Space News: Cassini shows why jet streams cross-cut Saturn

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.

Mill Fire estimated at 300 acres Saturday night; other fires burn around the region

NORTHERN CALIFORNIA – A fire in the Mendocino National Forest had reached an estimated 300 acres by Saturday night, according to forest officials.

The Mill Fire is burning near the Mill Valley Campground and Upper Letts Lake in Colusa County.

Mendocino National Forest officials reported that it was first spotted at about 3 p.m. Saturday and was estimated to have hit the 300-acre mark by 10 p.m.

Forest spokesperson Tamara Schmidt cautioned that the acreage is only an estimate until better maps are generated on the fire.

The U.S. Forest Service – with the assistance of Cal Fire – is working the fire, with Schmidt reporting that resources were continuing to make their way to the incident.

Reports from the scene indicated steep terrain that’s difficult for ground crews, with requests placed Saturday evening for additional air tankers and smokejumpers.

Twenty-five structures were threatened at the Board Camp summer home tract as well as the Mill Valley, Mill Creek and Letts Lake campgrounds, all of which Schmidt said were evacuated Saturday.

Visitors evacuating from the Letts Lake area on the west side of the fire are advised to take the Miner Ridge Road to Road M5 near Cedar Camp and follow M5 or County Road 42 out.

Schmidt said the road between Letts Lake and Fouts Springs, leading to Stonyford, is currently inaccessible, and forest visitors were asked to avoid the Fouts Springs and Davis Flat area due to increased fire traffic.

The cause of the fire remains under investigation, according to Schmidt.

In addition to assisting on the Mill Fire, Cal Fire personnel were engaged in fire suppression efforts around the state.

The Thunder fire, located off Highway 162 at Thunderhill Raceway near Willows in Glenn County, was fully contained on Saturday afternoon at 167 acres, Cal Fire reported.

Also on Saturday, the 140-acre Butte Fire – located off of Butte Mountain Road at Corning Road southwest of Corning in Tehama County – was 100-percent contained, as was the 1,200-acre Dale Fire southwest of Redding in Shasta County, according to Cal Fire.

Still not contained on Saturday were the Tin Can Canyon fire in San Diego County, at an initial size of 20 acres, and the Fish Fire in Inyo County, reported Saturday night to be 175 acres and 20 percent contained.

Email Elizabeth Larson at This email address is being protected from spambots. You need JavaScript enabled to view it. .

UPDATE: Mill Fire reaches 150 acres

NORTHERN CALIFORNIA – A fire burning in the Mendocino National Forest has reached 150 acres, according to a Saturday evening report from forest officials.

The Mill Fire is burning near the Mill Valley Campground and Upper Letts Lake in Colusa County, as Lake County News reported earlier Saturday evening.

Tamara Schmidt, a Mendocino National Forest spokesperson, said the fire was spotted on the forest at about 3 p.m. Saturday.

Shortly before 8 p.m., Schmidt reported that the fire had grown to 150 acres, with firefighters still in initial attack.

She said a Type 3 Incident Team was working on the fire Saturday night with resources scheduled to be coming in overnight and on Sunday.

Mill Valley, Mill Creek and Letts Lake campgrounds were evacuated, as well as the Board Camp summer home tract, Schmidt said.

The Mill Fire was threatening approximately 25 structures, including the campgrounds, according to Schmidt.

Schmidt said the road between Letts Lake and Fouts Springs, leading to Stonyford, is currently inaccessible.  

She said visitors evacuating from the Letts Lake area on the west side of the fire are advised to take the Miner Ridge Road to Road M5 near Cedar Camp and follow M5 or County Road 42 out.

Schmidt said visitors are being asked to avoid the Fouts Springs and Davis Flat area due to increased fire traffic.

Forest officials were not currently aware of any injuries associated with the Mill Fire, Schmidt said.

The cause of the fire is currently under investigation, Schmidt said.

Email Elizabeth Larson at This email address is being protected from spambots. You need JavaScript enabled to view it. .