Speculation is mounting among scientists that the previously thick, warmer atmosphere Mars had in ancient times may still be on the planet. Up till now, scientists have thought that the atmosphere has been slowly blowing away via the solar wind.
The controversy is rooted in the observation that the Martian atmosphere is only dissipating into space at a rate of 20 grams per second. At that rate, there only should have been a slight reduction in the contemporary atmospheric density.
Instead, Mars has an atmosphere currently less than 1% of Earth's. Thus, scientists are conceding the possibility that the remainder of the atmosphere is still on the planet, possibly hidden in huge underground reservoirs which remain undetected.Source