Of our planetary neighbors in space, Venus most closely resembles Earth in size and mass: it is only slightly smaller than our home planet. One would expect, therefore, that the internal structure of Venus is similar to that of Earth. The fact that Venus has a dense atmosphere shows that gravitational segregation has taken place and that considerable amounts of water must have been delivered to the surface of the planet. The question of whether much, little, or no water exists near the surface of Venus has only been settled very recently, mainly by direct probing. The reason is that the clouds of Venus, which give it the wonderful brilliance of the "morning star" (or "evening star"), obstruct observation of the surface. The space probes showed that the surface temperature of Venus is very high, almost 500 degrees Celsius. Iron glows dark red at this temperature. Water, of course, evaporates under such conditions. Thus, we would not expect an ocean, but a water vapor atmosphere.
Radar image of the surface of Venus
The shocker is that the heavy atmosphere of Venus (nearly 100 times the pressure of Earth's) is dry and consists almost entirely of carbon dioxide -- a gas which makes up only 0.03% of Earth's atmosphere! The clouds are made of sulfuric acid. Not a nice place.
In the atmospheres of both of Earth's sibling planets carbon dioxide is by far the dominant gas. On both planets, there is more CO2 in the air than on Earth. On Venus it is about 300,000 times more! On Mars, the atmosphere there has a pressure only one hundredth or so of that on Earth, but 95 percent of it is carbon dioxide. So, even on this small planet, with its thin atmosphere, there is much more carbon in the air than on Earth. Our home planet does have enormous amounts of carbon dioxide on its surface. The ocean holds about 60 times more carbon dioxide than the atmosphere. But the bulk of the carbon dioxide on the surface of Earth is elsewhere. Practically all of it is tied up in carbonate sediments and in coal and other organic matter. Shell-making organisms and plants and algae ultimately are responsible for this large-scale solidification of carbon dioxide within carbonates (limestone rocks) and also within organic materials.
Making coal and other organic matter also led to splitting the carbon from the oxygen, with much of the oxygen staying in the air. This produced an atmosphere fundamentally different from those of Venus and Mars - one that is greatly impoverished in carbon and greatly enriched in oxygen. If left alone, the oxygen would readily re-combine with the organic carbon in the near-surface sediments on Earth.
The atmosphere is chemically out of balance and therefore "unsustainable" except by ongoing life processes.
The Great Barrier Reef - A coral reef composed limestone produced by algae. (Source: U. of Michigan, UCAR)
When looking at the system in this way, we see that the low carbon dioxide values are a result of moving CO2 from the reactive reservoirs atmosphere-plus-ocean to the much less reactive reservoirs (limestone- plus-organic matter). When limestone rocks are heated (in mountain-making processes), the carbon dioxide is released back to the atmosphere, as volcanic emissions. Weathering and life processes then have to move carbon dioxide back into the long-term storage, to keep the atmosphere from filling up with the gas. This cycle runs on a time-scale of millions of years. Mainly, it involves reacting silicate minerals exposed near the surface of the Earth with carbonic acid (from the reaction of dissolved carbon dioxide with water) and precipitating the resulting dissolved ions as calcium carbonate (calcareous sediments, limestone rock) and siliceous deposits (diatomites, chert) in the sea.After being buried and sitting around long enough, calcareous sediments and opaline sediments become solidified into limestone rocks and chert, respectively. (Chert is also called "flintstone".)
The precipitation of the carbonate is by organisms (algae, corals, mollusks, foraminiferans) and the precipitation of the silica is by diatoms, radiolarians and sponges. Thus, the content of carbon dioxide in the atmosphere is largely tied to life processes, as is that of oxygen. By moving carbon dioxide out of the atmosphere and into solids, Life has changed the heat balance of the planet, making it cooler than it would be otherwise. (We are now reversing this process to some degree by burning coal and oil.)
Mars, the Red Planet
Oxygen is freed when carbon is tied up in coal and other organic matter. It has, in fact, another important source, that is, water. It is freed from water (oxygen-hydride) when ultraviolet light splits off the hydrogen. Hydrogen is the lightest gas, and its molecules tend to leave the planet more readily than any other. What remains is the oxygen. On Mars, leftover oxygen combined with the iron in basaltic rocks to make rust, which gives the planet a reddish appearance.
On Earth, gravity is strong enough to greatly slow the escape of hydrogen, and carbon dioxide therefore is the more important source of oxygen. Oxygen does not build up indefinitely, but tends to stabilize at present concentrations. One reason is that there are plenty of sinks for oxygen, in the re-oxidation of organic carbon, and in the oxidation of basaltic rock (which contains reduced iron eager to combine with oxygen) and of sulfides. The more oxygen, the more readily these reactions proceed, diminishing the oxygen. (This is called "negative feedback".) Also, at some point, if the oxygen rises much higher than present values, forest fires and all kinds of spontaneous combustion would increase in frequency and severity.
The fact that carbon and oxygen partake in the same kinds of processes links their cycles very closely. In addition, since both these cycles depend on life processes, they are also closely linked to the cycles of those elements that play an important role in living organisms. These are mainly the nutrient elements phosphorus and nitrogen, but also certain metals such as iron, calcium, barium, molybdenum and copper, to name but a few. Each of these elements has a cycle that is at least partly controlled by life processes.
The single most important process linking carbon and oxygen is photosynthesis:
CO2 + H2O + nutrients + energy → CH2O (nutrients) + O2