A small, bright feature that suddenly appeared in Saturn’s northern hemisphere late last year and grew to a diameter approaching Earth’s in a matter of weeks is a thunderstorm that’s still raging. The day it was first observed, the cloud measured about 1,500 miles across. Three weeks later, the storm system had grown to almost three times that distance across and sported a tail that eventually stretched around the planet. NASA’s Cassini probe, which has been circling Saturn since 2004, detected bursts of radio waves generated by lightning flashes that, at their peak, occurred at least 10 times per second, an international team of researchers reported this month.
The thermal energy released by the storm rivals that emitted by the entire planet in its quiet times, the scientists noted. Such storms, called “great white spots” due to their size and brightness, can be seen by Earth-based astronomers and occur on average every 30 years or so — approximately the length of a year on Saturn — but for unknown reasons this storm has appeared much earlier in the Saturnian spring than normal.
This article was produced by ScienceNOW, the daily online news service of the journal Science.