Earth

Earth's atmosphere is as unique within the solar system as the life and oceans it shelters. This should be no surprise: the atmosphere's composition, atmospheric pressure, and density, are all intimately related with the fact that Earth is covered by vast quantities of liquid water and is teeming with plant life that uses photosynthesis to extract energy from the sunlight, using up carbon dioxide and giving off oxygen in the process. The oceans provide an enormous heat reservoir that stabilizes Earth's climate and keeps the planet's surface temperature at optimal levels for the life forms inhabiting it.

Let us compare the composition of the Earth's atmosphere with that of the neighboring planets Venus and Mars. The atmospheres of both Venus and Mars are composed of approximately 96% carbon dioxide (CO₂), plus about 2-3% nitrogen (N₂). On Earth, nitrogen dominates, making up some 78% of the atmosphere, with oxygen holding a strong second at 21%, and with the remaining percentage made up of argon and traces of carbon dioxide, plus a variable small quantity of water vapor (air humidity). These differing atmospheric makeups can in large part be explained by the presence of life on earth. In the early history of our planet, before life became an important and ubiquitous factor, Earth's atmosphere was probably mostly made of carbon dioxide, just as on Mars and Venus. As plant and animal life proliferated in the oceans, photosynthetic algae started transforming the carbon dioxide into oxygen, while many forms of microscopic life started using the same gas and the calcium in the water to build their shells (shells are made of calcium carbonate, CaCO₃). As they died, the shells from these organisms fell onto the ocean bottom, rather than returning the CO₂ to the atmosphere.

In addition to their different gaseous makeups, the atmospheres of Earth, Mars, and Venus also differ in terms of atmospheric pressure. While Earth's atmosphere has a pressure of 1 bar, Mars has a thin atmosphere with 0.07 bars only (comparable to Earth's at 35 km of altitude). Mars has only 10% the mass of the Earth, and we can explain the thinness of its atmosphere through the hypothesis that Mars' weak gravity was unable to hold its atmosphere, and so much of that atmosphere was lost early in the planet's life. In contrast, Venus is subject to a crushing atmospheric pressure of about 90 atmospheres, comparable to the pressure on earth found 1 km deep in the ocean. Given that Venus is very similar to Earth in mass and size, we cannot explain the difference in their atmosphere as we did for Mars. It turns out that liquid water is the essential factor involved. Water on earth dissolves carbon dioxide, vastly lessening the amount of such gas in the air, and thus the atmospheric pressure. Since carbon dioxide is a very efficient greenhouse gas, it traps the heat of the planet without letting it escape into space. The almost complete removal of carbon dioxide from Earth's atmosphere meant that the temperature on our planet remained at much lower levels than on Venus, and made it possible for life to evolve.

Working in concert, water and life created an Earth atmosphere that is relatively thin, with very little carbon dioxide, a lot of oxygen, and an amount of nitrogen that remained roughly unchanged from the start.