Saturn’s Southern Bullseye

Saturn’s Southern Bullseye

Saturn’s rotating atmosphere creates distinct bands owing to different rotation rates. Image Credit: NASA/JPL

Saturn’s southern polar region exhibits concentric rings of clouds which encircle a dark spot at the pole. To the north and toward the right, wavy patterns are evident, resulting from the atmosphere moving with different speeds at different latitudes. A surprising feature of Saturn’s magnetic field is that the magnetic pole appears to lie exactly atop the north and south geographical poles. This is unlike Earth and Jupiter where there is a large tilt between the planet’s axis and the dipole axis, meaning that Earth’s magnetic north is not located at the north pole.

The Cassini team also hope to gather data that establishes precisely how long Saturn takes to rotate.

"At the moment we know it’s ten and a half hours, but our error is seven seconds either way," says Dr Dougherty. "Over two years of observations that makes an error in longitude of 70 degrees. So in a sense we’re trying to establish a reference frame of where zero degrees longitude is. This will mean we can accurately time the rotation and, intriguingly, establish whether the interior of Saturn is rotating at the same rate as the exterior." Cassini carries 12 instruments that will study the planet, rings and moons in extensive detail.

The image was taken with the Cassini spacecraft narrow angle camera on July 13, 2004, from a distance of 5 million kilometers (3.1 million miles) from Saturn, through a filter sensitive to wavelengths of infrared light centered at 889 nanometers. The image scale is 29 kilometers (18 miles) per pixel. Contrast has been enhanced slightly to aid visibility. Through a small telescope, Saturn is normally visible as a creamy yellow ‘star’. You may be able to see the ring system that the planet is famous for.

The weather report on Saturn is unlike any terrestrial equivalent, as a typical day on the ringed world features atmospheric hurricanes the size of Earth and liquid helium rain. The average temperature varies depending on height in the gas giant but plunges to a frigid minus 227 degrees Fahrenheit typically. Air pressure at the top of the atmosphere rivals the best vacuum achievable in a laboratory but increases to crushing densities just below the cloud tops.

Science operations continue for Cassini during its four-year nominal mission and 76 orbits of the ringed planet. Until Cassini, never had a spacecraft been put in orbit around Saturn and not since Pioneer 11 and Voyagers 1 and 2 in 1981 has one passed so close. At the end of this year, ESA’s Huygens spaceprobe will be descending through the atmosphere of Saturn’s largest moon, Titan, becoming the first spacecraft to land on a body in the outer Solar System.

Among its mission milestones logged so far in getting to Saturn over seven years, the probe has flown by Venus (within 176 miles in April and June, 1998) and Earth (within 727 miles in August, 1999). Both these gravitational boosters pushed up the craft’s speed by tens of thousands of miles per hour. Cassini was only the seventh probe to pass through the rocky asteroid belt between Mars and Jupiter (April 2000). In mid-June 2004, Cassini performed the closest flyby of the Saturnian moon, Phoebe, in hopes of revealing its dark-light reflection and its icy surface, while continuing on its tour of the rings and moons.

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 Office of Space Science, Washington, D.C. 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, Boulder, Colo.

Related Web Pages

Space Science Institute
Chronology of a Scientific Safari
Ring Recycling
Lord of the Rings
Long, Strange Trips
Titan’s Icy Bedrock
Saturn– JPL Cassini Main Page
Alien Landers: Extreme Explorers Hall of Fame