Cassini Shows Why Jet Streams Cross-Cut Saturn

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News feature: 2012-186 June 25, 2012

Cassini Shows Why Jet Streams Cross-Cut Saturn

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2012-186&cid=release_2012-186

Turbulent jet streams, regions where winds blow faster than in other places, churn east and west
across Saturn. Scientists have been trying to understand for years the mechanism that drives these
wavy structures in Saturn's atmosphere and the source from which the jets derive their energy.

In a new study appearing in the June edition of the journal Icarus, scientists used images collected
over several years by NASA's Cassini spacecraft to discover 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."

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.

By seeing for the first time how these eddies accelerate the jet streams at two different altitudes,
scientists found the eddies were weak at the higher altitudes where previous researchers had found
that most of the sun's heating occurs. The eddies were stronger deeper in the atmosphere. Thus, the
authors could discount heating from the sun and infer instead that the internal heat of the planet is
ultimately driving the acceleration of the jet streams, not the sun. The mechanism that best matched
the observations would involve internal heat from the planet stirring up water vapor from Saturn's
interior. That water vapor condenses in some places as air rises and releases heat as it makes clouds
and rain. This heat provides the energy to create the eddies that drive the jet streams.

The condensation of water was not actually observed; most of that process occurs at lower altitudes
not visible to Cassini. But the condensation in mid-latitude storms does happen on both Saturn and
Earth. Storms on Earth – the low- and high-pressure centers on weather maps – are driven mainly by
the sun's heating and do not mainly occur because of the condensation of water, Del Genio said. On
Saturn, the condensation heating is the main driver of the storms, and the sun's heating is not
important.

Images of one of the strongest jet streams and a figure from the paper can be found at
http://www.nasa.gov/cassini , http://saturn.jpl.nasa.gov and http://ciclops.org .

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the
Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of
Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission
Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed
and assembled at JPL. The imaging team is based at the Space Science Institute in Boulder, Colo.

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