NASA/JPL

Stormy spokes.
Dark radial features in Saturn's rings may be caused by thunderstorms.

Saturn's Spokes: Spawned by Storms?

PRESTON, U.K.--Dark, radial spokes in the rings of Saturn have puzzled planetary astronomers ever since they were discovered by the Voyager spacecraft in the early 1980's. Today, at the Royal Astronomical Society National Astronomy Meeting here, scientists described how the enigmatic features could be caused by thunderstorms and lightning. "It's one of the best theories I've heard so far," says Carl Murray of Queen Mary University of London.

The spokes are clouds of electrostatically charged dust particles that float above and below the ring plane. But there's no consensus on how they form. Astronomers have suggested that meteorite impacts or solar wind particles may do the charging, but no single theory has been able to explain all the observed characteristics of the spokes, such as their locations, shapes, clustering behavior, and--most notably--their puzzling absence between October 1998 and September 2005.

Enter the thunderstorm model, proposed by Geraint Jones of the Mullard Space Science Laboratory in Dorking, U.K., and colleagues, and presented at the meeting by team member Christopher Arridge. According to this idea, energetic beams of electrons produced above these storms are transported to the rings by Saturn's magnetic field, where they charge the dust and lift it out of the ring plane. If the storms occur at approximately 43 degrees latitude north or south, the electrons end up in a part of the ring that rotates at the same speed as the planet, so spokes can build up. Their absence between 1998 and 2005--about one quarter of a Saturnian year--may be just a seasonal effect in the occurrence of thunderstorms at this particular latitude, the astronomers speculate.

The thunderstorm model nicely explains why spokes occur in groups, says spoke expert Colin Mitchell of the Space Science Institute in Boulder, Colorado. But, he says, it doesn't account for the very narrow spokes that are also observed. "Spokes seem to be a pretty complicated problem," says Mitchell. "We definitely don't have the final word yet." Confirmation could come from NASA's Cassini probe, which is orbiting Saturn. If a thunderstorm is seen at the same time and the right location, Mitchell says, "that would be an indicator that the model is a good starting point."

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