Cloudy puzzle on Uranus.

Shrouded in darkness for 20 years, the northern hemisphere of Uranus is slowly emerging from the shadows. With sunlight once again illuminating the planet's northern face, astronomers are taking their first peek at a region on Uranus that they have never before seen with modern detectors.

Observers have been astonished to find that the planet's atmosphere, thought to be bland and nearly featureless, contains several enormous bands of bright clouds (SN: 11/29/97, p. 343).

New images, taken with the Hubble Space Telescope in orbit as well as at the WM. Keck Observatory and NASNs Infrared Telescope Facility on Hawaii's Mauna Kea, reveal even more of a puzzle. Each year since the mid-1990s, as the sun has illuminated more and more of the northern hemisphere, the clouds have appeared brightest along the northernmost, newly sunlit edge. But bright clouds seen at this boundary in 1998, which now should lie well in from the edge, are no longer detectable, reports Heidi B. Hammel of the Space Science Institute in Boulder, Colo.

Her team suggests that the northernmost clouds form when the first rays of sunlight strike the cold, dark region, causing vapors deep in the atmosphere to rise. In contrast, longer-lived southern clouds, observed on Uranus since 1994, may form by a different, still undetermined mechanism, Hammel says.

An additional hint that the northernmost and southern clouds have a different origin comes from near infrared images taken at Keck by Hammel's team and a team led by Imke de Pater of the University of California, Berkeley. The images show that the northern clouds extend to higher altitudes than those in the south. Follow-up spectra to determine the composition of the clouds and observations taken as the sun creeps farther north on Uranus may provide further insight into the variety of clouds.