I'm trying to create a hypothetical planet scenario, where there is a planet that has a similar atmospheric pressure to that of Earth, but in this case, it has a gas different than that of nitrogen. So far, I've found options like neon and argon. But, I'm not sure what gas to settle for. Plus, I want a scenario where the sky is a different color, instead of blue. If our atmosphere never had nitrogen, but instead had neon or argon, could life still have evolved to what it evolved to today? If so, what color would the sky be? If not, should I create a fictional elemental gas for this scenario?
Once again, the ubiquitous atmospheric escape chart needs to be broken out. It shows the sorts of gasses a planetary atmosphere might retain, given its escape velocity and surface temperature. It is a very simplistic model of atmosphere retention, but it is a reasonable starter-for-ten.
The problem with neon and argon is that they are monatomic gasses, and as such they have a relatively low molecular weight... about 20 and 40 amu, respectively. The nitrogen and oxygen that makes up Earth's atmosphere comes in forms with molecular weights of about 38 and 32 amu respectively.
You can see from the diagram that Earth is comfortably above the green band, and as such oxygen and nitrogen in the atmosphere can be reasonably expected to hang around for timescales of many billions of years. The blue band though is a little more lightweight, which means it has a much shorter escape timescale.
What this means is that atmospheric neon is very hard to hang on to. Neon's low molecular mass and boiling point mean that you would need a pretty massive world in order to provide a deep enough gravity well to hang on to it, and world that big are likely to develop into wet super-Earths or gas dwarfs.
Argon is much more workable, and indeed Earth's atmosphere is already about 1% argon. The problem is relative chemical abundance. You can look at a table of elemental abundances in the solar system, and see that argon is something like 30 times less common than nitrogen. Combine that with its unwillingness to form non-volatile compounds, and you can see that getting an atmosphere's worth of the stuff without also having several atmosphere's worth of nitrogen is going to be challenging. Not impossible by any stretch of the imagination, but you're going to have to work a bit to explain it.
if our atmosphere never had nitrogen, but instead had neon or argon, could life still have evolved to what it evolved to today?
AlexP's comment and "Escaped Lunatic"'s answer covers this well enough. Nitrogen is chemically very important for life on Earth (no nitrogen means no amino acids which means no proteins). That's not to say that an alternate form of life could have arisen without it, but given the relative abundance of nitrogen vs argon it would be surprising. If you have an explanation of why nitrogen removal from the atmosphere is so effective then you can handwave in lifeforms that include lots of nitrogen chemistry just fine. Maybe it the weird atmospheric balance has a biological explanation.
You'd still have the issue of things like denitrifying organisms, but maybe some kind of highly efficient and common nitrogen fixing organism is enough to prevent gaseous N2 from escaping into the atmosphere in significant volumes. The hands are yours to wave here.
If so, what color would the sky be?
Given Earthlike pressures and densities (which you'll basically have with an Argon/Oxygen main mix) and a Sunlike star, the sky will be more or less the same colour as it is on Earth. Argon has a very similar Rayleigh scattering cross-section to nitrogen, and so the resulting color of the sky will be slightly different to Earth, but not dramatically so.
Neon is probably more different, but I can't find any good information that would inform exactly what that difference would look like. Given that the color will still be dominated by the spectrum of light from the primary star, and Rayleigh scattering is strongly wavelength dependent, the color will inevitably still be "basically blue".
If you want different colors, you should probably get a different star, or change your atmospheric pressure and consult the excellent chart in this question. A radically different gas mix would also work, but gravity and toxicity will mean that it isn't going to be very human-friendly.
Note that there are other colorful effects that you can get from alternate atmospheres... electrical discharges such as lightning can be different. This would be very marked for a neon-rich atmosphere (red lightning!) though it is unlikely to be dramatically different for argon, which will look basically like lightning in a nitrogen-rich atmosphere.
If not, should I create a fictional elemental gas for this scenario?
That's strictly in the realms of very soft scifi. You can feel free to make stuff up if that's what you want and what your story needs, but it is a long stride away from plausibility. If you want to remain within spitting distance of science in your fiction, you probably shouldn't invent new elements that are so common you get whole atmospheres made of them but which somehow haven't been discovered on Earth and don't slot into the periodic table anywhere sensible.
I believe you could get by with most of the nitrogen being replaced by Neon or Argon, but even though Earth life is mostly Hydrogen, Oxygen, and Carbon, a certain amount of Nitrogen is mandatory.
So, if you want to visit such a world, take off your helmet, and go sight seeing, have fun. Want to try to set up an ecosystem for Earthly life without importing insane amounts of Nitrogen based fertilizers every year, please take videos so it can be featured in an upcoming episode of Failed Colonies.
I'm not an expert, but I think you could get by with less than 5% Nitrogen in the atmosphere. If there's no native life, you'll need some sort of excuse for having enough Oxygen to breath. Blame it on a yet-to-be-explored inorganic geochemical process around deep sea hydrothermal vents and not too many readers will complain.
I'm not sure what color sky you'll end up with. Let's hope for something interesting.
No plant life, no animal life
Animals live on plants. Without plants, animal life cannot sustain because:
- Plants are used as food
- Plant produce oxygen
Plants need nitrogen
Nitrogen is an essential nutrient for plant growth, development and reproduction.
Nitrogen is a major component of chlorophyll, the compound by which plants use sunlight energy to produce sugars from water and carbon dioxide and produce oxygen.
The Nitrogen Cycle
As explained here:
Nitrogen can go through many transformations in the soil. These transformations are often grouped into a system called the nitrogen cycle, which can be presented in varying degrees of complexity.
Argon and neon are non-reactive gases
Argon and neon cannot play any role in sustaining life as they are non-reactive.
There are many possible changes to the composition of an atmosphere which would not change the color of the sky. It is possible that some changes in atmospheric composiiton might affect the color of the sky, but I am not familiar with them.
Here is a link to many questions and answers about the color of alien skies. I typed "sky color" in the "search worldbuilding" section of the black bar at the top of the worldbuilding screen.
Possibly some of the answers will mention changes in atmospheric composition which might result in changes in sky colors.
The good news, found in my long answer, is that it is possible to make considerable changes to Earth's atmospheric composition while it remains breathable.
What is the atmospheric composition of Earth at sea level?
Dry air is 78.084 percent nitrogen, 20.946 percent oxygen, Argon 0.934 percent, Carbon Dioxide 0.0416 percent, Neon 0.001818 percent, Helium 0.000524 percent, Methane 0.000187, and Krypton 0.000114 percent. The atmosphere usually has between 0.0 and 3.0 percent water vapor.
There are many lifeforms on and in Earth that can life without that specific atmosphereic composiiton. But if your story involves people from Earth breathing the air of the planet, or beings with similar enviromental needs living on the planet, you need to make sure the atmospheric composition is similar enough.
Actually Earth life has existed in a radically different than the present one, and helped to make the present atmosphere.
Earth life goes back 3 billion years, or e maybe even 4 billion years. It evolved in the second atmosphere of Earth, containing carbon dioxide and inert gases from vulcanism.
When photosythetic organisms evolved, they emitted oxygen, eventually producing oxygen faster than it could be removed from the atmosphere by oxidation of minerals.
The third atmosphere resulted from the large amounts of oxygen in the atmosphere. There was enough oxygen in the air for multicelled animals since about 600 million years ago.
The average atmospheric pressure at sea level is defined by the International Standard Atmosphere as 101325 pascals (760.00 Torr; 14.6959 psi; 760.00 mmHg).
A mmHG is a millimeter of mercury.
In Habitable Planets for Man, 1964, Stephen H. Dole discuss the atmospheric requirements of humans on pages 13 to 19.
On page 19 he says:
To summarize, then, the atmosphere of a habitable planet must contain oxygen with an inspired partial pressure of between 60 and 400 millimeters of mercury and carbon doxide with an inspired partial pressure roughly between 0.05 and 7 millimeters of mercury. In addition, the partial pressures of the inert gases must be below certain specified limits and other toxic gases must not be present in more than trace amounts. Some nitrogen must be present so that nitrogen in combined form can find its way into plants.
In addition, a habitable planet would have lot of water and some water vapor would be needed for rain and to keep human airways from drying out.
Since an Earth atmosphere at sea level has a pressure of 760 millimeters of Mercury, and the combined limits of oxygen and carbon dioxide for humans are about 60.05 to 407 millimeters of mercury, the atmopshere of your planet needs about 353 to 699.95 millimeters of mercury to have Earth's atmosphere density.
Are there any gases which would be harmless to humans at such pressures.
According to table 2 on page 16, humans can tolerate up to 7 mmHG of carbon dioxide, 160 mm of Xenon, 350 mm of Krypton, 1,220 mm of Argon, or 2,330 mm of nitrogen. So a combination of 353 to 699.95 mmHG of various inert gases should be breathable. It is unfortunate that the question requires the elimination of Nitrogen from the atmosphere (except for the amount needed by plants).
In addition, the tolerable pressure of neon could be as high as 3,900 mmHG, and the tolerable pressure of helium could be as high as 61,000 mmHG.
To select the most desirable elements for the atmosphere of your planet, there are several factors that can be used to rank them.
Ranked by how much humans can tolerate, helium would be the big winner, followed by neon, nitrogen (excluded except for a small amount), and argon if only a single gas is in the atmosphere, while some krypton and Xenon could be mixed in with other gases.
Ranked by their relative abundance in Earth's atmosphere, and thus the relative ease of increasing their abundance in a slightly different world, they are argon, neon, helium, and krypton.
Ranked by their relative abundance in the universe, helium is first, followed by neon, argon, kyrpton and xenon.
They can also be ranked from the heaviest to the lightest gases.
The heavier an atom or molecule is, the slower it will move at any specfic temperature, like the temperature in the exosphere of a planet,the outermost atmosphere where gases escape into space.
On pages 34, and 35, Dole discusses the escape velocity a world needs to retain a gas in its atmosphere for geologic periods of time. There are formalas to calculated the time it takes for the amount of gas to drop to 0.368 of its orginal amount, relative to the ratio of the escape velocity of the world divided by the root-mean-square velocity of the gases in the exosphere.
Table 5 on page 35 shows that if the ratio is one or two, the amount of gas will reduce to 0.368 of the starting amount instantly.
If the ratio is 3, the amount of gas will reduce to 0.368 of the starting amount in a few weeks.
If the ratio is 4, the amount of gas will reduce to 0.368 of the starting amount in several thousand years.
If the ratio is 5, the amount of gas will reduce to 0.368 of the starting amount in about 100 million years.
If the ratio is 6, the amount of gas will reduce to 0.368 of the starting amount in an infinite time.
In descending order of atomic weight and thus the ability of a world to retain them in its atmopshere, they are xenon, krypton, argon, neon, and helium.
Argon and neon might be the best overall candidates for the atmosphere of your planet.
Because Helium is so much more abundant than any other of the gases, it might be the most likely for a planet to retain a lot of - if the planet has a high enough esape velocity.
Uranus is the planet with much helium in the atmosphere that has the lowest escape velocity, 21.29 kilometers per second, as compared to Neptune's 23.71, Saturn's 35.49, Jupiter's 59.54, and on the other hand Earth's 11.186.
But Uranus also has a lot of hydrogen in its atmosphere. Hydrogen is noted for burning in the presence of oxygen to produce water, so there should be only tiny trace amounts of hydrogen in your planet's atmosphere.
Thus an escape velocity between Earth's 11.186 kilometers per second and Uranus's 21.29 kilometers per second would be indicated tofor your planet to retain a lot of helium in the atmosphere.
I note that Earth's escape velocity is sufficient to retain helimum 4 much better than the lighter isotope helium 3, and that the complex processes which add and subtract atmospheric helium ae not well known. And of course the giant planets are are much farther from the Sun than Earth and thus would lose helium much faster if they were at Earth's distance from the sun.
On page 53 Dole stated that since he belived humans wouldn't want to live on a world with a surface gravity higher than 1.5 times Earth's surface gravity of 1 g, the maximum mass of a human habitable planet would have about 2.35 Earth mass, 1.25 Earth radius, and an escape velocity of 15.3 kilometers per second.
In "Exomoon Habitability Constrained by Illumination and Tidal Heating", Rene Heller & Roy C. Barnes (2013) also discuss the mass range of habitable worlds - in this case worlds habitable for any type of liquid water using life, not just for humans.
On page 20 they write:
Above a critical mass, the dynamo is strongly suppressed and becomes too weak to generate a magnetic field or sustain plate tectonics. This maximum mass can be placed around 2M4 (Gaidos et al., 2010; Noack and Breuer, 2011; Stamenkovic´ et al., 2011).
2M4 means 2 times the mass of Earth. So if this is correct your planet should need to have no more than about 2 times the mass of Earth.
Earth has a radius of 6,371 kilometers.
A planet with 2 times the mass of Earth and 1.3 times the radius would have a surface gravity of 1.18 g and an escape velocity of 13.874 kilometers per second.
A planet with 2 times the mass of Earth and 1.2 times the radius would have a surface gravity of 1.39 g and an escape velocity of 14.44 kilometers per second.
A planet with 2 times the mass of Earth and 1.1 times the radius would have a surface gravity of 1.65 g and an escape velocity of 15.083 kilometers per second.
A planet with 2 times the mass of Earth and 1.0 times the radius would have a surface gravity of 2 g and an escape velocity of 15.82 kilometers per second.
And an escape velocity in that general range is probably about as large as a habitable planet could have.
And maybe that would be high enough for a planet warm enough for liquid water using life to have an atmosphere rich in helium.
If not, you would have to go with an atmosphere rich in noble gases, preferably argon and neon, which are somehow many times more common than they are in Earth's atmosphere.