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I read somewhere that the reason why Venus has a runaway greenhouse effect and Earth doesn't is the former's inability to form separate tectonic plates due to slightly higher temperature softening the then-cooling tectonic plate, preventing its fracture and thus disabling the $\mathrm{CO}_2$ scrubbing function of tectonic activity, which absorbs $\mathrm{CO}_2$ from air by rock weathering and then puts it back into the mantle. And once $\mathrm{CO}_2$ is a runaway in the atmosphere $\text{H}_2\text{O}$ and $\text{SO}_2$ will join forces with it multiplying the greenhouse effect. Therefore, the surface temperature of Venus rose to astonishing values long before life formed on Earth. Is this theory at least possibly correct?

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Venus orbits the Sun at 0.723332 AU. As solar radiation follows an inverse square law, the intensity of the Sun at the top of Venus's atmosphere is almost twice that for the Earth ($\frac1{0.723332}^2\approx1.9113$). Even when the solar system was young and the Sun was considerably dimmer than it is now, Venus received 1/3 more solar radiation at the top of its atmosphere than the Earth does now. Unlike the Earth, Venus did not suffer a faint young Sun problem.

That faint young Sun was perhaps instead a blessing for Venus. Many have argued that shortly after cooling off, Venus started as wet planet, like the Earth. The intense and increasing solar radiation eventually resulted in Venus undergoing a wet greenhouse process. Instead of being a trace gas in the Earth's atmosphere, water vapor may well have been the dominant gas in Venus's atmosphere. Thanks to the potency of water vapor as a greenhouse gas, this resulting in a vicious positive feedback loop. The increased water vapor raised Venus's surface temperature, resulting in even more evaporation/boiling. Meanwhile, at the top of the atmosphere, solar radiation dissociated water vapor. The hydrogen slowly escaped, eventually leaving Venus more or less free of water in any form.

Water may be necessary as a lubricant for plate tectonics. Whether or not this is the case, Venus does not have plate tectonics. It instead has a stagnant lid geology, punctured occasionally by extreme vulcanism (Siberian traps level vulcanism, and then some). This eventually released some of Venus's deep carbon reserve into its atmosphere. The end result is a planet void of water with a very thick carbon dioxide atmosphere.

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