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Saturn lightning superbolts revealed

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NASA's Cassini spacecraft has revealed new photographs of powerful lightning flashing on Saturn after steadily watching a particularly stormy part of the ringed planet, according to a new study.

Lightning, a common phenomenon on Earth, has been an elusive target on Saturn, at times only indirectly identified by previous studies. The new photos show a series of bright flashes against a cloudy backdrop.

"We can learn the brightness and total power from the images," the lead researcher Ulyana Dyudina told "The brightest lightning on Earth, called superbolts – they're very bright and very powerful — these are comparable in the amount of light that they emit to what we see on Saturn, and what we see on Jupiter as well."

In the past, lightning images have been difficult to obtain because the planet's nights are typically very bright due the highly reflective nature of Saturn's rings. The light that reflects off of these rings makes it tough to visually distinguish flashes of lightning, researchers said.

Yet, scientists who had been monitoring a section of the planet at latitude 35 degrees south —which is referred to as "storm alley" because of the high level of storm activity — had suggested the presence of thunder and lightning on Saturn.

Additionally, Cassini's radio instruments picked up on static noise that likely signaled lightning coming from storm alley.

The Cassini spacecraft was able to obtain the new images of lightning on Saturn in the period around August 2009, during the planet's equinox. For the duration of the equinox, most of Saturn's rings are in shadow, which makes it possible to detect lightning flashes. The findings of the study were published May 15 in the journal Geophysical Research Letters.

Preliminary images of the lightning, including a short video, were released earlier this year in April.

Superbolts of Saturn lightning
Cassini detected the visible lightning flashes on Saturn on Aug. 17, 2009. The images were obtained by Cassini's imaging science subsystem at a latitude of about 36 degrees south and longitude of 11 degrees west over a period of 13 minutes.

"We had been hearing and seeing the storm, but not lightning itself for five years," said Dyudina, who is a researcher in the Division of Geological and Planetary Sciences at California Institute of Technology in Pasadena, Calif. "Now we have actually seen the light from that lightning on Saturn's night side. And we took actual images of the lightning."

Through close analysis, the researchers determined that the lightning flashes are consistent with a single cloud flashing once per minute, and the visible energy of a single flash is actually comparable to that on Earth and Jupiter, where lightning has also been detected.

Furthermore, Cassini's data and images allowed the researchers to measure the lightning and track where it occurs in the storm cloud.

"The spots that are produced by lightning have several hundred kilometers width, and to produce such large diffuse spots, the lightning should be down below the clouds by about 100 to 62 to 124 miles," Dyudina said. "So, this means that the lightning happens deep into the water cloud."

This lightning, said Dyudina, has similar properties to the alien lightning on Jupiter that has been confirmed.

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But, the most marked difference is in the location of Saturn's thunderstorm itself.

"On Saturn, we have a very unique situation in which we have just one thunderstorm at latitude 35 degrees south," Dyudina said. "It's a very bright cloud — so bright that even amateur astronomers can see it from Earth."

The researchers noticed that the bright cloud has a tendency to appear, then disappears for a few months, then appears again at the same latitude and remains for another several months. In fact, the storm that generated the lightning that Cassini imaged lasted from January to October in 2009.

"The gaps between the storms can be several years," Dyudina said. "The longest storm was nine months. But, it's always at the same latitude and we see it on just the one cloud. This is very unique."

Gas giant lightning
In contrast, on Jupiter, lightning happens in different places, and the phenomenon has been imaged at different latitudes.

Further studies will attempt to help scientists better understand the formation and characteristics of this peculiar lightning on Saturn.

Dyudina and her colleagues are already in the midst of conducting more lightning searches on Saturn, monitoring the planet for bright clouds, and trying to understand why the thunder and lightning storm occurs in one particular place.

In the meantime, this study has implications for other planets, where lightning may one day also be detected.

"This means that there is heat stored in the planets after the planet formation, and the way the planet cools is through thunderstorms and lightning," Duydina said. "These bring lots of heat that is down below to higher levels, and it's more efficient than just radiation. This cooling is important for the evolution of planetary systems."