Edition: U.S. / Global

Weather on Pluto, Too, Is Exercise in Conjecture

Published: August 27, 2002

The weather report for Pluto remains unclear.

Astronomers from the Lowell Observatory in Arizona and the Massachusetts Institute of Technology say the atmosphere has chilled 20 to 55 degrees in the last 14 years, the surface may have warmed a smidgen, and a low-lying layer of smog has cleared up.

French astronomers disagree. The air, they say, has cooled only slightly and is still hazy.

That the pronouncements seem no more reliable than those of Punxsutawney Phil is fitting. Like predictions of winter's persistence on Groundhog Day, the Pluto findings are all based on shadows.

Pluto, 2.8 billion miles distant and receding, is no more than a point of light to earthbound telescopes. The only time astronomers can deduce anything about its atmosphere is when, by heavenly coincidence, Pluto passes in front of a distant star and casts a faint shadow on Earth.

That happened once in 1988 and then, after a 14-year wait, twice this summer. The dimming of light from the eclipsed star tells of the density and temperature of Pluto's atmosphere. If Pluto is airless, the star would wink out suddenly. But the atmosphere, acting like a lens, bends starlight around the planet and causes the dimming to occur gradually. In the 1988 eclipse, the starlight dimmed steadily until it reached 40 percent of its usual brightness and then faded out quickly.

The dimming rate indicated that most of Pluto's atmosphere, which extends more than 100 miles above the surface, is largely uniform in temperature, from minus 280 degrees Fahrenheit to minus 260.

The sudden winking out indicated that above the surface was a layer of 15 to 35 miles of either smoggy haze that absorbed the light or much colder, denser air that bent the light more sharply, out of view from Earth. From other measurements, scientists know that Pluto's surface is minus 390 degrees.

On July 20, Pluto eclipsed the star, P126A, but cloudy weather thwarted all but two teams of observers in northern Chile. Astronomers were more fortunate last week, when Pluto eclipsed another star, P131.1, in full view of a multitude of major observatories in Hawaii and the western United States.

Dr. Marc W. Buie, an astronomer at the Lowell Observatory who made one of the two successful July observations in Chile, said the starlight dimmed in a manner to suggest a sharp drop in temperature from 1988. There was also no sign of the haze or cold layer, meaning that it had dropped in height by at least 15 miles or, perhaps, disappeared completely.

''We've got fundamental surprises,'' Dr. Buie said.

But the team that made the other successful Chile observation said its analysis showed that Pluto's atmosphere remained about minus 280 degrees.

''We didn't see many changes since '88,'' the team leader, Dr. Bruno Sicardy, an astronomer at the Paris Observatory, said. ''I don't understand how they could deduce anything about the changes.''

In addition, measurements of the August eclipse from the Canada-France-Hawaii Telescope on Mauna Kea showed that the haze layer was still there, Dr. Sicardy said.

Dr. Buie and his collaborator, Dr. James L. Elliot, an astronomy professor at M.I.T., have not yet analyzed information from the August eclipse from several Hawaii telescopes, the Lick Observatory in California and Lowell, which observed the eclipse at several wavelengths of visible and infrared light.

A clearer picture of Pluto should emerge in a few months. The infrared measurements should tell whether it is a layer of cold or haze in the lower atmosphere. Cold air would bend all wavelengths of light equally. Haze would absorb visible light, but let longer-wavelength infrared light pass through.

''Same reason as why sunsets are red,'' Dr. Sicardy said.

Now, Dr. Elliot said, neither explanation works very well. For haze to have caused the observed 1988 dimming, the haze particles would have to have been fairly large, too large to stay aloft in Pluto's thin atmosphere.

But computer models are also unable to construct an atmosphere with such a sharp drop in temperature. ''There was no physical model that could explain what we saw in '88,'' Dr. Elliot said.

The astronomers agree that the atmosphere appears surprisingly active, given how little sunlight it receives. In the August eclipse, Dr. Sicardy said, the starlight suddenly brightened about 10 times for a few seconds before dropping back down, indicating pockets of colder air.

''To create these variations, you need dynamical activity,'' he said. ''You need energy to maintain these structures.''

It is unclear how the new data fit into predictions that the cooling atmosphere will freeze and collapse in 20 years. That depends more on surface temperature, which determines how much of the Plutonian methane and nitrogen ice turns into gas. Dr. Buie said his computer models indicated that the surface might have actually warmed one- or two-tenths of a degree since 1988. The surface has become darker, possibly because of receding frost, allowing it to absorb more heat.

With Pluto becoming colder and farther away, that trend should eventually reverse. Astronomers do not know when the stars will provide another chance to look at Pluto's atmosphere and what will change before then.

Chart: ''A Faint Eclipse'' Pluto passed in front of a star on Aug. 20, creating a measurable eclipse. It was observed by telescopes in Hawaii, California and Arizona. (Source: Massachusetts Institute of Technology)