Once every 30 years or so, a massive storm rages on Saturn, mixing up the atmosphere and revealing some of the ringed planet's hidden secrets.
The most recent Saturn mega-storm arrived about 10 years ahead of schedule and lasted from December 2010 through August 2011, said Lawrence Sromovsky, a planetary scientist at the University of Wisconsin-Madison.
The storm grew quickly from a small white dot first detected by NASA's Cassini probe on Dec. 5, 2010, to a spot about the size of Earth by the end of the month. By the end of January 2011, the storm had totally encircled the planet at a latitude of 30 degrees.
The mega-storm raged on for seven more months, releasing static electric charges indicative of lightning and causing fierce vertical winds that blew up to 300 mph.
And aside from putting on a gripping show, Sromovsky said the storm also allowed scientists a rare opportunity to glimpse the hidden layers of Saturn's thick atmosphere.
Scientists have hypothesized that Saturn's atmosphere is stacked like a layer cake, with water vapor clouds at the bottom, followed by a layer of ammonia hydrosulfide clouds and pure ammonia clouds near the top. But this structure has been difficult to observe because the surface of Saturn is obscured by a thick haze that is difficult to penetrate.
The storm temporarily changed all that. As the storm's fierce winds cut through the haze, Cassini was able to observe the cloud tops in near-infrared light, revealing a very different infrared color signature than the one produced by haze particles in the surrounding atmosphere.
Sromovsky and his team recently analyzed that data and discovered that cloud particles at the top of the storm were probably made of water ice, ammonia ice and a third substance that might be ammonium sulfide.
This is the first time that water ice has been detected on Saturn.
Sromovsky, who published his findings in the journal Icarus, said the water ice probably originated from a water vapor layer more than 100 miles below the visible clouds.
"The water could only have risen from below, driven upward by powerful convection originating deep in the atmosphere," he explained in a statement. "The water vapor condenses and freezes as it rises. It then likely becomes coated with more volatile materials like ammonium hydrosulfide and ammonia as the temperature decreases with their ascent."
But there is still plenty more to learn. For example, scientists don't know whether clouds of the different chemicals are sitting next to one another or whether individual cloud particles are made of two or three of these chemicals.
Sromovsky will be about 100 years old by the time Saturn is due for its next mega-storm, but if he is lucky enough to see it, he said he would love to get higher spacial resolution spectrometer measurements of the cloud tops and learn the answer.
"That would be lovely," he said.