Extrapolating this figure back over 3.5 billion years, they estimate that only 0.2 to 4 millibars of carbon dioxide and a few centimeters of water could have been lost to solar winds. The rest may be locked up in hidden reservoirs on the planet, they claim.
According to the "warm and wet early Mars" model, liquid water once flowed on the red planet’s surface. Evidence from channels and gullies recently spotted on Mars suggest the water layer might have been more than half a mile deep in places. For Mars to keep that much water in liquid form, the planet must have had a much higher atmospheric temperature, which scientists think was made possible by a strong greenhouse effect in the planet’s past.The researchers reported their findings in the scientific journal Science.
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 that—about 0.008 bars, or about 0.7 percent of the average surface pressure at sea level on Earth.
Where the atmosphere went is still unclear, but the authors speculate that it might still be contained somewhere beneath the Martian surface.
"There are different alternatives," said study leader Stas Barabash of the Swedish Institute of Space Physics in Kiruna, Sweden. "One is that it is still stored somewhere on Mars in some hidden reservoir we cannot find."
Another, more controversial, idea is that Mars' atmosphere was blown away in a catastrophic impact with a giant asteroid or comet sometime in the planet’s ancient past. Barabash estimates that Mars would have had to have been struck by a space rock at least 6 miles (10 km) wide to obliterate its atmosphere.