Читать книгу Exploring the Solar System - Peter Bond - Страница 70

Earth's atmosphere is unique in the Solar System, since nitrogen and oxygen together make up 99% by volume. Other gases which are naturally present in small, variable, amounts include carbon dioxide, methane, water vapor, and ozone. Minor constituents include inert gases, particularly argon, along with human‐made pollutants such as sulfur dioxide and nitrogen dioxide. Near‐surface ozone and carbon dioxide are also generated by human activity.

Earth's original atmosphere was probably made up of hydrogen and helium from the solar nebula, but these light gases soon escaped. A secondary atmosphere was then created as the result of outgassing through volcanic activity. Its actual composition is uncertain, although its main constituents most likely included carbon dioxide, water vapor, ammonia (NH₃), and methane (CH₄). Large amounts of water vapor, organic (carbon‐rich) compounds, and other volatiles would also have arrived through the initial bombardment by comets and asteroids.