A monster storm that erupted on Saturn in late 2010 has already impressed researchers with its intensity and long-lived turbulence. Now, another facet of the storm’s explosive power has been revealed: its ability to churn up water ice from deep in Saturn’s atmosphere. This finding, derived from near-infrared measurements by Nasa’s Cassini spacecraft, is the first detection at Saturn of water ice.
“The new finding from Cassini shows that Saturn can dredge up material from more than 160 kilometres,” said Kevin Baines, a co-author of the paper published in the journal Icarus who works at the University of Wisconsin-Madison and Nasa’s Jet Propulsion Laboratory. “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.” Water ice, which originates from deep in the atmosphere of gas giants, doesn’t appear to be lofted as high at Jupiter.
Monster storms rip across the northern hemisphere of Saturn once every 30 years or so, or roughly once per Saturn year. The first hint of the most recent storm first appeared in data from Cassini’s radio and plasma wave subsystem on 5 December 2010. Soon after that, it could be seen in images from amateur astronomers and from Cassini’s imaging science subsystem. The storm quickly grew to superstorm proportions, encircling the planet at about 30 degrees north latitude for an expanse of nearly 300 000 kilometres.
The new paper focuses on data gathered by Cassini’s visual and infrared mapping spectrometer on 24 February 2011. The team found that cloud particles at the top of the great storm are composed of a mix of three substances: water ice, ammonia ice and an uncertain third constituent that is possibly ammonium hydrosulphide. The observations are consistent with clouds of different chemical compositions existing side-by-side, though it is more likely that the individual cloud particles are composed of two or all three of the materials.
The classic model of Saturn’s atmosphere portrays it as a layered sandwich of sorts, with a deck of water clouds at the bottom, ammonia hydrosulphide clouds in the middle, and ammonia clouds near the top. Those layers are just below an upper tropospheric haze of unknown composition that obscures almost everything.
But this storm appears to have disrupted those neat layers, lofting up water vapour from a lower layer that condensed and froze as it rose. The water ice crystals then appeared to become coated with more volatile materials like ammonium hydrosulphide and ammonia as the temperature decreased with their ascent, the authors said.
“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 Sromovsky. “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 colour signature that is very different from the haze particles in the surrounding atmosphere.”
In understanding the dynamics of this Saturn storm, researchers realised that it worked like the much smaller convective storms on Earth, where air and water vapour are pushed high into the atmosphere, resulting in the towering, billowing clouds of a thunderstorm. The towering clouds in Saturn storms of this type, however, were 10 to 20 times taller and covered a much bigger area. They are also far more violent than an Earth storm, with models predicting vertical winds of more than about 500 kilometres per hour for these rare giant storms.