I'm designing a lower gravity planet that I want to stay relatively warm despite having less atmosphere. My solution to this is to increase relative the amount of greenhouse gas, so this planet's atmosphere would have, say, 40% CO2. I was thinking it could be around 10-20% oxygen and 40-50% nitrogen. Would this be viable for life? Does complex life need as high a percentage of nitrogen in the atmosphere as Earth has or could it be substantially lower?
Atmospheric nitrogen is practically unavailable to most of life form, because of its low reactivity.
It takes nitrogen fixing organisms like bacteria and plants to make it available in a way that other life forms can process.
nitrogen-fixing bacteria, microorganisms capable of transforming atmospheric nitrogen into fixed nitrogen (inorganic compounds usable by plants). More than 90 percent of all nitrogen fixation is effected by these organisms, which thus play an important role in the nitrogen cycle.
Therefore also on a planet with lower atmospheric nitrogen life can thrive, as long as you have species which perform the task of fixing whatever nitrogen is present.
Given that you have supreme authorial fiat, I don't see any reason why you couldn't declare life to be able to develop there. There are various (largely bacterial) processes that makes atmospheric nitrogen available to other living things by converting it into forms that can be more easily taken up and used, and I don't believe that these need very high partial pressures of nitrogen in order to work, but even if they did you can handwave in a process that works with much lower partial pressures without worrying that you've made a chemical impossibility.
Having a very high partial pressure of CO2 is potentially problematic, though.
CO2 is soluble. This means that your clouds are going to be somewhat acidic (carbonic acid) and so any exposed surface life will have to be acid resistant, but it also means that your seas are likely to be highly acidic as they'll be able to absorb CO2 from the air until they reach some kind of equilibrium.
Acid oceans aren't uninhabitable necessarily, but they won't be hospitable for anything that expects to form carbonate shells, so no molluscs for you. That also means no chalk, and no limestone. You might get more complex forms of the silicate frustules that some kinds of marine microbiota produce, and hence other kinds of mineral formation, but the kinds of interesting landforms and cave systems you get on Earth in limestone are probably going to be entirely unavailable to you.
The density of CO2 at STP is ~1.977kg/m3, oxygen is 1.4290kg/m3 and nitrogen 1.2506kg/m3 (source). Gasses stay pretty well mixed in our atmosphere under normal circumstances, but it has been demonstrated in the real world that carbon dioxide can form low-lying gas pockets capable of causing asphyxiation:
Because it is heavier than air, in locations where the gas seeps from the ground (due to sub-surface volcanic or geothermal activity) in relatively high concentrations, without the dispersing effects of wind, it can collect in sheltered/pocketed locations below average ground level, causing animals located therein to be suffocated. Carrion feeders attracted to the carcasses are then also killed. Children have been killed in the same way near the city of Goma by CO2 emissions from the nearby volcano Mount Nyiragongo.
This makes various kinds of geographic depression potential deathtraps for aerobic life forms, along with other structures such as caves, mines and basements. Keeping your air well mixed could be problematic.
Elhammo could simply put his planet closer to its star, and/or make the star more luminous, so the planet will receive more radiation from the star to heat it up. If the planet receives enough radiation from the star, it might be as warm as Earth or warmer despite having no more greenhouse gases than Earth, or even less, in its atmosphere.
How close could Elhammo's planet get to its star? Obviously one should take the inner edge of the Sun's circumstellar habitable zone, and adjust it for the relative luminosity of the star compared to the Sun. That should be easy.
Here is a link to a list of estimates of the inner and outer edges of the Sun's circumsellar habitable zone.
Notice how "well" they agree. Some of the estimates require specific atmospheric compositions to have liquid water temperatures at the inner or outer edges. And those specific atmospheric compostions might be unsuitable for oxygen breathing organisms.
Here is a link to a table of the atmospheric composition of Earth.
I note that most of the atmosphere is nitrogen, and only a fraction of its present amount is necessary for life. So a planet with the same amount of oxygen, and with only a few percent as much nitrogen as Earth, could have a very thin atmosphere while retaining all the minor atmospheric gases, including the greenhouse gases.
And if the planet receives more heat from its star than Earth does, such an atmosphere should enable it to be at least as warm as Earth is.
I note that the carbon dioxide, methane, and water vapor in Earth's atmosphere are greenhouse gases.
The primary greenhouse gases in Earth's atmosphere are water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3). Without greenhouse gases, the average temperature of Earth's surface would be about −18 °C (0 °F),3 rather than the present average of 15 °C (59 °F).35
I see that nitrous oxide and ozone are also listed as atmospheric gases. And there are other greenhouse gases listed as being in Earth's atmosphere.
Habitable Planets for Man, stephen H. Dole, 1964, discusses the constituants of a breathable atmosphere - breathable for humans (and thus for beings with similar atmospheric requirements).
Dole discusses atmospheric composition on pages 13 to 19 and concludes:
To summarize then, the atmosphere of a habitable planet must contain oxygen with an inspired partial pressure between 60 and 400 millimeters of mercury, and carbon dioxide 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 andother toxic gases must not be present in more than trace amounts. Some nitrogren must be present so that nitrogen in combined form can find its way into plants.
Since a habitable planet must have bodies of water on its surface, it must also have water vapor in the atmosphere.
Table 2 on page 16 lists the approximate upper limits of various inert gases.
Table 3 on page 18 lists tolerable concentrations of other gases. I note that apparently the greenhouse gas methane could have a much higher concentration than in Earth's atmosphere before it would start to burn.
Alien life forms which evolved on a different planet could have much greater telerances for greenhouse gases in the atmosphere than humans. But probably the same biological reasons which would make them toxic to humans would also make them toxic to the alien lifeforms, just at greater concentrations. So you shouldn't assume that you can crank up the greenhouse gases as much as you need to make a planet at the distance of Pluto as warm as Earth, for example.
Anyway, a planet with oxygen near the lower limit, and very little nitrogen, should have plenty of leeway to increase the levels of greenhouse gases to as high as is humanly tolerable, while the total atmospheric pressure is about a third that of Earth. Once the greenhouse gases are all at their highest tolerable levels, a writer would probably have to increase the oxygen above the minimum level to get a total amospheric pressure one third that of Earth.
I note that calculating the temperature ranges on a planet with a specific atmospheric compositon receiving a specific amount of radiation from its star can probalbly be quite complex.
...Then I was wondering if this was a moon of a gas giant, if tidal effects and increased volcanism could keep the CO2 at proportionally high levels. The moon would be 0.25M earth, with maybe like a 0.3bar atmosphere. Not sure if it could stably maintain this atmospheric composition and pressure though.
If you are considering a habitable moon of a giant planet, you should remember that tidal heating not only releases greenhouse gases, which can heat up the air, it also heats up lower rock levels, which heat up higher rock levels, which heat up higher rock levels, and so on, until the heat reaches the surface of a world.
Scientists speculating about life on exomoons of giant exoplanets have calculated that tidal heating could be very significant. In fact they have calculated that tidal heating could be too strong and make a moon uninhabitable.
Too much tidal heating could produce so much vulcanism that moon would become a volcanic hell, like, for example, the Jovian moon Io.
A lesser amount of tidal heating might lead to a runaway greenhouse effect. If the surface becomes too hot, too much liquid water will become water vapor in the atmosphere. Since water vapor is a greenhouse gas, a lot of water vapor in the atmosphere will increase the temperures, evaporating more water vapor until all the water is in the atmosphere and none on the surface. Ultraviolet light in the upper atmosphere will separate water into the hydrogen and oxygen. The hydrogen will escape into space while the oxygen will combine to form solid matter on the surface of the world.
A really low amount of tidal heating will not contribute significant heat to the surface of a world.
And obviously, there is a range of tidal heating in between those extremes which is just right. Which can be enough to keep a world warm enough for liquid surface water without making it hot enough for volcanic hell or runaway greenhouse.
in this paper: https://arxiv.org/ftp/arxiv/papers/1209/1209.5323.pdf the authors create the concept of the "habitable edge". The "habitable edge" of a giant planet in the minimum possible distance for a moon to orbit around it without suffering from a runaway greenhosue effect.
In this paper: https://arxiv.org/abs/1309.0811 The authors also suggest a maximum distance for a habitable moon to orbits its planet.
The moon would be 0.25M earth, with maybe like a 0.3bar atmosphere. Not sure if it could stably maintain this atmospheric composition and pressure though.
One way to check whether a moon would maintain an atmosphere pressure would be to check Habitable Planets for Man, Stephen H. Dole, 1964.
On pages 34 to 35 Dole discusess a ratio that determines the time period required for a gas in a planet's atompshere to be reduced to 0.368 (1/e) of its former amount by escape of gas into space. The ratio is the escape velocity (not the surface gravity) of a world divided by the root-mean-square velocity of particles of that gas in the exosphere (outermost atmosphere) of the world.
The velocity of gas molecules depends on their temperatures, and the temperatures in the exospheres of worlds are usually much higher than their surface temperatures.
According to table 5 on page 35, multiplying the ratio a few times can make the difference between reducing a gas to 0.368 of its orginal amount in mere seconds and in reducing it to 0.368 of its original amount in billions of years.
Of course gas that escapes from an atmosphere into space can be replaced, but no doubt natural process couldn't replace any gas nearly as fast as the maximum rate it could be lost into space.
And there are other factors which can make a world lose atmospheric gases faster than by thermal escape into space alone.
Dole calculated a minimum possible mass for a world to retain an oxygen rich atmosphere for geological eras of time.
On page 53 Dole calculated the parameters of a world with a surface gravity of 1.5 g, which he considered the maximum for a colonizable world. He believed such a world would have a mass of 2.35 Earth mass, a radius of 1.25 Earth radius, and an escape velocity of 15.3 kilometers per second, 1.367 that of Earth.
On page 54 Dole said that the temperatures in Earth's exosphere are between 1000 K and 2000 K. Dole said that if the maximum exosphere temperatures of a world could be as low as 1000 K while still having liquid water temperatures at the surface, such a world could retain oxygen in its atmosphere while having an escape velocity of only 6.25 kiometers per second. Dole calculated it whould have a mass of 0.195 Earth, a radius of 0.63 Earth, and surface gravity of 0.49 g.
However, Dole didn't believe that such a small world could produce an oxygen rich atmosphere, and so eventually decided - correclty or not - that a world with 0.4 the mass of Earth, 0.78 Earth radius, and a surface gravity of 0.68 g would be necessary to produce an oxygen rich atmosphere. It would have an escape velocity of 8.01 kilometers per second, 0.716 thatof Earth.
In more recent times scientists have speculated about planetary habitability, but considering the more general case of habitable for liquid water using life forms in general, instead of the more specific case of habitable for humans and for beings with similar enviromental requirements.
So some of the worlds which scientists consider habitable for some liquid water using life forms might be swiftly deadly for humans or for beings who require the same type of environment.
Anyway, in this 2013 paper, various estimates of the mass range of habitable worlds are mentioned onpages 3 to 4.
A minimum mass of an exomoon is required to drive a magnetic shield on a billion-year timescale (Ms ≳ 0.1M⊕, Tachinami et al. 2011); to sustain a substantial, long-lived atmosphere (Ms ≳ 0.12M⊕, Williams et al. 1997; Kaltenegger 2000); and to drive tectonic activity (Ms ≳ 0.23M⊕, Williams et al. 1997), which is necessary to maintain plate tectonics and to support the carbon-silicate cycle. Weak internal dynamos have been detected in Mercury and Ganymede (Kivelson et al. 1996; Gurnett et al. 1996), suggesting that satellite masses > 0.25M⊕ will be adequate for considerations of exomoon habitability. This lower limit, however, is not a fixed number. Further sources of energy – such as radiogenic and tidal heating, and the effect of a moon’s composition and structure – can alter our limit in either direction. An upper mass limit is given by the fact that increasing mass leads to high pressures in the moon’s interior, which will increase the mantle viscosity and depress heat transfer throughout the mantle as well as in the core. 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 2M⊕ (Gaidos et al. 2010; Noack & Breuer 2011; Stamenković et al. 2011). Summing up these conditions, we expect approximately Earth-mass moons to be habitable, and these objects could be detectable with the newly started Hunt for Exomoons with Kepler (HEK) project (Kipping et al. 2012).
So they mention sources which claim a world could retain an atmosphere for long periods of time with a mass only 0.12 Earth mass. And possibly those sources have a differnt result than Dole because of different assumptions they make about various factors.
They believe that when all the factors are considered, exommons with mass greater than about 0.25 Earth mass are potentially habitable, though that is not a hard limit.
A world with a mass of 0.25 Earth, and 0.8 the density of Earth would have a volume of 0.125 Earth, a radius of 0.6786 of Earth (4,323.36 kilometers, aurface gravity of 0.54 g, and an escape velocity of 6.79 kilometers per second, 0.607 that of Earth.
And I guess that would not be too far from the minimum values for a habitable exomoon capable of retaining an atmosphere for geologic eras of time.