The Goldilocks Theory

Just as Goldilocks found the porridge that was just right, the Earth seems to be just right for conditions favorable to life. The reasons have to do with the fact that the Earth seems to be the perfect distance from the sun for abundant liquid water.

Venus is too close to the sun, and too hot for liquid water on its surface. In fact, it is so hot that the volatiles have all been driven into the atmosphere, so it has a thick and heavy atmosphere. Heat bakes the water out of the ground and prevents the special kind of hydrated minerals which allow for plate tectonics.

Mars is too far from the sun, and too cold for abundant liquid water on its surface. With no liquid water, the special kind of hydrated minerals which allow for plate tectonics are prevented from forming, and so atmospheric gases which become trapped within the ground stay trapped within the ground and are not recycled back to the atmosphere by way of volcanism. Thus over time Mars has developed a thin atmosphere, and all the water is frozen into the ground.

The temperature of Earth is just right for liquid water on the surface, and for the hydrated minerals which produce plate tectonics. With plate tectonics, atmospheric gases which become trapped within the ground are recycled back to the atmosphere in the form of volcanic emissions. These conditions favor repeated replenishment of the planet's atmosphere, which helps to maintain, via the greenhouse effect, surface temperatures just right for liquid water. These conditions are just right for abundant life

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