X-Rays from Saturn Pose Puzzles

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.

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. 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).

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.

Additional information and images are available at:
http://chandra.harvard.edu
and
http://chandra.nasa.gov