What powers Saturn's turbulent jet streams revealed
Washington: Scientists have finally discovered the mechanism that drives turbulent jet streams in Saturn’s atmosphere and the source from which the jets derive their energy, thanks to NASA’s Cassini spacecraft.
Turbulent jet streams, regions where winds blow faster than in other places, churn east and west across Saturn and scientists have been trying for years to understand what drives these wavy structures.
Using images collected over several years by Cassini, they have found that the heat from within the planet powers the jet streams.
Condensation of water from Saturn’s internal heating led to temperature differences in the atmosphere. The temperature differences created eddies, or disturbances that move air back and forth at the same latitude, and those eddies, in turn, accelerated the jet streams like rotating gears driving a conveyor belt.
A competing theory had assumed that the energy for the temperature differences came from the sun. That is how it works in the Earth’s atmosphere.
“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. The challenge has been coming up with ways to use the data so that we can tell the difference,” said Tony Del Genio of NASA’s Goddard Institute for Space Studies, N.Y., the lead author of the paper and a member of the Cassini imaging team.
The new study was possible in part because Cassini has been in orbit around Saturn long enough to obtain the 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,” said Carolyn Porco, imaging team lead, based at the Space Science Institute, Boulder, Colo.
“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,” Porco noted.
Rather than having a thin atmosphere and solid-and-liquid surface like Earth, Saturn is a gas giant whose deep atmosphere is layered with multiple cloud decks at high altitudes. A series of jet streams slice across the face of Saturn visible to the human eye and also at altitudes detectable to the near-infrared filters of Cassini’s cameras. While most blow eastward, some blow westward. Jet streams occur on Saturn in places where the temperature varies significantly from one latitude to another.
Thanks to the filters on Cassini’s cameras, which can see near-infrared light reflected to space, scientists now have observed the Saturn jet stream process for the first time at two different, low altitudes.
One filtered view shows the upper part of the troposphere, 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 capture images deeper down, at the tops of ammonia ice clouds, where solar heating is weak but closer to where weather originates. This is where water condenses and makes clouds and rain.
In the new study, which is a follow-up to results published in 2007, the authors used automated cloud tracking software to analyze the movements and speeds of clouds 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,” said co-author and imaging team associate John Barbara, also at the Goddard Institute for Space Studies.
The team’s findings provide an observational test for existing models that scientists use to study the mechanisms that power the jet streams.
The results appeared in the June edition of the journal Icarus.