In December 2010, amateur astronomers around the world noticed a growing bright spot on Saturn. Shortly afterwards, NASA's Cassini spacecraft captured the feature on camera: it was an enormous storm nearly as tall as Earth, churning away in Saturn's northern hemisphere and creating eddies of reflective, multi-hued clouds that wrapped almost entirely around the planet.
Now, further investigation by researchers from the University of Wisconsin-Madison and the University of Arizona has revealed that this monster storm not only created a spectacular display for scientists and space fans, but it also dredged up material from over 100 miles deep in Saturn's atmosphere, displaying water ice, ammonia ice, and "an uncertain third constituent that is possibly ammonium hydrosulfide."
The storm basically acted as a giant convective tower, similar to thunderstorms here on Earth except 20 times taller and much more powerful. It allowed scientists to get a look at components of Saturn's atmosphere that are usually hidden from view.
“We think this huge thunderstorm is driving these cloud particles upward, sort of like a volcano bringing up material from the depths and making it visible from outside the atmosphere,” said Lawrence Sromovsky, a senior scientist at UW-Madison and an expert on planetary atmospheres. “The upper haze is so optically pretty thick that it is only in the stormy regions where the haze is penetrated by powerful updrafts that you can see evidence for the ammonia ice and the water ice. Those storm particles have an infrared color signature that is very different from the haze particles in the surrounding atmosphere.”
This is the first confirmation of water ice in Saturn's atmosphere.
The observed presence of water ice helps to confirm that Saturn's storms are powered by the condensation of water deep in its atmosphere, about 120 miles (200 kilometers) down. As the water vapor rises, it freezes and collects other materials present in higher layers of Saturn's atmosphere.
“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,” Sromovsky said.
The planet-wrapping storm itself was an impressive event that typically only occurs one a Saturnian year -- which is 30 of our years -- and thanks to Cassini's extended missions and the Space Science Institute imaging team, this time around we got a front-row seat!
"It demonstrates in a very real sense that typically demure-looking Saturn can be just as explosive or even more so than typically stormy Jupiter," said Kevin Baines, a co-author of the paper who works at the University of Wisconsin-Madison and NASA's Jet Propulsion Laboratory.