Cassini scientists: Mystery of Saturn’s jet streams solved

The debate was whether Saturn’s own internal heat – or energy from the sun – drives Saturn’s jet streams.

Moon is near planet Saturn and star Spica on June 27 and 28, 2012. More info here.

To the human eye, the giant planet Saturn doesn’t appear as colorful – or as distinctly banded – as its neighbor planet, Jupiter. Yet Saturn has bands traveling east and west across its surface, and scientists have come to see them as turbulent jet streams in the atmosphere of this gas giant world. For years, scientists have scratched their heads, trying to understand what energy source drives Saturn’s jet streams. In June 2012, in the journal Icarus, they suggest that heat from within Saturn drives the jet streams.

Saturn’s jet streams are curious and yet reminiscent of earthly jet streams. Most blow eastward on Saturn, but some blow westward. Saturnian jet streams occur in places where temperature varies significantly from one latitude on Saturn to another.

Saturn’s atmosphere and its rings are shown here in a false color composite made from three images taken in near infrared light. You can see a particularly strong jet stream churning through Saturn’s northern hemisphere. Image credit: NASA/JPL-Caltech/SSI

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Tony Del Genio of NASA’s Goddard Institute for Space Studies in New York is lead author of the June 2012 paper on Saturn’s jet streams and a member of NASA’s Cassini spacecraft imaging team. His group used automated cloud tracking software to analyze the movements and speeds of clouds seen in hundreds of Cassini images from 2005 through 2012. These scientists say that condensation of water from Saturn’s internal heating leads to temperature differences in the atmosphere. The temperature differences create eddies, or disturbances that move air back and forth at the same latitude, and those eddies, in turn, accelerate the jet streams “like rotating gears driving a conveyor belt.”

Where is Cassini now?

Tony Del Genio said:

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.

To the human eye, Saturn does not appear as distinctly banded as it does in the false color image, above, or as the next planet inward, Jupiter. Yet, like Jupiter, Saturn is crossed by subtle bands, which are part of the planet’s weather. Image Credit: NASA/JPL/Space Science Institute

In other words, a competing theory assumed that the energy for the temperature differences in Saturn’s atmosphere came from our parent star, the sun. In fact, temperature differences in Earth’s atmosphere are driven by sunlight.

But there are profound differences between the atmospheres of Earth and Saturn. For one, Saturn is about 10 times farther from the sun than Earth. Plus Earth’s atmosphere is relatively thin, and lies atop a solid-and-liquid surface. In contrast, Saturn is a gas giant world, with nothing we can meaningfully call a surface.

So the mechanisms that create Saturn’s weather, including its jet streams, need not be the same as on Earth.

Saturn’s atmosphere is always changing, and the clouds at this latitude on the planet look different now than they did some years ago. Image credit: NASA/JPL-Caltech/SSI

Study co-author and imaging team associate John Barbara, also at the Goddard Institute for Space Studies, said:

… we’ve been able to extract nearly 120,000 wind vectors from 560 images, giving us an unprecedented picture of Saturn’s wind flow.

The team’s findings provide an observational test for existing models that scientists use to study the mechanisms that power the jet streams. In this way, they were able to pin down Saturn’s internal heat as the energy source of the planet’s jet streams.

Read more from NASA’s Jet Propulsion Laboratory

Deborah Byrd

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