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Is it possible to have a planet where the atmosphere is mostly water vapor instead of mostly N + O2 with solid ground, so not a gas giant?

In the sense - how close to the center star(s) should it be for water not to exist in greater bodies of liquid water or in any solid state as a result of water freezing below 0 degrees?

What effect would that have on the other parts of the planet ?

The planet is thought of as earth sized +/- 10% (not set in stone).

Where the people lives in domes and have humidity extractors to produce water for fields and oxygen to breathe

Could it be possible for people to exit the dome and survive just for lets say 20-60 seconds (beyond the time you can hold your breath) or maybe even longer? If so what would the effects be (such as water in the lungs?)

This is for a space-goth roleplaygame, roguetrader. So any mechanics or inventions can be advanced but should be kept in range of the universe as much as possible

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    $\begingroup$ Just one clarification:"water vapor" is water in it's liquid form, just in very, very tiny droplets distributed in the air. "Steam" is water in gaseous form. Walking into an atmosphere of steam would result in getting burned, but if "water doesn't exist in liquid form", then it's not water vapor, it's steam. Are we talking about water vapor or steam? $\endgroup$ – LindaJeanne Jun 10 '15 at 10:58
  • $\begingroup$ Steam is usually like 100+ degrees Celsius and would more or less instantly kill a person. By "liquid form" i mean it is liquid like mists but not liquid like lakes. $\endgroup$ – Magic-Mouse Jun 10 '15 at 11:11
  • $\begingroup$ Ah. I was confused by the "how close to the center star(s) would it be for water not to exist in liquid or solid state?". To me, that implied steam, while everything else in the question implied water vapor. (I think water vapor would require an atmosphere in which to be suspended? That if it were the 'primary atmosphere' it would just percipitate to the ground?) $\endgroup$ – LindaJeanne Jun 10 '15 at 11:13
  • $\begingroup$ It was more ment not ice, and hot enough to make mists but not hot enough to boil the planet ;) about the Atmosphere needed, sure, but how would that work ? Somehow like the integration of nitrogen and oxygen ? $\endgroup$ – Magic-Mouse Jun 10 '15 at 11:23
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    $\begingroup$ @LindaJeanne - No. ""water vapor" is water in it's liquid form, just in very, very tiny droplets distributed in the air." is just wrong. Water vapor is the gaseous form of water. You are confusing clouds, which are composed of tiny water droplets, with water vapor. In the case of clouds, the droplets condensed from water vapor, but that's not the same thing. $\endgroup$ – WhatRoughBeast Jun 10 '15 at 16:16
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If the planet were quite hot as @Formagella describes, say around 80°C, there likely would be no (constant) bodies of water and no steam. Whatever water the planet had, would mostly be in the atmosphere. If it's a lot of water, that means terribly bad storms, floods, F25 hurricanes, far worse than anything we see on Earth. If it's little water, it's going to be a very desolate place, mostly a barren, desert world.

The inhabitants could live in the mountainous regions, where the elevation reduces the temperature. Perhaps even above the "bad storms," on land or on airships. But there are some issues with these. It's hard to grow food at high elevations, mainly because it's too cold for us, but also because there's little flat, fertile soil. On airships, hydroponics would probably be used to reduce weight.

Since the "air" is going to be thinner at high elevations, so too will the water vapor content. So the "extractors" must be bigger and use more power. It is possible to disassociate water (H2O) into H2 and O2 by electrolysis. That too takes a lot of power. The good news is, a planet near it's star gets LOTS of solar energy, so sunlight (solar panels) are a suitable choice.

If many humanoids were consuming the oxygen, then twice as much hydrogen (H2O) has to go somewhere. Some of it could go to the airships to keep them afloat, however let's not forget the Hindenburg! On Earth, free hydrogen doesn't exist in the air. It either rises through the atmosphere and out to space, or combines with other atoms to form compounds. So for instance, if your (storming) atmosphere had nitrogen in it, then NH3 (ammonia) could form from lightning strikes, which is quite caustic and hazardous even at low levels. (It's flammable at higher levels!) Now if the inhabitants were intelligent and responsible enough to see ammonia levels rising and compensate for it ("ammonia scrubbers"), then it wouldn't be an issue. But that's more solar panels, more equipment...

Longetivity-speaking, if this civilization had lived and prospered there for a very long time, then the water vapor levels will drop as they are slowly consuming the vapor. So there would be legends and stories about the "really bad storms of old", and contrast to how much calmer the weather is now... and an uneasiness about the future, where the vapor is gone.

Also consider that plants here need sunlight, water, and CO2 (carbon dioxide) to grow. Sunlight, water, and water vapor aren't enough. Plants also produce oxygen. So the fact that this world has no oxygen, seems to infer that it has no plants. So maybe it should have oxygen, just very little of it, because it isn't a lush green world like Earth. Perhaps the inhabitants are in a rush to grow as many plants as they can, before the water vapor runs out?

The Earth only has 0.039% carbon dioxide in the air, but that's enough to grow all of the plants on the entire planet. And scare the "climate-change" theorists half to death.

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    $\begingroup$ Re the Hindenberg, hydrogen doesn't burn unless there is sufficient free oxygen to combine with it! $\endgroup$ – jamesqf Jun 10 '15 at 18:22
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    $\begingroup$ There should be some oxygen in the atmosphere if there is any plant life. $\endgroup$ – rdtsc Jun 10 '15 at 18:35
  • $\begingroup$ Why would the hydrogen accumulate ? Where the hell is the oxygen supposed to go? At some point we would either have bound it completly in CO2 and we die because there's no food left or it will be released back in the atmosphere (e.g. by plants) where it would slowly react back to water. If it's too cold to grow food on the mountain top then just go down to levels where the temperature suits you. The problem growing food in high altitudes might be the low pressure but an atmosphere off water vapor would have a higher pressure anyway so you need reduce it. This answer really needs revision. $\endgroup$ – Christoph Aug 9 '17 at 14:17
  • $\begingroup$ 1. "On Earth, free hydrogen doesn't exist in the air. It either rises through the atmosphere and out to space, or combines with other atoms to form compounds." 2. The O.P. seems to infer that there is no free oxygen. 3. If you go down in elevation where it is warmer, the weather severity would prohibit plant life ("terribly bad storms, floods, F25 hurricanes, far worse than anything we see on Earth.") 3. If my answer is so awful, why don't you answer it? $\endgroup$ – rdtsc Aug 9 '17 at 23:08
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from wikipedia, this is the maximum quantity of water in air at a given temperature, on earth, so since your planet is pretty similar it should apply too. Evaporation and condensation happen all the time but you can't add more water vapour without having an equal quantity precipitate.

As you can see, the temperature gets scorching hot long before you really get in the situation you describe. At 100°C you can have 100% water of course, but that's steam and you said you didn't want that, you just want water vapour.

I think it's easier if you think of salt in a glass of water: you can keep dissolving salt in it, until it's saturated, then any salt you add will cause precipitation of undissolved salt on the bottom. The water will be quite salty, but it's still water. Even if you put the temperature of the whole atmosphere at 40°C which is high and would already create a huge problem of thermal dissipation in the domes (if you're surrounded by something that is always at 40°C, keeping the dome cold would mean having some huge free power source and running a huge incredibly power-hungry AC system), at worst you're still getting the situation that you can encounter in a jungle during a particularly hot day. Which doesn't kill strong enough humans.

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Since an atmosphere of steam is out, then you have two possible conditions (that can happen here on Earth, so it will be at least possible on a local level on some of your planet).

  1. The air is cold and moisture condenses, creating a thick fog or mist. So long as there is a warm zone to evaporate water and feed it into the air and a cold zone for it to condense, this cycle can go on indefinitely. This suggests a planet tidally locked to its primary, so the "hot pole" is the source of all the water vapour, and the misty section takes place on the cold, dark hemisphere. I would think this would be below the equatorial "twilight region" to ensure it is cool enough for the fog to form. The winds would be fairly constant as well on a tidally locked planet, so the fog would be fanned out fairly evenly across the condensation zone.

  2. The air is very hot and you are in a coastal region, with the humidity at 100%. This isn't as visually exciting as a perpetual dense fog, but people living in places as diverse as Dubai and Costa Rica encounter this sort of climate. Unless there is a strong breeze, it is extremely uncomfortable and difficult to do any work, which explains why traditional culture in places like this often emphasized a siesta during the hottest part of the day, and work was done in the early morning and late evening hours. If we use our tidally locked planet again, then the region surrounding the "hot pole" will resemble this.

Your planet then could have two places where the atmosphere is pretty much saturated with water, a donut shaped ring of 100% humidity surrounding the "hot pole" and another one surrounding the "cold pole". There would be a fairly complex hydrological cycle needed to maintain the transportation of water from the hot pole to the cold pole and back again, otherwise, eventually the water will precipitate out on the cold side and the cold pole will be covered in miles deep glaciers, while the hot pole will be a desert hotter and drier than any known on Earth.

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Maybe, what you'd need if you wanted it livable would be a planet that was hot, as in radioactive, or very young, the ground would be hot enough that any water condensing and falling out would evaporate again almost immediately, while the atmosphere would stay relatively cold. The humidity on this world would be over 100% relative, you'd have constant pea-soup fog and condensation on every available surface including your lungs if you try to breath it for too long. As long as there was enough oxygen in the atmosphere and not too much in the way of sulfurous compounds boiled out of the ground, you could get away with being outside for a while before the heat and humidity got to you too badly, it would be a like trying to live here over the longer term, only more unpleasant.

Edit: Sorry forgot to mention this planet has to either be 1. quite far from it's primary or 2. orbiting a small Red Dwarf star in order that the atmosphere doesn't get much heat from the primary and remains breathably cool.

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It looks like the planet must have been hot as said earlier. Phase diagram of water shows that at human-friendly temperatures it is possible to make a virtually 100% water vapor atmosphere if pressure is low. In this case, a pressure suit and pressurized habitat are necessary. If water remains gaseous at high pressures, it is too hot for humans and habitats must be cooled.

Here is the diagram. Pay attention to the area between the red lines. https://en.wikipedia.org/wiki/Phase_diagram#/media/File:Phase_diagram_of_water.svg

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