Astronomers find abundant nitrogen on Pluto.

Astronomers for the first time have detected nitrogen and carbon monoxide ice on Pluto. Moreover, the findings indicate that nitrogen is the most abundant material on the planet's frozen landscape, making up about 97 percent of its surface. Carbon monoxide ice accounts for about 1 to 2 percent of Pluto's surface, and it now appears that methane -- the only material previously detected on Pluto -- has roughly the same low abundance.

"Planetary scientists were uncertain about which was the most abundant ice on Pluto -- methane or nitrogen," says Richard P. Binzel of the Massachusetts Institute of Technology. "Now that we know it's nitrogen, we can move forward to understanding more about the planet."

Given that other bodies in the solar system, such as Neptune's satellite Triton, contain nitrogen, the new findings were not unexpected, notes study collaborator Tobias C. Owen of the University of Hawaii in Honolulu. But he says the results are intriguing because Pluto has remained virtually unchanged since its formation several billion years ago.

Thus, Owen says, the mixture of compounds on the planet's surface offers a peek at the chemistry of the very early solar system. In particular, since astronomers believe that comets transported material from the outskirts of the solar system to the inner planets, the composition of distant Pluto may indicate the composition of the early Earth's atmosphere (SN: 9/5/92, p.150).

Before the current study, conducted in May using the United Kingdom Infrared Telescope atop Hawaii's Mauna Kea, astronomers had direct evidence only of methane ice on Pluto, based on that compound's telltale absorption of infrared light. Researchers suspected that molecular nitrogen and carbon monoxide also existed on the surface, but previous spectroscopic studies had failed to detect these molecules because they absorb only weakly in the infrared. The new observations were made with a highly sensitive spectrometer that can detect even very faint infrared absorption.

An international research team, which includes Owen, reported the work last week at the annual meeting of the American Astronomical Society's Division for Planetary Sciences in Munich, Germany.

The findings, says Owen, may shed new light on a long-standing puzzle: why researchers have detected so little nitrogen in interstellar clouds of gas and dust -- the raw material for stars -- even though stars themselves contain a high abundance of the element. He notes that the heat from stars would split molecular nitrogen, which consists of two atoms bound together, into single atoms, which more readily absorb light and are easier to detect.

Astronomers had theorized that much of the missing nitrogen in the chilly interstellar medium may lie hidden in its harder-to-observe, molecular form. But they lacked convincing proof. Because Pluto preserves primordial abundances of materials on its icy surface, detecting a significant amount of molecular nitrogen on the planet offers further support for the notion that the cold interstellar medium also contains lots of molecular nitrogen, Owen notes.