What apparently are brilliant strikes of lightning in the thunderclouds over Jupiter were photographed by the Voyager spacecraft that flew by the giant planet last Monday.
No fewer than 19 bright strokes of light show up in a photograph of the night side of Jupiter taken when Voyager flew behind the planet on its way to a November, 1980, encounter with Saturn. The lights lie thousands of miles below the northern limb of the planet, which stands out as a bright air glow of auroral light 20,000 miles across.
"This is the largest aurora ever seen," Dr. Allan Cook of the Harvard-Smithsonian Center for Astrophysics said today at the Jet Propulsion Laboratory, from which Voyager is directed. "If these are lightning strikes we see below the aurora, they are the equivalents of the super-bolts one sees on Earth at the tops of tropical thunderstorms."
The photograph was taken with the spacecraft's 200-millimeter lens from a distance of 320,000 miles. Two exposures of 35 seconds each and a third exposure of 85 seconds were made to produce the single picture, which reveals eight strokes of light in one region, seven strokes in another and four in a third.
Dr. Cook said the light output alone from each strike is 10 billion joules, the equivalent of 100 million light bulbs of 100 watts each flashing for one second. He said he had no idea what on Jupiter triggers lightning of that intensity except to say the lightning came from what appeared to be three different sets of thunderstorms.
"We were surprised to see lightning this bright," Dr. Cook said. "We thought that Jupiter's lightning would be buried in the clouds and out of sight of the spacecraft."
Dr. Cook pointed out that there is a chance the lights seen by Voyager are not lightning strikes at all. He said they could be the arcs of auroral discharge, the results of protons and electrons escaping from the radiation fields around Jupiter and striking other charged particles in the planet's ionosphere.
"But if they are aurora features, they're extraordinarily bright," Dr. Cook said. "We would not expect auroral arcing of this intensity."
The role of lightning on Jupiter has long been a topic of debate among scientists, in part because of the dark reds, browns and yellows seen in the striped clouds of the planet. Lightning could be the source of energy that creates complex organic chemicals from the ammonia, methane, hydrogen and acetylene detected in its atmosphere.
"Lightning might well be the mechanism that brings out the sulfur and phosphorous we think are striping the clouds with color," said Dr. Garry Hunt of University College of London. "Some dramatic energy source must be at work to bring out the colors we see in the clouds."
In a fresh measurement of the ring found around the planet, Voyager scientists said they now beileve it is no thicker than 500 meters. This explains why it was never seen from Earth.