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My previous question turned out to not ask the question I thought I was asking.

I have read that Venus began its heating-up around 700-750 million years ago.

When did the Venusian atmosphere get roughly as hot as it is now?

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    $\begingroup$ I suspect the answer is "nobody knows" (for sure) The link suggests that the heating-up could have started 750 million years ago, but this is presented as new and uncertain science. But I think the question "If there was a massive CO2 release that was the result of global vulcanism, 750 mya, then what?" is an interesting quetion. $\endgroup$
    – James K
    Sep 19, 2023 at 22:23
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    $\begingroup$ The temperature and hence runaway greenhouse is determined by its amount of $CO_2$ in the atmosphere - a whopping 90 bars of it! It's unknown whether volcanism unleashed this amount of $CO_2$ on Venus, or photolysis of water and subsequent oxidation of carbon-rocks - sorry too lazy to write another answer currently. $\endgroup$ Sep 19, 2023 at 23:12

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According to current models and computer simulations, Venus was a water world many billion years ago but due to the close proximity to it's host star, Sun or Sol, the water was Photocatalytically split from the UV rays. This split the water molecules and it broke down into hydrogen and oxygen. Now the hydrogen is less denser than the other gases so it had an hydrodynamic escape. Left with oxygen, the highly electronegative component reacted with carbon and that lead to the formation of $CO^2$, other geological processes like volcanic activity (that occurred 2.5 million years) also may have played a role. The CO2 absorbs sunlight efficiently since it is an greenhouse gas and has high heat capacity, this lead to a runaway greenhouse effect, heating the planet drastically.

However it started to maintain this sort of thermal equilibrium when CO2 started to heat up till it matched the temperature of the Sun, applied with inverse square law. This is due to the Conversation of Etendue, which means the maximum a ray of light can heat up an object is equal to the temperature of the blackbody that emitted it. So technically it can't heat up more because it is at it's max temperature with respect to it's CO2 level. Presently, This thermal equilibrium is thought to have been started when the CO2 level matched today. The atmospheric pressure of Venus = today might have been >2.5 million years because volcanic activity started at that time, and volcanic activity leads to rapid increase in CO2, which would heat it up further. Therefore thermal equilibrium established somewhere between from 2.5 million years upto the present day

enter image description here

Thank you, Hope it helps you!

Note: Venus is not in the habitable zone yet it once hosted water is due to the dimmer Sun, during that period the Sun was almost 20-30% dimmer, so technically at that time the Goldilocks zone radius was much smaller, however as time passed by the Sun brightened so the water evaporated.

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    $\begingroup$ "due to the close proximity to it's host star, Sun or Sol, the water was Photocatalytically split from the UV rays." did the lack of an internally generated magnetosphere also play a role here? $\endgroup$
    – eps
    Sep 20, 2023 at 16:29
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    $\begingroup$ "2.5 million years" That is the blink of an eye in geological time. Did you mean 2.5 billion years ? $\endgroup$
    – James K
    Sep 20, 2023 at 17:15
  • $\begingroup$ I think 2.5 million years is low as well. Venus' surface is thought to be relatively young, but estimates I've read on its planet-wide resurfacing is between 350 and 750 million years ago. solarsystem.nasa.gov/planets/venus/in-depth $\endgroup$
    – userLTK
    Sep 21, 2023 at 14:10
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    $\begingroup$ @eps New questions should be posted as a separate question, not in comments, but briefly, a magnetic field shields the planet's upper atmosphere from charged particles most commonly from coronal mass ejections, but not from UV rays. The UV rays can have enough energy to split molecules. $\endgroup$
    – userLTK
    Sep 21, 2023 at 14:17

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