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Suppose one fine day a mischievous god decided to eradicate all air molecules on Earth in an instant, my question is what will hapen to the ocean? Please kindly disregard (omit) your concern for life in your answer the extremophiles can have the last laugh I don't really care.

Kindly split your answer into 2 parts one for when the average air density is at 50% and another is when the air is totally gone, thanks.

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    $\begingroup$ If all the air is removed instantaneously, why would there be a halfway mark? $\endgroup$ – Frostfyre Jun 17 '15 at 5:12
  • $\begingroup$ @Frostfyre this instant occurs at no time at all for us but last eons for this divine entity so in order not to upset Poseidon in one go hence the trial. $\endgroup$ – user6760 Jun 17 '15 at 5:15
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At 50% not much happens, at 0% all the oceans begin to violently boil.

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The average ocean surface temperature is 62.6 °F. If the density is lowered by 50% the pressure is also 50%. At 0.50 bar, waters above around 188 °F (86 °C) will begin to boil, there are no oceans that hot.

At 0 bar all liquid water everywhere boils. At some point, that vapor pressure slows the water from completely boiling away and a water vapor atmosphere forms. I think what happens next is that atmosphere freezes on the dark side of the planet and falls into the boiling ocean and onto land and vapor from the light side fills the vacuum. It would be fairly chaotic and I'm not sure what, if any, the steady state would be.

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  • $\begingroup$ Can you estimate how thick this steam is going to be? $\endgroup$ – user6760 Jun 17 '15 at 5:47
  • $\begingroup$ @user6760 First, it's not steam, that comes from heated water. And, no, I can't estimate that, the water vapor won't be a steady state. It would be very chaotic. $\endgroup$ – Samuel Jun 17 '15 at 6:19
  • $\begingroup$ It's impossible to measure without temperature, but if the earth is a similar temperature to what it is now - figure, daytime temp of 20 degrees C - about, the chart gives water pressure would be about 50 mbar - give or take. 1/20th of 1 ATM. From there, it's a question of what the effect of a much thinner, pure water vapor atmosphere would be like - would it hold more heat or less. My guess would be less, and we'd have a snowball earth situation over time, but it's more of a quick semi-logical deduction - I wouldn't swear to that as an outcome in this improbable scenario. $\endgroup$ – userLTK Jun 17 '15 at 16:04
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    $\begingroup$ @userLTK Yes, it's a global system, so would be very complex. It also seems likely that many gasses would be released from the ocean and form up some non-water vapor atmosphere. But, it's very difficult to say how much and what its state would be. If it takes magic to create the situation then it's especially difficult to say what would happen. $\endgroup$ – Samuel Jun 17 '15 at 16:09
  • $\begingroup$ As Samuel alluded, the Earth's oceans possess quite a bit of dissolved gasses in them. As the water boiled, those gasses would be released too so you wouldn't end up with a pure water vapor atmosphere. Oceans contain between 0 - 8.5 mL $O_2$ / $L_{water}$, 8.5 - 14.5 mL $N_2$ / $L_{water}$, and $CO_2$ too. $\endgroup$ – Jim2B Jun 18 '15 at 3:11

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