NASA's Chandra X-ray Observatory has taken a long, clear look at the X-ray emission of Saturn — its unexpected findings are reported by J.-U. Ness and colleagues in Astronomy & Astrophysics (doi:10.1051/0004-6361:20035736; 2004). Although the rings and polar regions of the planet are ‘silent’, the equatorial region is beaming X-rays. Around 90 million watts of X-ray power is concentrated in the bright white spot in this image from Chandra (left, contrasted with an optical image of Saturn from the Hubble Space Telescope, right).

X-rays from other planets such as Mars and Venus are mostly only reflected rays from the Sun. But X-rays from Jupiter, Saturn's gas-giant neighbour, probably have an additional source. Earlier observations by Chandra have recorded strong X-ray emission from Jupiter's polar regions. This is believed to be a signature of auroral activity, caused by energetic heavy ions striking the jovian atmosphere and being directed by the planet's magnetic-field lines to the magnetic poles. At Earth, a similar light show is created by the energetic electrons of the solar wind as they hit the atmosphere. Despite the different origins, the aurorae emit X-rays in both cases.

So what is happening on Saturn? The Hubble Space Telescope has provided striking images of Saturn's auroral activity at ultraviolet wavelengths, showing small haloes at the poles. One explanation for the absence of polar X-ray emission could be that the auroral phenomena on Saturn simply occur at lower energies. This would imply a different mechanism for Saturn's aurorae, which is not entirely surprising — the presence of the rings could somehow alter the planet's magnetic-field lines.

The real puzzle is the brightness of the emission near Saturn's equator, compared with the darkness of its poles. If the Moon were scaled up to the size of Saturn and placed in its orbit, its X-ray brightness would still be about 50 times weaker than Saturn's. The emission from Saturn is, however, consistent with Jupiter's equatorial luminosity, so these gas giants are clearly much more efficient at reflecting solar X-rays than is the Moon.

The stage is now set for Cassini, the NASA spacecraft on a dedicated mission to study the ringed planet. Cassini will enter Saturn's orbit on 1 July, and begin a four-year tour of the planet. And in December, the European Space Agency's Huygens probe, which has ridden piggyback on Cassini on the seven-year journey to Saturn, will separate from the NASA craft and enter the cloudy, nitrogen-rich atmosphere of Titan, Saturn's largest moon.