# What effects would a massive amount of water vapor in the atmosphere have on an alien planet

I am an aspiring science-fiction novelist. I am experimenting with a world I’m trying to create and I wanted a couple of expert opinions on Atmospheric composition;

53% Nitrogen 27% Oxygen 3% Xenon 15% Water Vapor 2% Carbon Dioxide

Off the bat, the air isn’t supposed to be breathable for traditional earth-based life. It is inhabited by a couple of cultures (most primitive being Neolithic hunter-gatherers, most advanced Iron Age equivalent to the Viking Age), it has a lighter gravity than earth with twice the atmosphere and I am curious what would happen to an explorer from Earth if their breathing mask was removed.

Edit: by twice the atmosphere I mean the atmosphere is twice as thick, allowing more massive creatures to fly.

• Welcome to Worldbuilding, Jacob. Just to be clear, in your question you mention 'twice the atmosphere'; are you talking about twice the pressure? Atmospheric pressure and gravity don't always correlate (think Earth v Venus) so it would be useful to have that clarified via edit. – Tim B II Jul 24 '19 at 4:52
• well you will not have any solid land for a start, saturation ballance will cover the planet in a massive sea. – John Jul 24 '19 at 5:03
• Alright, interesting to know. I am going to edit the levels down a bit. – Jacob Badger Jul 24 '19 at 5:30
• Mind that this is not a negotiation game. Your second edit now invalidates the given answer, which is not what an edit should do. Edit rolled back. – L.Dutch - Reinstate Monica Jul 24 '19 at 5:35
• I advise you to take the tour and visit the help center to better understand our community and how it works. – L.Dutch - Reinstate Monica Jul 24 '19 at 5:36

They would cook.

You are asking for 0.3 atmospheres of water vapor. The temperature of water with that equilibrium vapor pressure is nearly 70 degrees Celsius, or 158 Fahrenheit. An unprotected human will not survive that for very long. And the fact that the air is already saturated with water means our natural heat rejection mechanisms won't work.

The carbon dioxide is also at toxic levels. Despite the high oxygen pressure, the elevated CO2 will cause blood acidosis, leading to hyperventilation and cognitive impairment. The xenon would also have a mild anaesthetic effect, further impairing cognitive function and motor skills.

Neither of those will kill you quickly, though. The real killer is the heat. The CO2 and xenon will merely contribute to making it more difficult for you to develop the presence of mind and motor coordination to actually do anything about the heat.

The oxygen is not up to toxic levels yet for most people, but it may be dangerous for some fetuses and newborns. That, however, will be the least of their problems if a fetus is exposed to this air.

• I will say I like that it isn't the unconscious in "30 seconds, dead in 4 minutes" though the picture you gave me with the impaired cognitive function and motor skills gave me the impression of the poor explorer stumbling around almost drunk or crawling like the Wolf of Wallstreet after the Quaaludes kick in as they slowly cook alive. Though I am curious if the heat and saturation problem you describe would remain the same if the water vapor dropped to 5%. – Jacob Badger Jul 24 '19 at 17:04
• @JacobBadger 5% with a twice-as-thick atmosphere makes for 0.1 atmospheres of pressure; that corresponds to a temperature of at least 48C / 120F. That is barely survivable for a human in a dry climate who can sweat with plenty of water (and indeed, there are places on Earth that occasionally get that hot)--but it'll still kill you, just slightly less quickly, when the air is saturated with water. – Logan R. Kearsley Jul 24 '19 at 17:30
• Ok still a slow death, I like it. So how long would an explorer have before that kills them? An hour? Longer? – Jacob Badger Jul 24 '19 at 18:44
• @JacobBadger That's tricky to figure out. You'd get hyperthermia in 10 minutes or less, I can't find anything that indicates exactly how long that will take to kill you, absent other medical conditions. That's probably a good thing to ask another question about--possibly on medicalsciences.stackexchange.com – Logan R. Kearsley Jul 24 '19 at 18:51
• @JacobBadger From what I've gathered, I'm assuming you want a human explorer to somehow end up on this planet and maybe die (maybe escape, who knows) But the heat and atmosphere aren't the only problem Mr. Explorer would encounter. Our human bodies evolved to metabolize Earth-based creatures who have a familiar DNA-structure, and on an alien world, the chances of the alien's equivalent of DNA being remotely recognizable by the digestive tract are extremely slim. If you want to make Mr. Explorer suffer even more, he shouldn't be able to eat anything he finds on this world. – Foosic17 Aug 1 '19 at 3:55