A storm on Saturn in 2004, which scientists believed to produce large lightning flashes due to its radio emissions.

Credit: Ulyana Dyudina/Caltech

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SYDNEY: A lightning storm on Saturn has been captured for the first time, proving the planet has Earth-like weather.

The Cassini spacecraft, which has been orbiting Saturn since June 2004, directly observed lightning during Saturn's equinox in August 2009, a team reported in the journal Geophysical Research Letters.

"The visible-light images tell us a lot about the lightning," said Ulyana Dyudina, planetary scientist at the California Institute of Technology in Pasadena, and lead author of the study.

Indirect evidence for lightning

"Now we can begin to measure how powerful these storms are, where they form in the cloud layer and how the optical intensity relates to the total energy of the thunderstorms," she said in a statement by NASA.

Scientists have suspected the existence of lightning on Saturn based on radio emissions termed Saturn Electrostatic Discharges (SEDs).

The Voyager missions, which passed Saturn in 1980 and 1981, and the Cassini spacecraft all detected such signals. SEDs are also correlated with convective-type clouds - like the updrafts that produce lightning storms on Earth.

'Ring shine' prevents measurement

Most of the year, however, sunlight reflects off of Saturn's rings, making its night side brighter than Earth under a full moon, said the researchers. This 'ring shine' makes the dark side of the planet too bright to detect lightning most of the time.

Researchers took advantage of one of the darkest night on Saturn last August. Around its equinox, sunlight primarily strikes the edges of the rings, making it dark enough to detect lightning optically.

Cassini's cameras captured flashed generated within a huge storm that roiled from January to October, 2009, at 35 deg S latitude.

The images allowed the scientists to compare the size, energy and frequency of Saturn's lightning to Earth's and Jupiter's.

The amount of energy in the lightning observed on Saturn is similar to the largest flashes on Earth, and comparable to Jupiter's.