NASA Cassini: Massive Saturn Storm Pulls Water and Ammonia Ices from the Depths

This series of images from NASA’s Cassini spacecraft shows the development of the largest storm seen on the planet since 1990. 

These true-colour and composite near-true-color views chronicle the storm from its start in late 2010 through mid-2011, showing how the distinct head of the storm quickly grew large but eventually became engulfed by the storm’s tail. 

Credit: NASA /JPL-Caltech /Space Science Institute

Once every 30 years or so, or roughly one Saturnian year, a monster storm rips across the northern hemisphere of the ringed planet.

In 2010, the most recent and only the sixth giant storm on Saturn observed by humans began stirring. It quickly grew to superstorm proportions, reaching 15,000 kilometers (more than 9,300 miles) in width and visible to amateur astronomers on Earth as a great white spot dancing across the surface of the planet.

Now, thanks to near-infrared spectral measurements taken by NASA's Cassini orbiter and analysis of near-infrared colour signatures by researchers at the University of Wisconsin-Madison, Saturn's superstorm is helping scientists flesh out a picture of the composition of the planet's atmosphere at depths typically obscured by a thick high-altitude haze.

The key finding: 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.

According to the Wisconsin researchers, the observations are consistent with clouds of different chemical compositions existing side-by-side, although a more likely scenario is that the individual cloud particles are composed of two or all three of the materials.

Writing in the current edition (Sept. 9, 2013) of the journal Icarus, a team led by UW-Madison Space Science and Engineering Center planetary scientists Lawrence Sromovsky, and including Kevin Baines and Patrick Fry, reports the discovery of the icy forms of water and ammonia.

Water in the form of ice has never before been observed on Saturn.

"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," explains 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 colour signature that is very different from the haze particles in the surrounding atmosphere."

"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 adds.

The interesting effect, he notes, is that in Saturn's massive storm, at least, the observations can be matched by having particles of mixed composition, or clouds of water ice existing side-by-side with clouds of ammonia ice.

In the latter scenario, water ice would make up 22 percent of the cloud head and ammonia ice 55 percent.

The remaining fraction would be made up by the third constituent, which though less certain, is believed to be ammonia hydrosulfide.

"Up until now, there have been no quantitative calculations of spectra for cloud structures and compositions that matched the observed spectrum of an actual storm cloud feature," says Sromovsky.

Journal Reference:
L.A. Sromovsky, K.H. Baines, P.M. Fry. Saturn’s Great Storm of 2010–2011: Evidence for ammonia and water ices from analysis of VIMS spectra. Icarus, 2013; 226 (1): 402 DOI: 10.1016/j.icarus.2013.05.043