SPACE.com -- Mars' Missing Air Might Just be Hiding:
"Rather than having had its air knocked out into space, Mars might just be holding its breath.
New findings suggests the missing atmosphere of Mars might be locked up in hidden reservoirs on the planet, rather than having been chafed away by billions of years' worth of solar winds as previously thought.
Combining two years of observations by the European Space Agency’s Mars Express spacecraft, researchers determined that Mars is currently losing only about 20 grams of air per second into space.
Extrapolating this measurement back over 3.5 billion years, they estimate that only a small fraction, 0.2 to 4 millibars, of carbon dioxide and a few centimeters of water could have been lost to solar winds during that timeframe. (A bar is a unit for measuring pressure; Earth’s atmospheric pressure is about 1 bar.)"