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Ignoring how this sort of atmosphere would come about, what are some things that happen regularly on Earth that would be altered by this sort of atmosphere? Example: Would firearms still ignite normally with that much argon present? Would humans have trouble breathing? How easily would light pass through without reflection/refraction?

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It seems like the main change you want to make is the replacement of atmospheric diatomic nitrogen by argon. I propose, for minimal disruption and to keep the scope of the question contained, that this is be the only change you make. (Getting rid of CO2 would be disastrous for plant life, getting rid of water vapor impossible if water is present, and getting rid of the lesser trace components... we could have an unexpected dependency on some trace gas that we can't fulfill from diet that we don't even know about yet because we've never been in atmosphere w/out the trace gas.) – SudoSedWinifred Jan 11 at 21:44
    
That said, replacing all atmospheric nitrogen by argon would be a nightmare for all life on Earth. Atmospheric nitrogen is our source of fixed nitrogen (ammonia and derivatives, usable by a broad range of life, are produced by some bacteria and archea, and by electrical storms.) So, on Argon World, you at the very least need an alternative source of fixed nitrogen. It is colorless (throughout the range of earthly atmospheric temperatures, anyway), odorless, and tasteless. Asphyxiation is a risk, as it is significantly more dense than oxygen, so it will accumulate lower in the atmosphere. – SudoSedWinifred Jan 11 at 21:54
    
Inert gas narcosis is a thing. Argon is 2.33 times more narcotic than nitrogen so humans would suffer mild effects. I'd assume this would be fixed by evolution, so the actual effect would be everything having nervous tissue more resistant to narcosis. – Ville Niemi Jan 12 at 2:04
    
@SudoSedWinifred - Argon has a molecular weight of 40, oxygen 32 (2 atoms per molecule), Nitrogen 24. The density difference wouldn't be significant, as we don't have issues with Oxygen sinking relative to Nitrogen now. – Oldcat Mar 2 at 22:28
    
The density of argon at stp is 1.784 g/L, whereas diatomic oxygen and nitrogen have respective stp densities of 1.429 g/L and 1.251 g/L. Oxygen is denser than nitrogen, so to the extent that density matters, oxygen sinks relative to nitrogen. But the lowest 100km of our atmosphere is well-mixed by air currents. However, heavy gasses can accumulate in poorly-ventilated ground and subterranean structures. Oxygen being denser than nitrogen, no problem. But with argon denser than oxygen... an Ar-based atmosphere could be dangerous. – SudoSedWinifred Apr 26 at 2:48

You've basically replaced nitrogen in Earth's atmosphere with argon, but left the ratios otherwise intact.

Argon is slightly denser than nitrogen; as a result, there would be slightly more light refraction in your argon atmosphere, but unfortunately I can't tell you what that would end up looking like to an observer. The speed of sound would also be ~10% slower, which isn't likely to be noticeable until timed.

Combustion (including firearms) would not be significantly affected, since that's really just oxidation of a fuel and you've left the ratio of oxygen in the atmosphere untouched. One thing that does change, however, is that you've eliminated less common byproducts of combustion, such as NO and NO2

The biggest problem is that you've eliminated the nitrogen cycle, which makes impossible the biology we know here on Earth. Nitrogen is crucial to every living organism we know, with atmospheric nitrogen being a staple of the process: Plants take it in, animals eat the plants, other animals eat those animals, and everything releases it back into the air during decomposition. Nitrogen is a critical piece of our biology, being a foundation of amino acids, and without it in the atmosphere we have no nitrogen cycle, and thus no (or at least insufficient) nitrogen to live on.

Unfortunately, argon is a noble gas, so it can't plausibly replace nitrogen in a hypothetical "argon cycle"-based biosphere. You have to either come up with a biology that doesn't depend upon nitrogen, or find a way to get it back in somehow (which, since it's a gas at anything above 77K, is pretty much impossible without it getting back into your atmosphere).

The good news is that argon is inert, so since there's adequate oxygen it's almost certain that humans could visit this planet and breath without needing full spacesuits and carrying heavy oxygen tanks. The slightly denser atmosphere would result in their voices sounding slightly deeper (think the opposite effect of inhaling helium, but much less pronounced), but otherwise they'd not really notice anything. They may just need to have access to foods that were part of the nitrogen cycle, and/or ship in nitrogen supplements for long-term habitation.

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Wouldn't a slightly denser atmosphere mean slightly faster speed of sound? – Draco18s Jan 11 at 22:25
    
@Draco18s I would have thought so as well, but among the chemical properties of argon and nitrogen I found the speed of sound in each, with the former being about 90% of the latter. – Kromey Jan 11 at 23:10
    
Interesting. Good research. :) – Draco18s Jan 12 at 3:27
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Your point on the nitrogen cycle is spot on. Fixed nitrogen already limits plant growth in many places. That is why it is in so much fertilizer. It would become precious. Animals would have to evolve to recycle much more of it instead of excreting it as urea after breaking down proteins. – Ross Millikan Jan 12 at 5:33

I think plants would get into trouble. Plants need nitrogen compounds, which many of them get from bacteria that take the nitrogen from the air. Without nitrogen in the air, those bacteria cannot create nitrogen compounds, and thus those plants are in trouble.

Note that the nitrogen compounds in dung indirectly come from plants, too. So that's no way to get new nitrogen into the cycle. Ultimately all that nitrogen comes from the air.

And the nitrogen in artificial fertilizer is taken from the air as well, so unless there's another source available, even that won't work.

Note that Argon, as a noble gas, cannot replace nitrogen in chemical compounds.

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Argon is an inert gas. It takes a great deal of effort to get it to react with anything.

Would firearms still ignite normally with that much argon present?

Firearms (specifically, bullets) include their own oxidiser. That's why many guns will happily fire while underwater.

Would humans have trouble breathing?

A little bit. Argon is ~43% more dense than Oxygen. We'd have a harder time breathing, but our bodies would very likely adapt to that change just fine.

How easily would light pass through without reflection / refraction.

The refractive indices of Argon and Nitrogen are pretty similar. Interestingly, Argon's absorption spectra is quite a bit more lively than Nitrogen's. Despite it being a colourless gas, I imagine an atmosphere full of it would tint the sky with a tiny amount of green.

Argon's thermal conductivity is a lot lower than Nitrogen. Blowing on your hot coffee won't be as effective.

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Argon is inert, meaning that in doesn't really interact much with anything. Like the other noble gases, it is much less reactive than the major components of the atmosphere. Therefore, to look at changes in this atmosphere, it's important to look at what we'd lose if the nitrogen was this substantially reduced.

That said, atmospheric nitrogen (N2) doesn't do much, either - it's also quite inert. So indirect results are what we need to look at - specifically, the nitrogen cycle.

Things that are impacted by the nitrogen cycle:

  • No nitrogen fixation, leading to a lack of ammonia produced by nitrogen-fixing bacteria.
  • Highly reduced amino acid synthesis (it needs that ammonia)

Really, atmospheric nitrogen isn't that important. Other sources of nitrogen exist, and organisms get quite a lot of their nitrogen from the ground and elsewhere.

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Argon wouldn't asphyxiate a human when mixed with 20% oxygen. It's inert, so it wouldn't have any noticeable effect at all on human respiration. – Monty Wild Jan 12 at 1:08
    
The nitrogen that is in the ground is fixed by bacteria that take it from the air. – Ross Millikan Jan 12 at 5:29

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