A new study of the Martian atmosphere has added strength to a popular theory of why there is so much carbon dioxide in the red planet's air. Researchers have detected hydrogen molecules in the atmosphere, which had been predicted but not found for decades.

Carbon dioxide makes up about 95 percent of Mars' very thin atmosphere, roughly nine times more overall than contained in Earth's much thicker atmosphere. Laboratory work showed that the Sun's energy should split this carbon dioxide into its components -- carbon monoxide and oxygen.

So about 30 years ago, scientists suggested that the Martian atmosphere must contain tiny amounts of water vapor. The water would trigger chemical reactions and put carbon monoxide and oxygen back together again, creating carbon dioxide. A continuous cycle would develop whereby carbon dioxide was created, destroyed, and its components recombined.

For three decades, however, a piece of the puzzle remained unfound. The theory also predicted that the cycle should produce hydrogen molecules, commonly referred to as molecular hydrogen.

The new research, by Vladimir A. Krasnopolsky of the Catholic University of America and Paul D. Feldman of Johns Hopkins University, employed NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft to study the upper Martian atmosphere in ultraviolet light, confirming the presence of molecular hydrogen.

"It was expected," said Hunten, who was not involved in the new discovery. "But it's a nice confirmation."

The researchers will publish their results in the Nov. 30 issue of the journal Science.

Krasnopolsky and Feldman also used the finding to speculate that Mars may once have had more water per pound of dirt and rock than Earth. Other researchers, however, prefer to use evidence of present-day water vapor to make such calculations.