The Atmosphere of Mars

Clouds and wind-blown dust are visible evidence that Mars has an atmosphere. Analyses of the atmosphere show that it is mostly carbon dioxide(95%) with small amounts of nitrogen(3%), oxygen, and water. Astronomers have measured the density of the atmosphere and have found that it is very low-only about 1% the density of Earth's atmos-phere. Its density is so low that it cannot trap heat. This, and the fact that Mars is far from the Sun, make the planet very cold. The average temperature is 218 Kelvin (-67 F). sO, although water exists on Mars, most of it is frozen-either on the surface of the planet as permafrost(ground that is always frozen) or in the palar caps. Clouds of dry ice(frozen CO2) and water-ice crystals (H2O) drift over Mars, but no rain falls from the Martian sky because the atomsphere is too cold and it contains too little water.