Big chill in the solar atmosphere.

Earth intercepts only a minuscule fraction of the light and heat radiated by the sun, but it's enough to sustain life on the planet. How the sun's energy, originating in nuclear fusion reactions at its core, gets to the sun's surface and through its atmosphere into space has long interested solar astronomers.

New data now suggest that the chromosphere--that portion of the sun's atmosphere between its surface, or photosphere, and its corona -- contains a large component of gas much colder than many researchers had supposed in their traditional models of energy transfer in the atmosphere.

"This really changes our whole picture of what the chromospher is all about," says astronomer Thomas R. Ayres of the University of Colorado at Boulder.

Ayres, William C. Livingston of the National Optical Astronomy Observatories in Tucscon, Ariz., and S.K. Solanki of the Swiss Federal Institute of Technology in Zurich report their findings in the Jan. 7 SCIENCE.

Astronomers had viewed the chromosphere as a relatively uniform blanket of gas with a temperature of between 5,000 and 7,000 kelvins. Starting at roughly 6,000 kelvins at the sun's surface, the solar atmosphere's temperature falls to nearly 4,300 kelvins at an altitude of 500 kilometers before rising to typical chromospheric temperatures. At 2,000 km, near the chromosphere's upper boundary, the temperature again rises steeply, eventually reaching coronal temperatures as high as 1 million kelvins.

Models based on this picture successfully predict the intensity and color of light emitted by a variety of atoms in the sun's chromosphere. But they cannot readily match measurements of the intensity of certain wavelengths of infrared light emitted by carbon monoxide molecules. The carbon monoxide emissions appear to come from gas that is significantly colder than 6,000 or 7,000 kelvins, perhaps as cold as 3,500 kelvins.

Because carbon monoxide molecules form only at temperatures lower than 5,000 kelvins, researchers can use the presence and characteristics of carbon monoxide emissions to map low-temperature features in the solar atmosphere.

In April 1993, astronomers using new equipment at Kitt Peak, near Tucson, Ariz., for the first time obtain high-resolution measurements of infrared emissions from the sun's edge, or limb, where emissions from the photosphere can't muddle the observations.

Because carbon monoxide emissions could be seen above the sun's limb, Ayres contends that cold gas exists at fairly high altitudes inthe chromosphere. Moreover, the atmosphere's minimum temperature may occur at roughly 1,100 km above the sun's surface rather than at 500 km, he argues.

Ayres suspects that this cold gas forms a patchy layer, punctuated by hot filaments of gas, in the lower part of the chromosphere. But his suggestions remains controversial.

"The last word isn't in yet," says Robert W. Noyes of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. For example, the clouds of cold carbon monoxide molecules may result from transitory, local phenomena--sharp temperature drops caused when blobs of gas, perhaps ejected from the photosphere, expand rapidly.

"We know a great deal about the structure of the photosphere and its dynamical behavior, and we know very little about the structure of the region a few hundred kilometers up, where the temperature minimum is," Noyes says. "This is an important region of the atmosphere to understand."

New spectrographic equipment and cameras at Kitt Peak will allow researchers to make high-resolution, two-dimensional infrared images of the sun's disk to determine how pervasive the cold carbon monoxide clouds are and how they change over time. "Over the next couple of years, various groups will be making these kinds of observations, and they will likely lay this problem to rest," Noyes says.