Monster Storm on Saturn Dredges Up Water Ice

A super-massive storm which has been raging across Saturn since 2010 is so powerful it is churning up water in the form of ice from the depths of the ringed planet, according to the latest research on the cosmic weather event.

The storm, which is some 9,300 miles in width (about 15,000 kilometers) and stretches in a band encircling the planet at an expanse of about 190,000 miles (300,000 kilometers), is so big that amateur astronomers can see the great white band from Earth. Megastorms like this one are believed to occur in Saturn's northern hemisphere every 30 years or so, roughly once per Saturn year. This giant storm is only the sixth giant storm on Saturn to be observed by humans.

NASA's Cassini spacecraft, which has been orbiting Saturn for the past nine years, recorded the storm data using near-infrared measurements, finding that the storm is dredging up water ice and ammonia ice from deep in Saturn's atmosphere. It is the first time water ice has been detected in Saturn's atmosphere, according to a NASA report.

Saturn is what's known as a gas giant. Although it may have a solid core, the huge planet's exterior is not composed of rock or other solid matter, but rather an atmosphere of gases. The classic model of Saturn's atmosphere is one of layers, with water clouds at the bottom, a middle section of ammonia hydrosulfide clouds, and pure ammonia clouds near the top. Above these three layers is Saturn's upper troposhpere, the layer which gives the planet its haze.

"The new finding from Cassini shows that Saturn can dredge up material from more than 100 miles (160 kilometers)," said research co-author Kevin Baines, who works at the University of Wisconsin-Madison and NASA's Jet Propulsion Laboratory in California. "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."

Baines published the work in the journal Icarus along with colleagues, including co-author Lawrence Sromovsky, a planetary scientist at UW-Madison.

"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," Sromovsky said. "The upper haze is so optically 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."

Vertical winds at speeds of 300 miles per hour did their part in driving the water and ammonia ice upward.

"It starts at the water cloud level and develops a huge convective tower. It is similar to a big thunderstorm, only 10 to 20 times taller and covering an even greater area," Sromovsky explained.

Sromovsky said presence of water ice supports the idea that Saturn's superstorms are powered by condensation of water and originate deep in the atmosphere, about 200 kilometers below the visible cloud deck.

"The water could only have risen from below, driven upward by powerful convection originating deep in the 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 added.