Remember too that solar wind another factor. Solar wind strips the upper layers of the atmosphere off of the planets...abit slowly. The earth is protected from the effects of solar wind because of its magnetic field. Venus is losing its atmosphere slowly because it has a very weak magnetic field and is closer to the sun so the flux of solar wind is higher. But whatever process (probably rampid volcanism) that gave it its thick atmosphere did a swell job. Mars wasn't so lucky (weak magnetic field as well).