 11.2 minutes on the remote chance—no one took the possibility very seriously—that Jupiter had some kind of ring. To everyone's amazement, Voyager's time exposure produced a streaky image that the scientists could explain only as a ring of boulder-size debris. The findings seemed so unlikely that the NASA team delayed making the information public for several days while the data were checked and rechecked. Saturn was long the only planet known to have rings and considered to be the only one that could have them. In 1977 that theory was shattered with the discovery of rings around the planet Uranus. Jupiter itself was surveyed earlier by the Pioneer 10 and 11 spacecraft, but it is easy to see why no Jovian ring was found. Jupiter's is almost paper thin, perhaps 1 km (0.6 miles) high, and impossible to view from earth.
Like other successful space probes, Voyager 1 has raised as many questions as it answered. It managed to look deep into Jupiter's Great Red Spot but provided no explanation for what causes this huge, hurricane-like storm center. Yet scientists are convinced that the $400 million mission will pay off in valuable new insights into the solar system. As Caltech's Edward Stone points out, "We may learn something about the evolution of the earth and where it is going."
Voyager is now going to keep a November 1980 date with Saturn. After that it will head farther out into space. Though its nuclear-powered instruments will no longer be functioning, it bears tidings from earth: a golden record that will play greetings in 60 languages—if anyone out there is willing to listen.