The Appearance of Life on the Primitive Earth Favourable Conditions

Lying 1 AU from the Sun, Earth has benefited, ever since its formation, with a temperature that enabled liquid water to exist. Subsequently, it also acquired the conditions that allowed it to preserve this favourable environment. One essential process is plate tectonics, which ensures that atmospheric carbon dioxide is recycled. Plate tectonics, driven by the Earth's internal energy (whose origin is the radioactive decay of unstable isotopes within the interior), recycles oceanic crust by carrying it down into the mantle at subduction zones, and regenerating it at mid-ocean ridges. The presence of a moderate amount of CO2 in the terrestrial atmosphere ensures that there is a greenhouse effect sufficient to raise the planet's equilibrium temperature by some thirty degrees, which allows the water in the oceans to remain liquid. The mass of the Earth plays a vital role here, because it is what provides, via radioactivity, sufficient internal energy to drive plate tectonics and volcanism.

We may note that the Earth's geological record appears to show that the planet has, in the past, suffered from almost complete glaciation. The cooling process is one that is self-reinforcing: if the surface temperature decreases, the area of the polar caps increases, which increases the planet's overall albedo. So the planet absorbs less solar energy and its temperature tends to decrease even farther, which increases the glaciation. It seems that several episodes of global glaciation took place between 750 and 580 million year BP. It was probably a violent volcanic episode that reestablished the greenhouse effect by injecting a large amount of carbon dioxide into the atmosphere.

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