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What would have been the approximate surface temperatures and pressure on Venus if its atmospheric composition was like the Earth's that is ,being composed primarily of nitrogen and oxygen,rather than CO2 ?

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  • $\begingroup$ This question isn't really answerable, because it would not be possible long-term for Venus' atmospheric composition to be like the Earth. There are too many factors that are different between Earth and Venus (oceans, life, rotation, magnetic field, to name a few) that any naïve calculation of such an average temperature would not produce a useful result, in particularly not if you are thinking of "terraforming". $\endgroup$ – gerrit Nov 11 at 10:04
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Other factors being equal, planetary temperature (measured on an absolute scale e.g. Kelvin) falls with the square root of distance from the sun.

With a semi-major axis of 0.723 AU, we would expect Venus to have an average planetary temperature about 1/0.85 that of Earth’s 288 K: 339 K or about 66°C or 150°F.

Since surface temperature is largely regulated by atmosphere and hydrology, which effects are in complex feedback loops with temperature, the actual average temperature at the surface would likely be very different — and with enough water vapor in the air it might still go into a runaway greenhouse feedback loop.

In any case it would likely be substantially higher than human body temperature, making it unsurvivable without active cooling.

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  • $\begingroup$ Other factors being equal, planetary temperature (measured on an absolute scale e.g. Kelvin) falls with the square root of distance from the sun. Can you source this? I was going to do a computation using the StefanBoltzmann law, which I think may give the same answer. $\endgroup$ – barrycarter Nov 10 at 18:30
  • $\begingroup$ I don’t have a good source to cite, unfortunately (I learned it from a sourcebook for a science fiction RPG!) but I believe it follows simply from Stefan-Boltzmann versus the inverse square law of radiation. $\endgroup$ – Russell Borogove Nov 10 at 18:57
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    $\begingroup$ 150 = 147 for large values of 147. $\endgroup$ – Russell Borogove Nov 10 at 22:22
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    $\begingroup$ Shouldn't you multiply this factor with the Earth-without-the-greenhouse temperature, then add the greenhouse effect? So 1/0.85 * 255 + 33 = 333 K? And I would question whether the number of severe simplifications in this estimate are so numerous that the result is not actually useful. $\endgroup$ – gerrit Nov 11 at 10:00
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    $\begingroup$ Or (ignoring magnetic fields and all other differences) Venus would need to develop unicellular photon-absorbing life (bluegreen algae, etc) faster and in far greater quantities than on Earth. $\endgroup$ – Carl Witthoft Nov 11 at 17:02

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