23:16 09 November 2006
NewScientist.com news service
Maggie McKee

A hurricane-like storm two-thirds as wide as the Earth is raging on Saturn's south pole, new images from the Cassini spacecraft reveal. Such clear hurricane-like features have never before been seen on any other planet, but scientists are not sure what is causing them.

The dark eye of the "hurricane" spans about 8000 kilometres and is surrounded by rings of clouds that tower about 30 to 75 kilometres above it. Watch a 0.6 mB video of Saturn's storm taken over a period of three hours.

These eye-wall clouds have never been seen anywhere other than on Earth, where they form in a process of convection when moist air flows across an ocean and rises. They drop rain in a ring around a region of falling air, which is the eye of a hurricane.

But Saturn's storm also differs from hurricanes on Earth because it is fixed in place – above the south pole – and is not powered by an ocean, since Saturn is a gaseous planet.

"It looks like a hurricane, but it doesn't behave like a hurricane," says Andrew Ingersoll, a member of Cassini's imaging team at Caltech in Pasadena, US. "Whatever it is, we're going to focus on the eye of the storm and find out why it's there."

It is unclear how long the storm has been there because Cassini has never before seen the pole at such a high resolution. And scientists are still puzzling over how it formed. 

SOURCE: New Scientist 

23:16 09 November 2006
NewScientist.com news service
Maggie McKee
   
Warm spot

"I don't think we really have a good idea of what's sourcing it," says Cassini team member Richard Achterberg, a planetary scientist specialising in atmospheres at NASA's Goddard Space Flight Center in Greenbelt, Maryland, US.

The storm may be a seasonal phenomenon that changes over the course of Saturn's year, which lasts 29 Earth years. It is currently summertime in the planet's southern hemisphere, and both ground-based observations and higher-resolution data from Cassini's Composite Infrared Spectrometer (CIRS) show the pole is about 2° Celsius warmer than its immediate surroundings.

Mission members will try to observe the storm again with CIRS to study the chemical composition of the atmosphere at the pole. "We're going to see if there's water or something being dragged up from below," Achterberg told New Scientist. In particular, the team will look for changes in the storm over the next few years, as the southern hemisphere moves into autumn.

Achterberg says the storm appears to be unique in the solar system. Jupiter's famous storm, the Great Red Spot, for example, does not have an eye or any surrounding eye-wall clouds. It also drifts slowly around the planet with the winds and is colder than its surroundings, while Saturn's storm is warmer.

Whatever its cause, Achterberg says the most striking thing about the storm is simply its appearance. "When you look at it, the shape of those clouds surrounding the pole ... it's an amazing image."

SOURCE: New Scientist 

NASA's Cassini spacecraft has seen something never before seen on another planet -- a hurricane-like storm at Saturn's south pole with a well-developed eye, ringed by towering clouds.

The "hurricane" spans a dark area inside a thick, brighter ring of clouds. It is approximately 8,000 kilometers (5,000 miles) across, or two thirds the diameter of Earth.

"It looks like a hurricane, but it doesn't behave like a hurricane," said Dr. Andrew Ingersoll, a member of Cassini's imaging team at the California Institute of Technology, Pasadena. "Whatever it is, we're going to focus on the eye of this storm and find out why it's there."

A movie taken by Cassini's camera over a three-hour period reveals winds around Saturn's south pole blowing clockwise at 550 kilometers (350 miles) per hour. The camera also saw the shadow cast by a ring of towering clouds surrounding the pole, and two spiral arms of clouds extending from the central ring. These ring clouds, 30 to 75 kilometers (20 to 45 miles) above those in the center of the storm, are two to five times taller than the clouds of thunderstorms and hurricanes on Earth.

Eye-wall clouds are a distinguishing feature of hurricanes on Earth. They form where moist air flows inward across the ocean's surface, rising vertically and releasing a heavy rain around an interior circle of descending air that is the eye of the storm itself. Though it is uncertain whether such moist convection is driving Saturn's storm, the dark "eye" at the pole, the eye-wall clouds and the spiral arms together indicate a hurricane-like system.

Distinctive eye-wall clouds had not been seen on any planet other than Earth. Even Jupiter's Great Red Spot, much larger than Saturn's polar storm, has no eye or eye-wall and is relatively calm at the center. 

This giant Saturnian storm is apparently different fromhurricanes on Earth because it is locked to the pole and does not drift around. Also, since Saturn is a gaseous planet, the storm forms without an ocean at its base.

In the Cassini imagery, the eye looks dark at infrared wavelengths where methane gas absorbs the light and only the highest clouds are visible.

"The clear skies over the eye appear to extend down to a level about twice as deep as the usual cloud level observed on Saturn," said Dr. Kevin H. Baines of Cassini's visual and infrared mapping spectrometer team at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "This gives us the deepest view yet into Saturn over a wide range of wavelengths, and reveals a mysterious set of dark clouds at the bottom of the eye."

Infrared images taken by the Keck I telescope in Mauna Kea, Hawaii, had previously shown Saturn's south pole to be warm. Cassini's composite infrared spectrometer has confirmed this with higher-resolution temperature maps of the area. The spectrometer observed a temperature increase of about 2 Kelvin (4 degrees Fahrenheit) at the pole. The instrument measured high temperatures in the upper troposphere and stratosphere, regions higher in the atmosphere than the clouds seen by the Cassini imaging instruments.

"The winds decrease with height, and the atmosphere is sinking, compressing and heating over the South Pole," said Dr. Richard Achterberg, a member of Cassini's composite infrared spectrometer team at NASA's Goddard Spaceflight Center, Greenbelt, Md.

Observations taken over the next few years, as the south pole season changes from summer to fall, will help scientists understand the role seasons play in driving the dramatic meteorology at the south pole of Saturn. 

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 was designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo. The visual and infrared mapping spectrometer team is based at the University of Arizona. The composite infrared spectrometer team is based at Goddard.

For a movie, high-resolution images, infrared images and Saturn temperature maps, visit: http://saturn.jpl.nasa.gov, http://www.nasa.gov/cassini and http://ciclops.org .

Carolina Martinez (818) 354-9382
Jet Propulsion Laboratory, Pasadena, Calif.

Erica Hupp/Dwayne Brown (202) 358-1237/1726
NASA Headquarters, Washington

Preston Dyches (720) 974-5859
Cassini Imaging Central Laboratory for Operations
Space Science Institute, Boulder, Colo.

2006-137

SOURCE: NASA Sees into the Eye of a Monster Storm on Saturn

See Also:

• Saturn's Surprisingly Stormy South  11.09.06
• The Hole at the Pole - 11.09.06