“Ghost atmospheres?” you might ask. Well…conventionally when we worldbuild planets, in the context of harder science fiction, we assume that atmospheres of terrestrial planets will be primarily carbon dioxide/nitrogen mixtures, or else will have been CO2/N2 mixtures but modified by life; to wit, Earth is thought to have had several bars worth of nitrogen originally and even more carbon dioxide, with negligible free oxygen. Our oxygen, famously, was generated by photosynthetic life over millions of years. But where did all the carbon dioxide and nitrogen go? It was sequestered, by geological processes yet, but also by biological activity. Hence why our atmosphere today is not only of a considerably different composition but is also much thinner than it was billions of years ago. And this is a continuing process.
Consider that, as relayed by Wikipedia’s timeline of the far future, it’s expected that in around 500 million years carbon dioxide levels will have fallen to the point where C3 photosynthesis is no longer possible; naïvely we’d expect 99% of plant life to die out, but 500 million years is a long time (500 million years ago there were no trees; really), so I suspect plant life will adapt. But the point is future Earth’s atmosphere should be thinner than present-day Earth’s. Oxygen may well be a higher percentage, the primordial nitrogen inventory will draw down, and carbon dioxide will lessen the greenhouse effect. And that’s very much a good thing, since our sun’s luminosity is gradually brightening (indeed this process is thought to be why the Earth’s climate is not significantly warmer now than it was 4 billion years ago, despite the sun’s luminosity having increased 30%; the greenhouse effect weakening over time has offset this factor).
Anyway, what does all this have to do with ghost atmospheres? Consider that we have so much nitrogen left over because the N2 molecule is pretty heavy as far as common elements go, so it doesn’t escape an Earth-mass planet’s atmosphere easily once it’s generated (in stark contrast to e.g. hydrogen, which tends to leak away; it takes a more massive planet with greater gravity to retain lighter molecules). Nitrogen is used by some forms of life, but not very aggressively, so it just tends to draw down very gradually in our biosphere. CO2 has shown this tendency to a much more pronounced extent.
But what if Earth’s life drew down the atmospheric components much more aggressively? Nitrogen is not reactive in the form of N2 but it’s not totally inert either; certain metabolisms do indeed make use of it (e.g. “nitrogen fixation”), the net effect being to sequester the nitrogen out of the atmosphere and into the ground. Repeat this process for carbon dioxide, and of course the ever-reactive oxygen, and what do you have left? Let’s say our biosphere was hyper-active and drew down every component that could possibly react in metabolism…what would be left?
Well…it turns out Earth’s atmosphere is not all usable components. You’d have some left over! Dry air, by volume, is 78.084% nitrogen, 20.946% oxygen, 0.934% argon, 0.0412% carbon dioxide, 0.00182% neon, 0.000524% helium, 0.000179% methane, 0.000114% krypton, together accounting for all components whose prevalence is more than 1 part in 1 billion.
Nitrogen? It can be used. Oxygen? We breathe it; so do aerobic life-forms. That can be sequestered away, in principle. Ditto for carbon dioxide, even methane. But the argon, the krypton, and the neon? Those are noble gases; they don’t react and would play no chemical role in any organism’s processes. They’re inert! So even in a maximally aggressive drawdown, these gases remain.
The nifty part is that over geologic time these gases are increasing in prevalence, largely because of the buildup of argon; our argon is primarily comprised of the isotope argon-40, which is less cosmically abundant isotope (in the wider universe the slightly lighter argon-36 is far more common). The reason for this is that our argon is produced by the decay of radioactive potassium-40 in the Earth’s crust, and as a heavier gas argon does not escape our gravity easily once released. Ergo, it builds up. 4 billion years has built levels up to 0.934% of our atmosphere, or a partial pressure of 0.009 bars.
Strip away all the active components, and you’re left with a relic atmosphere that’s nigh-totally inert, 99.7% argon, with neon, helium, and krypton being the remaining 0.3%. Partial pressure? 0.0094 bars, or 0.94% as much as today. Which sounds extremely thin, but it’s still not quite enough for water to boil away as is the case on Mars. The boiling point of water, due to the lower pressure, would be 42 degrees Fahrenheit, or 6 degrees Celsius (keep in mind this would cause ebullism in unprotected humans, since human body temperature is 98 degrees Fahrenheit, i.e. the bodily fluids would start to boil away).
Realistically temperature fluctuations during the day would easily send the oceans to a boil, unless they freeze first…as they very likely would. Without a greenhouse effect, Earth’s temperature would gradually draw down to its equilibrium temperature, which today stands at 0 Fahrenheit. As ice sheets grow, the Earth’s surface becomes more reflective, dropping equilibrium temperatures further. Ice easily reflects 70% of incoming sunlight (Earth’s surface currently reflects about 30%); adjusting for that factor, Earth’s temperature drops to perhaps -89 Fahrenheit. Coincidentally, about the same as present-day Mars. Even this “ghost atmosphere” though would still be 9 millibars in pressure, slightly thicker than Mars’s 6 millibars.
The really interesting scenarios start to come into play when you consider a much more massive planet. Scale up Earth to five times the mass and suppose a planet that’s twice as old. Now you’ve got ten times as much degassing of argon from the crust, maybe more, given super-Earths should have more vigorous geological activity than our world. Now you’ve got not 9 millibars worth of argon, but 90 millibars. The lighter and less common noble gases should be considerably more common too; helium might be an appreciable fraction here, for example. So in this scenario you’ve got a world with no greenhouse effect, but one where the atmosphere is thick enough to push water’s boiling point up to 112 degrees Fahrenheit. That’s a decent range at which water remains liquid! You could have oceans on such a world, a full water cycle…only the atmosphere would be thin and inert for biology as we know it.
Still more exotic processes are possible; if the atmosphere becomes this drawn down, then it doesn’t take an implausible amount of biological activity for its metabolism’s gaseous “waste” products to start becoming appreciable fractions of the ambient air. Some really creative possibilities can be found in this space.
And keep in mind this general category of world might be genuinely common, especially among older planets where biology has had more time to fully extract the fuel (so to speak) that ambient atmospheric chemistry gives them. Atmospheres that are both very thin and whose composition cannot be easily explained by conventional geology might, if you’re staring at the squiggly lines on a spectrum, be a more reliable “biosignature” than, say, the presence of oxygen (which can easily be split off from water by stellar radiation without life even needing to be in the picture). Biology has already modified the atmosphere of our world, so much heavier and more exotic modification on other planets by no means should be ruled out.
It gets even better: if you’re dealing with removing carbon dioxide, nitrogen, and oxygen, organisms that make use of all these compounds and sequester them as CNO polymers into the ground could be posited as a viable metabolism, in which case you’d have a planetary surface dominated by this dark tough material that would be eroded into grains, perhaps blowing in the wind like the black sand (which would help to darken the planet, absorbing more light even as the greenhouse effect diminishes, in an interesting echo of the Gaia mechanism posited for life on Earth — biospheric homeostasis).
Keep in mind also as a worldbuilder reasoning out plausible biochemistries, that while full sequestration of all chemically active components is possible, it’s by no means the only outcome. How many “hycean worlds” might there be out there that are very old and have had all that free hydrogen, abundant fuel for methanogens, for example, sequestered away or transformed into other compounds over billions of years? They’d still have their primordial helium fraction, these worlds, but the rest of the atmosphere could easily be very different from pure hydrogen.
And keep in mind that an Earth-like world is not the only world that could undergo a biospheric consumption runaway. Take Venus, for example. It’s speculated that microbes could be chilling out in those white clouds up high right now. What if their metabolism was much more aggressive at sequestering atmosphere?
Take Venus’s current atmosphere. 96.5% carbon dioxide, with the other 3.5% being nitrogen, along with trace gases. Let’s remove the carbon dioxide. Now you have an atmosphere that’s 99% nitrogen. The pressure? 3.2 bars, several times Earth’s. It’s thought that Venus’s nitrogen inventory might have been similar to what Earth possessed originally, but its just wasn’t sequestered.
Anyway, a thick almost all nitrogen atmosphere isn’t too interesting, so let’s strip away the nitrogen as well. Let’s say it’s all sequestered. What’s left over on Venus? 54% sulfur dioxide, 25% argon, 7% water vapor, 6% carbon monoxide, 4% helium, 2% neon, and apparently 0.2% hydrogen chloride (of all things). Pressure? 0.02 bars. Yes, even without any carbon dioxide or even nitrogen, Venus’s atmosphere would still weigh in at 25 millibars, considerably thicker than Mars’s 6 millibars, or ghost Earth’s 9 millibars.
Obviously a lot of these components in Venus’s case are still biologically active; some forms of life on Earth make use of carbon monoxide and sulfur dioxide, for example. Indeed, on an alternate ghost Venus, where the top layer of crust is a thick mass of CNO polymers blowing in the wind, sulfur chemistry might dominate local life’s metabolism, since SO2 is an abundant and (to some forms of life) usable gas. At 0.02 bars water boils at 63 degrees Fahrenheit. The liquid range is a bit tight, but there are scenarios where it actually works; if the planet reflected 50% of all light that hit it, average temperatures would be 35 Fahrenheit or so. Enough to maintain liquid water…what there was of it. Most likely you’d see a vast desert with scattered lakes, since Venus was so water-depleted to begin with. Nevertheless, something like this is a stable equilibrium, geochemically and biochemically. And one that might well occur on very old versions of Venus in other solar systems.
Heck, how can we be so sure that local life isn’t making use of the abundant CO2 now and sequestering it away in some form? Venus’s greenhouse effect might be attenuating over geologic time as surely as the Earth’s is, with the world lying in wait for its transformation just as Earth’s atmosphere transitioned into the thin chilly oxygen-rich envelope we know and love today.
Only a detailed investigation of the planet’s geological record would even show such a thing, and inconveniently the surface is hard to access (the high pressure and temperature crushed our best landers in hours (weirdly only the Soviet Union ever tried to land anything on Venus, let alone succeed)), and we’ve never sent geologists beyond the Moon to date (and even the lunar expeditions were very superficial compared to our investigation of Earth’s geology). Makes you think, doesn’t it?
I’m sure some have wondered what the Earth or other airworlds would be like without their most common gases, but I think even fewer have run the numbers and figured out that even without the bulk common gases that are normally the primary constituents of rocky planets’ atmospheres…you still have enough air to retain oceans, to drive winds and weather, and potentially serve as a playground for biochemistry to produce all sorts of fascinating mixtures. Science fiction at its finest.
Crack open an atmospheric composition table, look up viable metabolisms, and project forward a few billion years…the world you build may well be a fascinating one to explore…