麻豆传媒

鈥淪uperbolts鈥� of lightning burst in erratic thunderstorms on Saturn, reveals new data from the Cassini spacecraft, challenging previous observations of static storms.

The orbiting spacecraft has observed unpredictable storm patterns, not observed by the Voyager spacecraft more than 20 years ago. Cassini has also revealed a new radiation belt just above the planet鈥檚 cloud tops 鈥� far closer than any previously observed radiation belts around the ringed planet.

Lightning produces radio emissions as well as visible light, and in 1980 and 1981, NASA鈥檚 two Voyager spacecraft each spent a few days listening to Saturn鈥檚 storms during flybys. The storms seemed eerily unchanging, firing off the same number of lightning bursts as they whipped around the planet every 10 hours.

Now, astronomers eavesdropping with the US-European spacecraft Cassini, which went into orbit around Saturn on 1 July 2004, are hearing a different story.

鈥淲e鈥檙e seeing lightning strokes during some rotations but not others 鈥� sometimes they seem very intense, others not,鈥� says Bill Kurth, an astronomer at the University of Iowa in Iowa City, US, who is studying the storms with Cassini鈥檚 radio and plasma wave instrument.

The storms appear to take 40 minutes longer to travel around Saturn than those 24 years ago. That suggests they are riding slower jet streams at higher latitudes, which hints at a cause in the planet鈥檚 famous rings.

Sun-kissed areas

Saturn orbits the Sun every 29 years, and as it does so, the 27-degree tilt of its axis make the rings cast different shadows on the planet. Today, it is summer in Saturn鈥檚 southern hemisphere, and the rings cast a widespread shadow across the planet鈥檚 northern face.

But during the Voyager flybys, the Sun hit the rings nearly edge-on so that Saturn鈥檚 equator was constantly shaded. The drastic temperature difference between the cool shaded belt and the surrounding sun-kissed areas may have fuelled regular convection patterns that drove Saturn鈥檚 static storms, says Kurth. But he cautions the theory is just a 鈥渉unch鈥�.

鈥淚 can think of other theories,鈥� agrees Andrew Ingersoll, a planetary scientist at the California Institute of Technology who has studied Jupiter鈥檚 lightning storms.

鈥淭here鈥檚 a certain element of luck when you talk about weather. It鈥檚 possible there were no equatorial storms during the past few months but there was one when Voyager went by.鈥�

Water and ice

鈥淪ometimes you get fooled observing over short periods of time,鈥� concedes Kurth. 鈥淭hat鈥檚 one of the advantages of being in orbit and hanging around so you can see if the storm systems evolve over months or years.鈥� At the end of Cassini鈥檚 mission in 2008, the rings鈥� orientation will be similar to that seen by the Voyagers, so Kurth can test his shadow theory.

Jupiter鈥檚 storms, which may be similar to Saturn鈥檚, span about 1000 kilometres and feature 鈥渟uperbolts鈥� of lightning about three times brighter than the brightest bolts on Earth.

But aside from the storms鈥� super size, they are probably quite similar to Earth鈥檚 鈥� born of charge separation between water and ice in clouds and driven by convection.

The rain clouds are buried 80 kilometres into the atmospheres of Jupiter and Saturn, but astronomers have seen faint flashes from lightning on Jupiter鈥檚 dark side. Saturn鈥檚 rings reflected too much sunlight to reveal the flickers to the Voyager probes, but imaging instruments on Cassini may be able to capture them.