September 4, 2013
Cassini Uncovers New Data About Saturn’s Atmospheric Composition
[ Watch the Video: Saturn Storm Reveals Its Explosive Power ]
Lee Rannals for redOrbit.com - Your Universe OnlineA new analysis of near-infrared images taken by NASA's Cassini orbiter is providing more data about the composition of Saturn's atmosphere.
Scientists, publishing a paper in the journal Icarus, say they have discovered that cloud particles at the top of Saturn's great storms are composed of a mix of three substances, including water ice, ammonia ice and an uncertain third constituent that could be ammonium hydrosulfide.
One Saturnian year is about 30 Earth years, and during this time a monster storm always rips across the northern hemisphere of the planet; the most recent witnessed was from 2010, when a large Saturn storm stretched more than 9,300 miles in width. This storm was so big that even amateur astronomers on Earth could view the great white spot across the surface of the planet.
The team analyzed Cassini's near-infrared color signatures in order to help determine the composition of the planet's atmosphere at depths typically obscured by a thick high-altitude haze.
"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 Lawrence Sromovsky, a senior scientist at University of Wisconsin-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."
Scientists say Saturn's atmosphere consists of layers, with a deck of water clouds at the bottom, ammonia hydrosulfide clouds in the middle, and ammonia clouds near the top. The upper tropospheric haze at the top contains an unknown composition that obscured nearly everything. However, the latest storm and Cassini's images helped the team unveil what sat below this layer.
The massive storm works like the small convective events on Earth, where air and water vapor are pushed high into the atmosphere, creating towering clouds of a thunderstorm. On Saturn, these clouds are much bigger and far more violent, with models predicting vertical winds of more than 300 miles per hour.
The latest study helps validate the models of Saturn's great storms as well as previous observations that detected water and ammonia in vapor form. Sromovsky says the presence of water supports the idea that Saturn's superstorms are powered by condensation of water and originate deep in the 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.
He said the interesting effect is that in Saturn's massive storm, 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. The team believes that water ice makes up 22 percent of the cloud head and ammonia ice 55 percent. The remaining fraction is made up of the third constituent -- likely 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.
This finding shows how Cassini is still able to uncover the secrets of Saturn and its moons.
"The new finding from Cassini shows that Saturn can dredge up material from more than 100 miles [160 kilometers]," said Kevin Baines, a co-author of the paper who works at the University of Wisconsin-Madison and NASA's Jet Propulsion Laboratory, Pasadena, 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."