From across the vast expanse of our solar system, gas giants Jupiter, Saturn, Neptune and Uranus appear serene. Their gaseous surfaces are unscarred by the meteor impacts that have gouged their rocky brethren in the inner solar system and the deep hues of browns, reds and blues misleadingly suggest a sense of calm.
In 2010 and 2011, Saturn once again gave us a demonstration of how far from the truth this portrayal is -- a giant storm 15,000 kilometres in width and 300,000 kilometres long churned up the northern hemisphere.
Nasa's space probe Cassini had an unprecedented front row seat on the action, and detected for the first time the presence of water ice in Saturn's atmosphere, according to a paper published in the September issue of Icarus.
Saturn's atmosphere is believed to consist of a series of layers of different types of gas, with the pressure and temperature increasing as you move closer to the core. However, direct measurements of the various layers is difficult, as the planet is obscured by the uppermost layer, a hazy gas of unknown composition.
When the giant storm of 2010/11 came along and churned everything up, scientists had an opportunity to peer beneath, through a gap in the haze. These storms occur once every 30 years, which is the same time it takes Saturn to orbit the Sun, and have been observed before. 2010 was the first time we were able to observe these giant storms from orbit.
Using near-infrared measurements from Cassini, which has orbited Saturn since 2004, the team was able to use spectral analysis -- matching missing wavelengths of light in data with corresponding elements -- to identify the different composite materials in the storm.
What they found demonstrated the power of those once-in-a-generation storms. The storm, with vertical winds of 500 kilometres an hour, reached hundreds of kilometres down into Saturn's hot lower atmosphere, scooping up water vapour and throwing it high into the cold upper atmosphere, where it froze and was detected as water ice.
In the swirling mix, Cassini also detected ammonia ice and a third constituent, possibly ammonia hydrosulphide. "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 of the University of Wisconsin-Madison, who led the study.