ATMOSPHERIC EVOLUTION OF THE TERRESTRIAL PLANETS

The major atmospheric gases on Earth, Venus, and Mars were probably CO2, H2O, and N2. Most of the Earth's CO2 is tied up in minerals such as limestone, and Venus has lost most of its H2O, leaving the CO2 in the atmosphere. Much of Mars' atmosphere may have been eroded in impacts by large meteoroids early in solar-system history. Noble gases are very underabundant everywhere, and must have been lost during an early period; they were probably dragged along during rapid loss of massive amounts of hydrogen. The tenuous atmospheres of Mercury and the moon have lifetimes of a few days or less and must be continuously replenished from internal or external sources.