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Adhering to natural laws and physics as much as possible[1], what could cause Venus to go from a habitable planet to its current state, and how long would it take from the apocalyptic event to achieve the current state?

In this scenario, human life evolved on Venus, the apocalyptic event took place, and humans moved to earth. Events after that do not matter and do not need to line up with actual human history.

Venus Pre Apocalypse

  • Habitable
  • Chemical composition same or similar to earth
  • Able to support human life[2]
  • Hosts Type 1 civilization[3]

Apocalyptic Event

  • Can be man made[4]
  • Can be natural[5]
  • Can be both
  • Can be neither[6]

After TBD amount of time post Apocalypse

  • Venus is how it is now

Specific sub-questions

  • What would slow the rotation of the planet?
  • What would cause the drastic chemical change to acidic atmosphere?

  1. Does not have to be perfectly scientific, but reasonably realistic. Type 1 Civilization-esque level of technology and “science” allowed.

  2. Or a predecessor that could micro evolve to live in the similar chemical composition of earth. If different, please outline said differences.

  3. Space-faring but not yet colonizing. See this SE answer for plausible reasons for delayed space travel.

  4. What political landscape would be in place and/or cause such an event?

  5. Nothing is out of the question so long as it resolves into the current state of the solar system

  6. If you are clever enough to think of what that might possibly be. I’m not.

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A runaway greenhouse effect. Just like early life started producing oxygen that changed the atmosphere, perhaps some metabolism or geologic force produced a super-greenhouse gas. Once the state was pushed beyond some point, a positive feedback cycle was initiated and the thick carbon dioxide atmosphere at high temperature was the new stable equilibrium point.

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    $\begingroup$ Said super-greenhouse gas might have been as mundane as water vapor. Venus likely used to be similar to earth, with liquid water and plate tectonics. Increasing luminosity from the sun or fluctuations in the carbon cycle might have caused widespread evaporation of water. Water vapor is a good greenhouse gas, so this would have resulted in increasing temperatures and more evaporation. Enough repetitions of this cycle might have dried up enough water to halt plate tectonics, causing any carbon cycle it might have had to break down, which would result in a buildup of carbon dioxide. $\endgroup$ – emo bob Jun 30 '16 at 16:55
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Lack of continents. Venus has one solid shell while earth has continents. Earth releases extra energy through cracks at plates and the continents themselves moving. Venus does not, and as such this extra energy can only be release through volcanoes. Volcanic event releases a huge amount of sulphur and the runaway warming converts Venus to what it is now. But this doesn't explain the rotation of Venus.

More fun but further out there....Venus developed where it is now, but earth did not. Earth originally formed where the asteroid belt did. Venus was a habitable planet at this point. A cosmic scale collision occurs and the planet is now relocated to where earth is now. The presence of earth slowed and eventually reversed Venus's rotation and brought it to where it is now. Magnetism and a planets magnetic field protects the planet from harsh solar radiation and a planets rotation is required to support that magnetic field. When Venus stopped, the magnetic field weakened to the point where solar rays fried all life on Venus leaving it where it is today.

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  • $\begingroup$ I don't think your cosmology is all right. How would Earth affect Venus's rotation? Lack of magnetic field will cause the atmosphere to erode; how is that relevant to a crazy dense atmosphere? $\endgroup$ – JDługosz Jun 30 '16 at 2:04
  • $\begingroup$ Earth and venus can be considered tidal locked...3 days on venus is 2 trips around the sun for the earth. Tis a stretch, but earth and venus gravitationally effect each other and have billions of years to do so. Lack of magnetic field lets all radiation from the sun through, destroys all life capabale of refining amd shaping the atmosphere and all that left is gas from volcanoes over millioms of years. $\endgroup$ – Twelfth Jun 30 '16 at 2:31
  • $\begingroup$ See this question, @twelfth. It sounds like you are running into that same outdated piece of information. There's no way such passes could affect the rotation rate to such a magnitude, anyway. $\endgroup$ – JDługosz Jun 30 '16 at 2:34
  • $\begingroup$ Earth had life under the sea long before it had an ozone layer. I don't think lack of a magnetic field would prevent life in general, just land life. Sulpher from volcanoes cools the planet; you have that backwards or are thinking of the wrong stuff. $\endgroup$ – JDługosz Jun 30 '16 at 2:38
  • $\begingroup$ @jdludosz Sulphur is the dense heavy atmosphere comment, not the warming, i can see the confusion i left there. Ozone layer amd magnetic field are not the same things and ive yet to refer to ozone layer in this answer...solar winds would rip the atmosphere away after prolonged exposure (including ozone and water vapour) and leave behind an atmosphere regenerated by volcanoes. Not counting a one time event like one good CME from the sun that strikes a planet with no magnetic field would fry much of it. $\endgroup$ – Twelfth Jun 30 '16 at 16:48
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The positioning of Venus is somewhat important but in reality you need to do one thing, alter the atmosphere. The greenhouse effect on Venus is many, many times what is on earth and this is highly related to the CO2 content of the planet. There are many of realistic ways to induce massive amounts of CO2, from standard pollution to warheads, basically anything that releases it as a byproduct. A type I civilization should definitely have it within its means to induce such an apocalyptic event.

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  • $\begingroup$ We'll soon see. $\endgroup$ – A. I. Breveleri Jun 30 '16 at 4:49

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