Out of the shadows: a new map of Pluto.

Taking advantage of several eclipses of Pluto by its moon Charon during the past several years, astronomers have constructed a new map of the surface brightness of the distant, frozen planet. The map depicts an unusually bright patch at Pluto's south pole and indicates that the planet may undergo seasonal changes in surface brightness as it moves closer to and farther away from the sun during its 248-year orbit.

Richard P. Binzel of the Massachusetts Institute of Technology in Cambridge began the study in 1985, when he became the first to detect eclipses of Pluto by its recently discovered moon. Taking advantage of an alignment of Earth, Pluto and Charon that occurs for a few years each century, Binzel viewed several eclipses from 1985 through 1990, using telescopes at McDonald Observatory near Fort Davis, Texas.

Each four-hour partial eclipse always dims the same face of Pluto. During Binzel's years of observations, Charon blocked different banana-shaped swatches of the planet. The first several eclipses dimmed Pluto's north polar region; later events blocked the planet's equatorial and south polar regions.

The changing eclipse pattern enabled Binzel and a graduate student, Eliot F. Young, to create a luminosity map of one entire face of the planet -- even though neither the Hubble Space Telescope nor ground-based instruments can resolve Pluto's tiny surface. Measuring the overall drop in Pluto's luminosity during each eclipse, and keeping precise track of what portion of the planet Charon had blocked during each encounter, the researchers calculated the contribution of each banana-shaped section to the planet's overall surface brightness. They presented their map last month at a meeting of the American Geophysical Union in Montreal.

Pluto's south polar cap is extremely bright, while the north polar cap appears dim. Young attributes this asymmetry to seasonal variations during the planet's elliptical orbit, which takes Pluto as close to the sun as 30 times the distance between Earth and the sun, and as far away as 50 times that distance.

During years when Pluto recedes from the sun, the planet's south pole lies in darkness, which could enable highly reflective methane frost to settle there and account for the region's brightness. When Pluto approaches the sun, its south pole receives constant sunlight, but the region's reflective methane coat -- akin to wearing white in summer -- might allow the pole to maintain a cool temperature and retain a frost layer, the researchers speculate.