Evolution of the atmosphere of Mars. Illustration of three mechanisms proposed to explain changes in the atmosphere of Mars over time. Mars initially had a relatively thick atmosphere, but now has a thin atmosphere that is mostly carbon dioxide. At upper left and upper centre, a meteor impacts the surface resulting in the ejection of gas from the atmosphere. At upper right, rain falls from clouds (blue arrows) leading to carbon dioxide being absorbed and deposited as carbonate minerals below the surface (lower left). The final mechanism is the effect of sunlight, with high-energy solar radiation (light blue arrows, lower centre) splitting gas molecules apart, followed by the solar wind (yellow arrows, lower right) ejecting lighter particles from the upper parts of the atmosphere.