Wednesday, March 2, 2011

Lightning Flashing on Saturn

This image from NASA's Cassini spacecraft -- the first of its kind -- shows lightning on Saturn's night side flashing in a cloud that is illuminated by light from Saturn's rings.

The cloud, whose longest dimension is about 3,000 kilometers (1,900 miles), does not change perceptibly over the 16 minutes of observations covered by the 10-second movie. The lightning flashes are the bright spots within the cloud, and are about 300 kilometers in diameter. The lightning strikes last for short periods of time (less than one second before the time line of the movie was compressed).

The energy output of the visible light from the lightning is comparable to the brightest lightning flashes on Earth.

At Saturn, there are three types of clouds that might produce lightning. The top layer is made of ammonia ice; the middle layer is made of a compound of hydrogen sulfide and ammonia; the bottom layer is water. The light has to diffuse up through this cloud system, which is over 100 kilometers (60 miles) thick. The width of the lightning spot at the top of the cloud is proportional to the depth where the flash originated. The observed widths indicate that the lightning is originating either in the hydrogen-sulfide-ammonia cloud or in the water ice cloud. The lightning does not appear to originate at the deepest levels of the cloud system, where water is liquid.

Interested in how early researchers came to understand lightning? PV Scientific offers reprints of classic texts on the subject of atmospheric electricity on our Classic Reprint Page.

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