The Earth's atmosphere is extraordinary; comparing the Earth to other planets in the Solar System illustrates just how extraordinary. The inner planets are often referred to as the terrestrial planets because they consist of rocky masses surrounded by gaseous atmospheres (with the exception of Mercury, whose atmosphere has long since been lost to space because of its proximity to the Sun). Venus and Mars both have substantial atmospheres and make excellent comparisons for the atmospheric evolution of Earth.
The atmospheres of Venus and Mars have evolved to consist primarily of carbon dioxide (CO2), with some nitrogen and hardly any oxygen. Although Venus and Mars have important differences in their atmospheric chemistry, their atmospheres are essentially similar. Roughly speaking, both atmospheres consist of 95 percent CO2 and 3 percent nitrogen. The amount of free oxygen on both planets is less than 1 percent. Earth's atmosphere may have evolved in a similar way: it was formed at roughly the same time from roughly the same material. Instead, Earth's atmosphere is markedly different. It contains hardly any CO2 (although CO2 plays a crucial role in regulating the planet's atmosphere and climate), consisting mainly of nitrogen, with a comparatively high oxygen content of around 21 percent. These differences result in an important chemical distinction between the atmospheres. The atmospheres of Venus and Mars are highly oxidized, containing a lot of chemically combined oxygen (mainly CO2). In contrast, Earth's atmosphere is highly oxidizing, containing large amounts of free oxygen.
That one fifth of the Earth's atmosphere is comprised of oxygen is curious. Oxygen is a highly reactive gas, and the atmosphere and surface of the Earth contain many materials with which oxygen can react. Furthermore, the amount of time it would take for all the oxygen in the air to undergo these reactions is much shorter than the time the oxygen has been there. In short, there should not be anywhere near as much oxygen as there is in Earth's atmosphere; the atmosphere is a long way from chemical equilibrium.
From a chemical perspective, the Earth's atmosphere is like a bucket that, despite having a hole in the bottom, remains half full. It is impossible, unless water is flowing into the bucket at the same rate that it is leaking through the hole. In the same way, the composition of Earth's atmosphere is impossible, unless something is constantly adding oxygen to counteract the rate at which it is being lost through chemical reactions. The factor acting on the atmosphere to keep it in its state of chemical disequilibrium is life.
Biological activity on the planet uses energy from the Sun, during photosynthesis, to produce vast quantities of free oxygen. In the absence of life, there is no way for oxygen to have built up to the levels observed today. The evolution of the Earth's atmosphere has taken a very different path from that of Venus and Mars, and this path has led to an atmosphere that is outstanding among all the other known atmospheres. In this evolution, life has played a critical role.
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