The Carbonate Circuit Process

The most prominent volatile substance on terrestrial worlds is water, followed by carbon dioxide and then nitrogen. Water becomes liquid if the planet is cool enough to allow for this; carbon dioxide reacts with basic minerals (metal oxides and silicates) forming carbonates, if the planet is not too hot. Therefore, Earth's atmosphere is mainly composed of the remaining gas: nitrogen. Water is mostly in the oceans, carbon dioxide bound as carbonates. Now, both liquid water and carbonates coexist with water vapour resp. gaseous carbon dioxide in a dynamic balance, a circuit process. The water circuit is simple: water evaporates from oceans, lakes and rivers, and rains down again. But there is also a carbonate circuit. If carbonates are carried into deeper layers of the Earth's crust, they are broken up, and carbon dioxide is released into the atmosphere through volcanoes. But it does not accumulate there indefinitely. It gets washed out by rain, and forms carbonates again when the dissolved carbon dioxide gets in contact with basic minerals.

Now, carbon dioxide is the most important variable greenhouse gas in terrestrial atmospheres. This means that the more carbon dioxide there is in the planet's atmosphere, the warmer the planet is. The trick is that the warmer the planet is, the more rapidly the water circuit is turning, and the more carbon dioxide is washed out of the atmopshere, cooling the planet. (One can safely assume that geological processes are not significantly influenced by changes of the planet's temperature by a few degrees, which means that the rate at which the washed-out carbon dioxide is replenished through volcanoes can be assumed to be constant.) On the other hand, if the planet cools down for some reason, the carbon dioxide concentration in the atmosphere rises, warming the planet. This keeps temperatures on Earth (or a similar planet) stable in the long run.

However, this has its limits. The upper temperature limit is marked by the point where no more carbon dioxide is left to be washed out. At that point, a further increase of temperature can no longer be compensated. With the carbon dioxide greenhouse effect being reduced to zero, the planet's temperature is now governed by another greenhouse effect which is normally overshadowed by the carbon dioxide greenhouse effect: the one of water vapour. Unfortunately, this one does not stabilize the planet's temperature: the warmer the planet is, the more water evaporates, enhancing greenhouse effect and warming the planet even more, until the oceans start boiling away.

This cataclysm pushes the water vapour greenhouse forward, heating the planet up to perhaps 200 or 250 degrees C. At those temperatures, there is of course no longer a water circuit. This means that carbon dioxide accumulates again, adding to the greenhouse effect. Now the planet is so hot that the carbonates on the surface are slowly broken up, releasing more carbon dioxide and pushing up the temperature, until there are no carbonates left. The result is a superdense carbon dioxide atmosphere and temperatures that would melt lead. Venus is an example for this.

The other extreme is that the planet becomes so cold that the water freezes. This, however, brings the water circuit to a halt, letting carbon dioxide accumulate again, which might stop the planet from freezing over. However, there is a limit to even that, set by a run-away albedo effect. When a planet cools down, its polar ice caps increase in size. Ice is white, it reflects light very well. This means that the planet receives less radiation from the sun and becomes colder. More areas freeze over, more light is reflected, ...