Cassini shows why jet streams cross-cut Saturn

Jun 25, 2012
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 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 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 ," 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 .

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.

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User comments : 5

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Noumenal
1 / 5 (1) Jun 25, 2012
"or disturbances that move air back and forth at the same latitude"

Urm, I seriously doubt that that is "air" which is moving about.
Silverhill
4.5 / 5 (2) Jun 25, 2012
If you restrict the definition of "air" to "the atmospheric gases surrounding Earth", you would be right. It's a bit cumbersome, however, to keep referring to "the atmospheric gases" of another planet; "air" is perfectly understandable in context.
Noumenal
not rated yet Jun 26, 2012
ohh, well, yes, I was referring to definition. I think it was a reasonable point, no need to vote it to a 1.

Unfortunately some people lack the right kind of edumacation to even understand that there is a difference and how so, and for others, it takes a way a point that could be interesting for further study, 'what are "the atmospheric gases" of Saturn?'

In a way it is a lazy covering up of a phenomenon. Especially considering it was only used twice, and the second time 'gases' would have sufficed.

So, I disagree with your point.

Silverhill
not rated yet Jun 26, 2012
Consider "air" to have been a loose usage, then, for which the writer may well be forgiven.
(I don't know why someone would give you 1 star for your remark; such a ranking was quite unnecessary....)
GSwift7
not rated yet Jun 27, 2012
ohh, well, yes, I was referring to definition. I think it was a reasonable point, no need to vote it to a 1.


and

(I don't know why someone would give you 1 star for your remark; such a ranking was quite unnecessary....)


If you click on your own name in the comments, you can see who has rated your comments. The 1/5 you recieved came from a user named "Parsec".

I would suggest that you just ignore the star ratings on comments. They are meaningless because there are several people who use multiple accounts to up-rate themselves and down-rate people who disagree with them. You may have received that 1/5 because you disagreed with Parsec on some other thread. The trolls can click on your name and go find all of your recent comments, then give them all 1/5 without even reading them. I'm no expert, but that seems like some kind of antisocial or perhaps sociopathic problem. A couple of them are a bit scary sometimes.