The planet Saturn, its rings tipped about as wide open as they ever get, glows prominently in the evening sky this season. Astronomers at one of the world's largest telescopes have taken advantage of the opportunity to test out their newest instruments. The result: perhaps the sharpest image of the ringed world ever obtained from Earth's surface, and one showing a surprise at the planet's south pole.

The European Southern Observatory's Very Large Telescope (VLT), high atop Cerro Paranal in the Chilean Andes, consists of four identical reflectors each 8.2 meters in diameter. The last of these to come online, named Yepun ("Venus" in the language of the indigenous Mapuche people), has been undergoing commissioning tests since November 2001 with a French adaptive-optics system and German near-infrared camera.

While putting the devices through their paces last December 8th, ESO scientists and engineers turned the telescope toward the ringed giant some 1.2 billion kilometers (750 million miles) away amid the stars of Taurus, the Bull. They made two exposures, each 10 to 12 seconds long, at wavelengths of 1.6 and 2.2 microns, while the adaptive-optics system compensated for blurring caused by atmospheric turbulence. Then they combined the resulting images and false-colored them to produce the composite view shown here.

Details as small as 0.07 arcsecond (410 km) are visible on the planet's disk and in its rings. The brightening along the equator is all that remains of a giant storm that erupted in the planet's atmosphere about 5 years ago. The most unusual feature in the image is a dark spot near the south pole. Some 3,000 km across, it is not an image-processing artifact, because observers at Pic du Midi in the French Pyrenees have detected it visually. But what it is remains a mystery pending additional study at the VLT and elsewhere.