Mars' brush with Earth last August -- closest in 60,000 years -- gave astronomers a peek into its atmosphere that revealed why oxygen levels remain too low for humans to breathe.

The culprit, researchers are reporting today, is hydrogen peroxide, H 2O 2, -- the liquid compound familiar on Earth as an antiseptic for cuts.

R. Todd Clancy, who headed the research, in an interview described it as the first detection of hydrogen peroxide in the martian atmosphere.

Clancy headed a research team from the Space Science Institute in Boulder, Colo., that made the discovery. They used the James Clerk Maxwell Telescope, a radio telescope located near the 14,000-foot summit of Mauna Kea in Hawaii.

The big three ingredients in Mars's atmosphere are carbon dioxide (95 percent), nitrogen (4 percent), and water vapor (0.02 percent).

Martian chemistry breaks carbon dioxide into carbon monoxide and oxygen. Hydrogen peroxide then acts as a catalyst. It pushes key chemical reactions that reassemble carbon dioxide.

What about the possibility of short-circuiting hydrogen peroxide so that oxygen does build up, making Mars more habitable for humans?

Clancy noted that low oxygen is only part of Mars's problem. Low atmospheric pressure is another. Even with more oxygen, Mars' pressure would be too low for humans to stroll the surface.