Andrew Ingersoll, a member of the Cassini imaging team, told Astronomy
the imaging team used the spacecraft's view of Saturn's night side to search for lightning flashes illuminating the planet's clouds from below. "If it was totally dark, then any light at all would be from lightning," he says. The trouble is, Saturn's rings reflect a lot of light onto the planet, washing out any optical flashes from lightning.
Sunlight at Saturn is 100 times dimmer than at Earth due to the ringed planet's 10-times-greater distance from the Sun. Nevertheless, says Ingersoll, the light the rings cast onto Saturn's cloud tops is 10 times brighter than that of a Full Moon on Earth. But while "ringshine" obscured lightning flashes, it was bright enough to let Cassini's cameras make out atmospheric features and image the storm.
"Since we've been in orbit, we've detected maybe four or five such storms," explains Kurth. "This is by far the strongest that we've detected with Cassini. And it's even stronger and there's more activity than the storms that were detected by the two Voyager spacecraft back in the early 1980s."
The new storm lies in a region of Saturn's southern hemisphere scientists refer to as "storm alley" because of the numerous systems it has produced since Cassini's 2004 arrival. The storm spans about 2,175 miles (3,500 kilometers) — larger than Earth's Moon — in its north-south dimension.
"It's really the only large storm on the whole planet," Ingersoll notes. "It's in the right place and it appeared at the right time to match the radio emissions, so it has to be the right storm."
As revealed by Cassini's cameras, the storm bears a slight resemblance to a hurricane, with four spiral arms arcing toward its center. "Every day, those arms are changing," Ingersoll says. "It's very dynamic."
"With Cassini, we have learned that lightning storms can emerge suddenly and last for several weeks or even a month," says Georg Fischer, a postdoctoral researcher at the University of Iowa. "On the other hand, we have only observed a single smaller lightning storm throughout 2005, which is remarkably different compared to what we know about terrestrial thunderstorms."
"Saturn seems to do this," says Ingersoll. "It's quiet for a long time, and then 'burps' with a single giant storm."