The Evolution of Planetary Atmospheres
The Earth did not always have the kind of atmosphere it has today. In fact scientists now suggest that originally it had no atmosphere at all, and that once an atmosphere formed, its chemical composition evolved gradually to that of the present. The question of how the Earth's atmosphere arose and changed is extremely important, because the history is inseparable from understanding the origin and evolution of life on our planet.
The early Earth may have collected some gases by gravitational attraction. During the early formation of the Sun large amounts of material and radiation were thrown off. This flood of materials would have blown off any atmosphere the Earth had accumulated. Thus, for all intents and purposes, the early Earth atmosphere was an airless ball of hot (or even molten) rock.
During this period of cooling that followed the great bombardment and melting, large amounts of water vapor, carbon dioxide, and other gasses would have been released from deep within the solid Earth. Countless volcanoes and fissures, all belching steam and other materials, would have blown their gases into the newly forming atmosphere. In other words, the gases that formed the ancestor of today's atmosphere was swept away. Subsequently, a completely new atmosphere was released by a process called outguessing.
Outgassing, which was violent and rapid early in the Earth's history, has not ended today. We tend to think of volcanic eruptions as involving the flow of red–hot lava, but if you remember pictures of eruptions, you probably recall large clouds of smoke and steam that accompany the glowing lava flows. Even today, more than 4.5 billion years after the planet's formation, volcanoes release large amounts of gases from the interior of the Earth.
One estimate is that the principle result of outgassing in the early Earth was the production of an atmosphere composed primarily of methane (CH4), ammonia (NH3), carbon dioxide (CO2), hydrogen (H2) and water (H2O), though scientists are engaged in a spirited debate on the subject. Presumably, the same sort of process was occurring on the outer terrestrial planets. For a time, the atmosphere was probably too hot for water to condense from a gas to a liquid, but eventually the atmospheric temperature drooped and torrential rain began to fill the ocean basin.
Once a planet has acquired an atmosphere by outgassing, there are several ways that its atmosphere can evolve and change. The simplest is gravitational escape. The molecules in an atmosphere heated by the Sun many move sufficiently fast so that appreciable fractions of them can actually escape the gravitational pull of the planet. The Moon, Mercury and Mars are examples of bodies that had denser atmospheres early in their history but lost much of these gases through gravitational escape long ago. Most of the light elements such hydrogen and helium were presumably lost in the same way from the Earth, but the heavier gases such as carbon dioxide, and water vapor remained because they were lost too heavy to escape Earth's gravitational force.
A second cause for atmosphere change—one that operates only on the Earth—is the effect of living things. To the best of our knowledge, no life exists anywhere else in the solar system (although some scientists argue that there may have been life at one time on Mars). By the time the Earth was one billion years old, photosynthetic organisms evolved to use the Sun's energy to power chemical reactions essential for life. In photosynthesis, carbon dioxide and water are taken into the structures of living things, and oxygen is the waste product. As life flourished on the planet the amount of free oxygen increased. Today it comprise about 20% of the atmosphere.
