There's lots of surface evidence that the planet Mars had a breathable atmosphere billions of years ago. Astronomers have theorized that either due to Mars' small mass or solar winds, the atmosphere eventually leaked away into the vacuum of space - or did it?
Rather than having had its air leak away into space over millions of years, Mars may be holding its breath. Findings from the Mars Express spacecraft currently orbiting the red planet suggest that Mars' missing atmosphere might be socked away in underground gas and ice reservoirs around the planet.
Researchers discovered last year that Mars is currently losing a mere 20 grams (.04 pounds) of air per second into space. That's not a very big leak.
Thus, when space researchers extrapolated 20 grams per second back over the 3.5 billion years of martian geohistory, they found only a tiny fraction, 0.2 to 4 millibars of carbon dioxide and a few centimeters (less than one inch) of water, was lost. A note about millibars: A bar is a unit of the liquid-element mercury for measuring atmospheric pressure; Earth's atmospheric pressure is about one bar.
Liquid water may still flow for brief moments across Mars' surface as NASA scientists discovered in 2006. In fact, a martian water layer might be half-a-mile deep in places and feed the transient surface flows with powerful, albeit brief, aqueous outpourings. With this much liquid, it becomes apparent that Mars had a higher atmospheric temperature and pressure - perhaps enough to form a moderate greenhouse effect similar to Earth.
Mars' atmosphere must have been between one to five bars to maintain that kind of greenhouse effect, scientists think. But Mars' atmospheric pressure today is only a small fraction of that - about 0.008 bars or about 0.7 percent of the average surface pressure at sea level on Earth, according to a news report.