Earth's atmosphere is primarily composed of nitrogen and oxygen with water vapor comprising 0.25 percent of the atmosphere by mass. It has five layers: troposphere, stratosphere, mesosphere, thermosphere and exosphere.
Earth's atmosphere is the only one in the solar system capable of supporting life. Venus's atmosphere is mostly carbon dioxide with thick clouds of sulfur, which render the surface too hot to support any known form of life. The atmosphere of Mars is also mostly carbon dioxide with trace amounts of argon and nitrogen, but it is much thinner than that of Venus. The thin atmosphere fails to trap greenhouse gases, which leaves Mars much cooler than Earth.