Pluto's atmosphere: more than methane.

Pluto's atmosphere: More than methane

Astronomers first detected methane on Pluto several years ago from spectral measurements, but could not tell whether it existed in the form of an atmosphere or solid frost on the surface. When Pluto passed in front of a star last June 9, the gradual refraction and dimming of the star's light as the planet got in the way showed a planetary atmosphere (SN: 6/18/88, p. 391), but researchers remained uncertain of its composition.

Now Roger V. Yelle and Jonathan I. Lunine of the University of Arizona in Tucson report in the May 25 NATURE that there is not only methane but also another constituent, a substance whose molecules are considerably heavier. Others have suggested the existence of such a constituent, based on assumed abundances of other gases in the cosmos, but Lunine describes the results of the new analysis as "the first real piece of evidence showing that there is heavier gas there."

The authors cite observations of the star's occultation made by other researchers from the University of Tasmania Observatory in Hobart, Australia, and from NASA's Kuiper Airborne Observatory. The atmosphere's average temperature, they calculate, is about 106 kelvins, rather than the 60 K surface temperature measured in 1983 by the Infrared Astronomy Satellite.

The researchers compared the amount of the sun's heat absorbed by the methane in the atmosphere with the amount reradiated upward to space and conducted down to the surface. They find that the resulting "energy balance" shows the thin mixture of gases around Pluto to have a mean molecular weight of about 25, meaning that "a molecule heavier than (and in addition to) methane must be present in the atmosphere."

Yelle and Lunine conclude that the likeliest candidate is carbon monoxide, with a molecular weight of 28 compared to methane's 16. Other possibilities include argon and molecular nitrogen. Carbon dioxide would probably freeze onto the surface instead of remaining as a gas, though Lunine notes cosmic rays might produce carbon monoxide from the CO.sub.2.

The change in the starlight measured as Pluto blocked it off showed a "kink" that other observers have interpreted as evidence of a layer of haze in the atmosphere. In work reported at the recent American Geophysical Union meeting in Baltimore earlier this month and now submitted to the journal ICARUS, William B. Hubbard of the University of Arizona suggests together with Yelle and Lunine that the kink could result from the effect of Pluto's cold surface on its atmospheric density.

One implication of the cold plutonian surface could be that there are no cumulus clouds overhead, adds Lunine, because the atmosphere, unlike Earth's, gets warmer with altitude so that moisture does not condense into droplets, though there could be early morning ground fogs.

Hubbard and his colleagues suggest that Pluto's radius may be about 1,180 kilometers, though the presence of a dust layer could make the planet as small as 1,140 km. The Voyager 2 spacecraft's upcoming flight in August past Neptune's moon Triton, where methane has also been detected, could help understand Pluto, says Lunine, in the question of "how cold, thin atmospheres work."