>>11922800>Being close to the sun does not explain its dense atmosphere.Wrong.
Venus probably started off Earthlike with an Earthlike atmosphere, except with more nitrogen (Venus today has ~2x the nitrogen partial pressure as Earth, and would have had almost exactly the same when it formed). Basically it would have had twice the atmospheric pressure as Earth when it formed, with nitrogen as the primary component.
However, while in the early solar system Earth froze over, since the Sun was not as bright, Venus would have been fairly balmy from the beginning. In fact, given its thicker atmosphere, it would already have had a significant greenhouse effect just from the water vapor in the atmosphere.
Over hundreds of millions of years the Sun slowly brightened and more and more CO2 from Venus' interior would have been outgassing through volcanism into the atmosphere, increasing surface temperatures. For a while this would have been a slow process as dissolved minerals in the primordial oceans would have reacted with the CO2 to form limestone, but eventually due to the lack of fresh mineral uplifts the rate of CO2 outgassing would outstrip the rate of sequestering enough that a tipping point was reached.
This point came when the planet became too warm for the oceans to circulate vertically, and instead the warm upper layers became saturated with CO2 and stopped allowing any more CO2 to react with any more minerals in the water. This lead to an accelerated rate of CO2 concentration increase in the air, thus increasing the temperature in a faster loop of heat leading to increased evaporation trapping more heat, until the surface of the planet became so hot everywhere that liquid water could no longer exist at the surface.
The resulting blanket of hot water vapor and nitrogen would have caused the temperatures at the surface to skyrocket, and prevent any further carbon sequestering. Fast forward billions of years and the water has been lost while the CO2 remains.
