Voyager II has found lightning striking across Saturn's rings, generating as much electricity in each strike as a large nuclear power plant does on Earth.
"What we have here is 100,000 times more powerful than terrestrial lightning," Dr. Joseph Romig said today at the Jet Propulsion Laboratory here, where the spacecraft's flight is directed. "We hear electrical discharges at Saturn that range from 100 to 1,000 megawatts." (A thousand megawatts is 1 million kilowatts.)
Romig, of Radiophysics Inc. in Boulder, Colo., identified the source of the lightning strikes as the planet's "B" ring.
"B" is the middle of Saturn's three largest rings, measuring 16,000 miles across and tinged with a reddish color. At least one scientist using Earth-bound telescopes has said that tinge is caused by traces of iron in the B ring.
"We don't see this lightning, we hear it like you hear the effects of lightning as static on your car radio when a thunderstorm is nearby," Romig said. "The discharge at Saturn is so powerful that we began to get its static on our radio antenna when the spacecraft was still 200,000 miles from the planet."
Romig said that Voyager II's radio antenna picked up thousands of lightning strikes as the spacecraft crossed the rings last week. He played a portion of a tape recording of the strikes as they were heard on Earth. The lightning came loud and clear through the background hum of the spacecraft antenna, sounding like snaps, crackles and pops every few seconds along the passage of the rings.
The lightning strikes occur along the same regions of the B ring where Voyager's cameras have observed dark spoke-like formations. These spokes appear each time the rings emerge from the shadow of the planet and disappear when the rings go back into the shadows.
"We have no information that would correlate the spoking with the lightning but it's hard to believe that the lightning discharge doesn't have something to do with the formation of the spokes," Romig said.
Romig said the source of the lightning is either the line-up of dust particles in the B ring or a moonlet inside the ring that is invisible to Voyager's cameras because it is too small -- no larger than six miles across.
Romig said the red color and the suggestion of iron in the B ring also suggest the dust and the still-unseen moonlet are made up at least in part of an iron-bearing mineral called magnetite, which could build up enormous amounts of static electricity inside the ring by interacting with Saturn's enormous magnetic field.
"We're talking about millions of volts of potential," Romig said. "We're talking about voltages so high they can even be discharged through a vacuum such as the one that exists in the region of the rings."
Photographs released today of the moon of Saturn called Tethys show it is marked by a huge crater whose floor rises in an arc shape as much as 40 miles higher than the ramparts of the walls of the crater.
Scientists said that the floor of the crater on Tethys is the only one seen so far in the solar system whose floor is higher than its rim.
"The inside of Tethys had to be mobile enough and warm enough so it could flow to the surface when this crater was formed," Dr. Laurance Soderblom of the U.S. Geological Survey said. "At some ancient time, this entire moon expanded, froze and cracked and the warm interior bubbled to the top and formed this egg shape above the crater's rim when it froze, too."
Voyager II has only one more photographic assignment at Saturn. On Friday, it will take photographs when it crosses the orbit of Phoebe, a dark-colored Saturn moon that's moving in the opposite direction of the planet's 16 other moons. Scientists believe Phoebe, orbiting Saturn at a distance of 8 million miles, is the nucleus of an errant comet.
After Friday, Voyager II heads for Uranus, the seventh planet out from the sun and one that has never been visited by a spacecraft from Earth. Less than half Saturn's size, Uranus is so little understood that scientists do not know how fast it rotates.