There's a Ring, By Jupiter

And some startling moons too

He may have had his hands full with more down-to-earth problems last week, but even President Carter took time out to watch an otherworldly show as the Voyager 1 spacecraft made its closest approach to the giant planet Jupiter. Coming within 278,000 km (172,400 miles) of the swirling Jovian cloud tops, the robot survived intense radiation, peered deep into the planet's storm-tossed cloud cover, provided startling views of the larger Jovian moons and, most surprising of all, revealed the presence of a thin, flat ring around the great planet. Said University of Arizona Astronomer Bradford Smith: "We're standing here with our mouths open, reluctant to tear ourselves away."

There was every reason for exhilaration. As Voyager curved around the sun's largest planet at speeds up to 104,600 km (65,000 miles) per hour, the craft performed nearly flawlessly, its probing eyes and instruments shifting between Jupiter and its moons. As one startling picture after another flashed onto the screens at Caltech's Jet Propulsion Laboratory, even Cornell's irrepressible Carl Sagan was left nearly speechless. Said he: "This is almost beyond interpretation. There's different chemistry, different physics, different forces at work out there."

The close encounter lasted 39 tense hours, during which Voyager sent back enough data to fill up miles of magnetic tape and keep scientists busy for years ahead. But Voyager has already opened up new worlds for them. Ablaze with colors of every shade and hue, speckled with strange, often puzzling features, the Jovian moons prompted oohs and aahs from even the most seasoned scientists.

As the center of a kind of mini-solar system, Jupiter is surrounded by at least 13 moons, and possibly a 14th. The four largest--lo, Europa, Ganymede and Callisto--are the so-called Galilean moons (named after their discoverer). Like the earth's moon, they are large enough to be considered small planets, but appeared as little more than fuzzy blobs in earth-bound telescopes. Now, Voyager's cameras have found that these moons are not only complex but also markedly different, their surfaces varying greatly in age, composition and appearance. Observed the U.S. Geological Survey's Laurence Soderblom: "There is no such thing as a boring Galilean moon."

The surface of Callisto, the outermost of these moons, is riddled with craters, apparently the result of pummeling by meteorites for some 4 billion years. Although it is mountainless, Callisto has a feature never before seen in the solar system: a huge, smooth, circular basin rimmed with concentric ridges that look almost like a frozen tsunami (tidal wave). Appearances may not be entirely deceiving: the scientists speculated that these ridges were created when a particularly large meteorite hit, melted subsurface ice and caused the water to spread out from the place of impact, only to freeze rapidly again.

Neighboring Ganymede, like Callisto, is at least half composed of water and ice. It shows sinuous ridges and crisscrossing fractures that look like earthly fault lines--possibly caused by what Soderblom calls "water quakes." Ganymede's surface is less cratered than Callisto's and only a fourth its age, about 1 billion years.

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.

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