Saturn in X-Rays

The first clear detection of X-rays from the giant, gaseous planet Saturn has been made with NASA’s Chandra X-ray Observatory. Chandra’s image shows that the X-rays are concentrated near Saturn’s equator, a surprising result since Jupiter’s X-ray emission is mainly concentrated near the poles. Existing theories cannot easily explain the intensity or distribution of Saturn’s X-rays.

Scientists would like to know more about the origin of the ring ‘spokes’ imaged on Saturn by Voyager, but not so far by the current Cassini images. Image Credit: Cassini Imaging Team/Boulder/

Chandra observed Saturn for about 20 hours in April of 2003. The spectrum, or distribution with energy of the X-rays, was found to be very similar to that of X-rays from the Sun.

"This indicates that Saturn’s X-ray emission is due to the scattering of solar X-rays by Saturn’s atmosphere," said Jan-Uwe Ness, of the University of Hamburg in Germany and lead author of a paper discussing the Saturn results in an upcoming issue of Astronomy & Astrophysics. "It’s a puzzle, since the intensity of Saturn’s X-rays requires that Saturn reflects X-rays fifty times more efficiently than the Moon."

The observed 90 megawatts of X-ray power from Saturn’s equatorial region is roughly consistent with previous observations of the X-radiation from Jupiter’s equatorial region. This suggests that both giant, gaseous planets reflect solar X-rays at unexpectedly high rates. Further observations of Jupiter will be needed to test this possibility.

The weak X-radiation from Saturn’s south-polar region presents another puzzle (the north pole was blocked by Saturn’s rings during this observation). Saturn’s magnetic field, like that of Jupiter, is strongest near the poles.

Disperse X-ray image from Chandra superimposed on the visible outline for comparison with polar axis and ring system. Image Credit: Chandra

X-radiation from Jupiter is brightest at the poles because of auroral activity due to the enhanced interaction of high-energy particles from the Sun with its magnetic field. Since spectacular ultraviolet polar auroras have been observed to occur on Saturn, Ness and colleagues expected that Saturn’s south pole might be bright in X-rays. It is not clear whether the auroral mechanism does not produce X-rays on Saturn, or for some reason concentrates the X-rays at the north pole.

"Another interesting result of the observation is that Saturn’s rings were not detected in X-rays," noted Scott Wolk of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA, a coauthor of the paper. "This requires Saturn’s rings to be less efficient at scattering X-rays than the planet itself."

The same team detected X-radiation from Saturn using the European Space Agency’s XMM-Newton Observatory. Although these observations could not locate the X-rays on Saturn’s disk, the intensity of the observed X-rays was very similar to what was found with Chandra and consistent with a marginal detection of X-rays from Saturn reported in 2000 using the German Roentgensatellite ( ROSAT).

What’s Next

On May 18, the space probe named Cassini officially enters the Saturn planetary system.

On that day, the gravitational pull of Saturn begins to overtake the influence of the Sun and the probe crosses the outer limits of the most distant group of Saturnian moons, only weakly bound to Saturn and located tens of millions of kilometers from the planet. A few days later, a 3.5 week long campaign of Titan movie sequences commences to measure atmospheric winds and begin mapping its surface as Titan rotates. On June 11, Cassini makes a close, 2000-kilometer approach to Phoebe, collecting detailed images during the encounter. At 220 kilometers across, Phoebe is the largest of Saturn’s outer moons and believed to be a captured asteroid.

The seven year voyage will end when Cassini’s main engine is fired, the spacecraft is slowed, and the probe enters Saturn orbit on July 1, 2004.

"We very much want everyone to enjoy Cassini’s tour of this magnificent planetary system," said Dr. Carolyn Porco, leader of the Cassini imaging science team at the Space Science Institute in Boulder, Colo. "And I can say right now the views out the window will be stunning," Porco said.

The research team, which used Chandra’s ACIS instrument to observed Saturn, also included J. Schmitt (Univ. of Hamburg) as well as Konrad Dennerl and Vadim Burwitz (Max Planck Institute, Garching Germany). NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for NASA’s Office of Space Science, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Related Web Pages

JIMO Science, JPL
Titan in a Test Tube
Primordial Recipe: Spark and Stir
Saturn– JPL Cassini Main Page
Cassini Imaging Team
Voyager: Beyond the Great Beyond
Titan’s Oily Lake
Titan’s Icy Bedrock
Saturn– JPL Cassini Main Page
Alien Landers: Extreme Explorers Hall of Fame
Titan: Biological Birthplace?
Solar System Bodies: Titan (NASA JPL)
The Probe Mission (NASA JPL)
Why Titan? (ESA)