Atmospheric Evolution of the Terrestrial Planets

The major atmospheric gases on Earth, Venus, and Mars were probably CO 2 , H 2 O, and N 2 . Most of the Earth's CO 2 is tied up in minerals such as limestone, and Venus has lost most of its H 2 O, leaving the CO 2 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.