Neptune's northern half grows brighter.

After several nights of poor viewing atop hawii's Mauna kea, astronomer Heidi B. Hammel finally captured clear images of Neptune on July 12, her last night of observing with the University of Hawaii's 2.2- meter telescope. But her elation turned to puzzlement when she examined the first picture. It looked as if the instrument's optics had somehow produced an upsidedown portrait of the distant planet. How else to explain that Neptune's northern hemisphere appeared brighter than the southern - a phenomenon never before abserved?

After further observations convinced her the feature was real, "I was dancing around the (telescope) control room," recalls Hammel, of the Massachusetts Institute of Technology. The new findings offer further proof that Neptune has one of the most variable atmosphere of any outer planet.

The near-infrared and visible-light images taken by Hammel and graduate student Ray LeBeau indicate that the brightening occurred at the cloud tops of the planet. Last month, John R. Spencer of Lowell Observatory in Flagstaff, Ariz., and Mark A. Shure of the University of Hawaii in Honolulu found similar brightening when they observed Neptune with the NASA infrared Telescope Facility on Mauna Kea.

Hammel doesn't know what caused the brightening. Images she took a year ago show the planet's northern half fainter than the southern half-as it has been for the past eight years. She adds that a few bright features had previously appeared on Neptune's northern hemisphere - perhaps a prelude to the increase in reflectivity now observed. Neptune's northern half was bright in images from the late 1970s, but its southern half had always appeared at least as bright. Hammel notes that astronomers have only measured the brightness of Neptune's northern and southern latitudes since the early 1970s.