June 26, 2012
Saturn’s Jet Stream Process Revealed By Cassini
Lee Rannals for redOrbit.com
Scientists have been trying to understand the mechanism that drives Saturn's jet streams, which blow faster in other places, and churn east and west across the planet.
The jet streams in Saturn's atmosphere slice across the face of the planet and are visible to the human eye, and also visible at altitudes detectable by Cassini's near-infrared camera filters.
Scientists used images collected by Cassini to discover that heat from within the planet helps power the jet streams. Condensation of water from Saturn's internal heating led to temperature differences in the atmosphere.
The temperature differences created eddies that move back and forth at the same latitude, which in turn accelerate the jet streams like rotating gears that drive a conveyor belt, according to the scientists.
"We know the atmospheres of planets such as Saturn and Jupiter can get their energy from only two places: the sun or the internal heating," said Tony Del Genio of NASA's Goddard Institute for Space Studies, N.Y. in a press release. "The challenge has been coming up with ways to use the data so that we can tell the difference."
The study was made possible because Cassini has been in orbit around Saturn long enough to obtain a large number of observations required to see subtle patterns emerge from the day-to-day variations in weather.
"Understanding what drives the meteorology on Saturn, and in general on gaseous planets, has been one of our cardinal goals since the inception of the Cassini mission," Carolyn Porco, imaging team lead, said. "It is very gratifying to see that we're finally coming to understand those atmospheric processes that make Earth similar to, and also different from, other planets."
Saturn is a gas giant with a deep atmosphere that is layered with multiple cloud decks at high altitudes.
While most jet streams blow eastward, some blow westward, and they occur on Saturn in places where the temperature varies significantly from one latitude to another.
Scientists were able to use Cassini to observe Saturn's jet stream process at two different, low altitudes, for the first time.
One of the spacecraft's filters shows the upper part of the troposphere, which is a high layer of the atmosphere where Cassini sees thick, high-altitude hazes and where heating by the sun is strong.
Views through another filter show images deeper in Saturn's atmosphere, where solar heating is weak but closer to where weather on the planet originates.
In the new study, the scientists used automated cloud tracking software to analyze the movements and speeds of clouds that were seen in hundreds of Cassini images from 2005 through 2012.
"With our improved tracking algorithm, we've been able to extract nearly 120,000 wind vectors from 560 images, giving us an unprecedented picture of Saturn's wind flow at two independent altitudes on a global scale," co-author and imaging team associate John Barbara said.
NASA said that the team's findings help provide an observational test for existing models that scientists use to study the mechanisms that power the jet streams.
Scientists found during the study that the eddies were weak at the higher altitudes where previous researchers had found that most of the sun's heating occurs.
They were able to discount the heating from the sun, and infer instead that the internal heat of the planet is ultimately driving the acceleration of the jet streams.
NASA said the mechanism that best matched the observations involve internal heat from the planet stirring up water vapor from Saturn's interior. This water vapor condenses in some places as air rises, and releases heat as it creates clouds and rain.
The condensation of water was not actually observed, according to the space agency. It said most of that process takes place at lower altitudes that are not visible to Cassini.
Image 2 (below): This figure examines a particularly strong jet stream and the eddies that drive it through the atmosphere of Saturn's northern hemisphere. Data from NASA's Cassini spacecraft were used to create this figure. Image Credit: NASA/JPL-Caltech/SSI