A particularly strong jet stream churns through Saturn’s northern hemisphere in this false-color view from NASA’s Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI |
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.
This figure examines a particularly strong jet stream and the eddies that drive it through the atmosphere of Saturn’s northern hemisphere. Image credit: NASA/JPL-Caltech/SSI |
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.
