Saturn's spokes may be made by lightning
Thunderstorms on the giant planet may cause puzzling streaks in its rings.
The mysterious spokes in Saturn's rings may be created by massive thunderstorms in the planet's atmosphere.
If the theory is right, these faint features are the signature of awesome events: lightning strokes ten thousand times more energetic than those on Earth, releasing beams of electrons that surge up from Saturn's surface to whack into the rings and blast out jets of electrically charged dust.
The idea, proposed by Geraint Jones of the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, and his colleagues in Geophysical Research Letters1, remains speculative. No one has ever seen storm-induced electron beams on Saturn. But the researchers say that the theory would explain some puzzling features of the spokes, and that it fits with what is known about the effects of thunderstorms on Earth.
The spokes of Saturn's rings were first discovered by the Voyager I and II spacecraft when they passed close to the gas giant planet in 1980 and 1981. The spokes are "a very complex phenomenon, and a difficult thing to untangle", says Carolyn Porco of the Space Science Institute in Boulder, Colorado, who was the first person to work on these mysterious features back in the Voyager days.
The spokes appear as bright or dark streaks about 100 km wide and up to 20,000 km long that cross the rings, making them look as though they have been smeared by a giant finger. They are transient, building up over a period of a few hours and then disappearing again several hours later. And they aren't always there at all: between 1998 and 2004, when NASA's Cassini spacecraft reached the planet, there was no sign of any. Spokes reappeared in September 2005, when Cassini took snapshots of them.
Researchers agree that the spokes are made of dust grains that have become electrically charged and are pushed up out of the rings in long streamers by electrostatic repulsion. But they don't know what causes the charging.
One theory is that this happens when meteorites plough into the rings and whip up a cloud of charged plasma, which then charges up the rings' dust particles. But this, says Jones, can't explain why the spokes build up gradually it's implausible that several meteorites would strike the rings in the same place in close succession.
An alternative explanation is based on the way that thunderstorms on Earth can generate electron beams in the upper atmosphere. While regular lightning bolts travel between clouds and the ground below, high-energy particles streaming from space called cosmic rays can trigger electron discharges above the clouds. These electrons stream outwards into space along the field lines of the Earth's magnetic field. This can produce flashes of gamma rays as well as glowing atmospheric features called sprites.
Thunderstorms are also known to happen within the thick atmosphere of Saturn, and Jones and colleagues say that these are likely to have the same effect cosmic rays coming in towards Saturn should likewise form outgoing electron beams that could hit the rings, charging up dust grains.
Hit it again
The magnetic field lines above the Saturnian latitudes from 43-52° N and from 38-46° S would carry the electron beams into Saturn's so-called B ring, where the dust and pebbles rotate at the same rate as the planet itself. This means a storm would keep whacking the same part of the ring over and over again, allowing a spoke to build up. Storms at other latitudes would hit different parts of a ring as it moves above the planet's surface, so these spokes would be quite diffuse. The researchers say that spokes in the B ring do indeed seem sharper than those that spring up elsewhere.
Porco says that there was a suggestion in the Voyager data that Saturn's spokes happen with about the same frequency as the planet's magnetic field and storms, so there's tentative evidence for a link. "It's a good thing that this connection is now being investigated," she says. She is currently analysing the Cassini results to search for a firmer link.
So far, Jones and colleagues can only point to tantalizing hints of more direct evidence: Cassini did detect two beams of electrons while orbiting Saturn that could have been unleashed by thunderstorms beneath, they say.
Firmer support for the hypothesis could come from the detection of thunderstorms at locations on the surface where they would be linked by magnetic field lines to spokes above. But this won't be easy. "I don't know exactly what a thunderstorm on Saturn looks like," Jones admits, "and I'm not sure anyone else does either."
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- Jones G. H., et al. Geophys. Res. Lett., 33 . L21202 (2006).
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