Rare events illuminate Pluto's atmosphere. (Pluto and the Occult).

Twice in the past month, astronomers were given a rare opportunity to peer through the tenuous atmosphere of Pluto. For a few minutes on July 20 and again on Aug. 21, Pluto passed directly between Earth and stars in the constellation Ophiuchus.

During such events, called occultations, Pluto casts a moving shadow across Earth as it blocks a star's light. Starlight passing through Pluto's atmosphere bends, or refracts, and some is absorbed. The amount of dimming and the specific wavelengths absorbed reveal information about the temperature, density, and composition of the frigid planet's atmosphere.

Scientists are divided about whether Pluto's atmosphere has changed significantly since 1988, the last time the planet occulted a star as seen from Earth. But scientists do agree that the data are precious. Pluto is the only planet never visited by a spacecraft, and so, for now, occultations are the sole means to study its atmosphere.

Because the star eclipsed on July 20 turned out to be a trio of closely orbiting bodies, researchers pinpointed the location of Pluto's shadow only days before the event. Marc W. Buie of Lowell Observatory in Flagstaff, Ariz., and Oscar Saa of the Cerro Tololo Inter-American Observatory in La Serena, Chile, hurriedly set up a 14-inch, visible-light telescope in Mamina in northern Chile.

According to an analysis by Buie and James L. Elliot of the Massachusetts Institute of Technology, the temperature of Pluto's upper atmosphere--the region beginning about 30 kilometers above the planet's surface--has declined by 5 to 15 kelvins, down to about 85 kelvins, over the past 14 years. Perhaps most surprisingly, given that Pluto is now 3 percent farther from the sun than it was in 1988, the team finds that Pluto's surface is now slightly warmer.

Buie speculates that the surface is warmer because the planet has tipped its northern hemisphere toward the sun during the past 14 years. With more sunlight hitting the northerly regions, highly reflective layers of nitrogen frost may have evaporated and left behind darker material that absorbs more heat from the sun.

Using a smaller telescope near Arica in northern Chile, Francois Colas of the Observatory of Paris and his colleagues also observed the July 20 occultation. Although that team has found no evidence for any major change in Pluto's atmosphere since 1988, the researchers also measured an increase in Pluto's surface temperature.

The shadow of the second occultation passed over several large telescopes atop Mauna Kea in Hawaii. A first look at near-infrared observations taken with the Canada-France-Hawaii Telescope there gives no evidence of a change in Pluto's atmosphere, says Bruno Sicardy, who collaborates with Colas at the Observatory of Paris. Sicardy and Buie expect to report more information on Pluto's atmosphere in October at a planetary-science meeting.

The difficulty in studying Pluto's atmosphere highlights the need to send a spacecraft to the planet, Buie asserts. Although NASA had cut funds for such a mission, which could take 8 to 10 years to reach Pluto, a report by the National Research Council supports such a project (SN: 7/27/02, p. 62).