Theres 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 spaceor 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 month that Mars is currently losing a mere 20 grams (.04 pounds) of air per second into space. Thats not a very big leak. Thus, when European 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 1 bar.) Liquid water may still flow for brief moments across Mars's surface as NASA scientists discovered last year. 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 pressureperhaps enough to form a moderate greenhouse effect similar to Earth. Mars' atmosphere must have been between 1 to 5 bars to maintain that kind of greenhouse effect, scientists think. But Mars' atmospheric pressure today is only a small fraction of thatabout 0.008 bars or about 0.7 percent of the average surface pressure at sea level on Earth, according to a European Space Agency news report. What happened to Mars' atmosphere and water has been a well guarded cosmic mystery. One idea was that the martian atmosphere was eroded over billion years by powerful solar winds. A more controversial idea is that Mars' atmosphere was blown into space by a catastrophic asteroid or comet impact in the remote past. A space rock at least 6 miles (10 kilometers) wide could have been enough to obliterate Marss once abundant atmosphere. However, scientists may overplay the impact hand when explaining planetary change. Whats in the Sky: Dec. 29 look east after sunset for brilliant Mars. The star Capella is to Mars's upper left, orange Aldebaran to Mars's upper right, orange Betelgeuse to Mars's lower right, and the Castor-and-Pollux twins to Mars's lower left.