Why is most of the atmosphere nitrogen?
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The composition of an atmosphere depends on a couple of factors. The four most important are:
* The gravity of the planet.
* The temperature of the planet. Higher temperatures lead to higher speeds in the gas particles. Together with gravity, this affects what gases a planet can prevent from “rapidly” (in terms of the lifespan of a planet) escaping into space. The bigger and cooler the planet, the lighter the gases it can hold onto.
* The available elements. Some elements could form an atmosphere in principle, but are just too rare to actually appear in any large amount.
* Reactivity. Components of an atmosphere usually need to be very non-reactive, or they’ll react and leave the atmosphere. The exception to this is if they’re being constantly replenished by some process, as oxygen is by living things here on Earth (you may be surprised to know that Earth did not originally have oxygen in its atmosphere!)
If we apply these factors to Earth:
* Earth is small-medium sized and warm-ish as planets go.
Earth isn’t big or cool enough to hold onto hydrogen or helium. Hydrogen exists in an atmosphere mostly as dihydrogen (H2), with a weight of ~2, and helium exists as isolated atoms with a weight of ~4. Compare that to dioxygen (O2, weight ~32), dinitrogen (N2, weight ~28), or carbon dioxide (CO2, weight ~48) and you can see why they’d be hard to hold onto.
That’s why Earth did not end up as a gas giant, like Jupiter or Saturn did, or an ice giant (“ice” here is light molecules like methane and ammonia), like Uranus and Neptune did. Earth is, however, big enough to hold onto oxygen, carbon dioxide, nitrogen, and – critically for living things like you and me – water vapor.
* Nitrogen, oxygen, and carbon are very common elements. Heavier gases, like xenon, are rarer.
Xenon is in fact a small portion of Earth’s atmosphere, but it is overwhelmed by the far-more-common oxygen and nitrogen.
* Nitrogen is extremely unreactive.
That means that very few processes remove nitrogen from the atmosphere. Only a few living things (“nitrogen fixating” living things) can force nitrogen into reacting and forming biological molecules. All other living things get their nitrogen from those living things. You, for example, get most of your nitrogen from proteins, which are ultimately from the nitrogen in plants and thus from the nitrogen fixed by soil bacteria.
If not for living things, Earth’s atmosphere *wouldn’t* contain oxygen, because oxygen *is* reactive. (And as mentioned above, Earth’s original atmosphere did not contain oxygen.) It would react with rocks on the surface of the Earth and end up locked up in minerals. (And in fact, it does do so; almost all minerals on Earth’s surface are oxygen-containing in one form or another.) But living things provide an unusual geological process on Earth that sustains the reactive oxygen – but not at the same level of nitrogen.
The last mystery here is why we don’t have much carbon dioxide (a medium-weight, non-reactive molecule made of elements common on Earth), but living things solve that one too. Earth’s early atmosphere had plenty, but it was consumed by living things. Life has existed on Earth for so long that Earth’s own geology is inseparable from it, and living things and tectonic plates actually work together in a massive many-million-year cycle that shuffles carbon back into the Earth’s mantle rather than it building up in the air.
TLDR: Earth couldn’t gather hydrogen and helium because it’s too small and too warm. It started with nitrogen and carbon dioxide. Living things processed the carbon dioxide into our modern oxygen atmosphere and locked up much of Earth’s surface carbon in living or formerly-living things.
Nitrogen in the atmosphere exists as molecules consisting of two atoms connected with a triple bond. This bond is extremely strong, and for that reason atmospheric nitrogen is very hard to get to chemically react with anything, which would be necessary to remove it from the atmosphere. So once that nitrogen got into the atmosphere at an early point in Earth’s history, it then just stayed there and remains there to this day.
Living things (certain microorganisms) have the ability to extract nitrogen from the atmosphere, which is very important since building any organism requires nitrogen. But that nitrogen eventually goes back into air through the action of other microorganisms, so nitrogen content in the atmosphere remains the same.
The two major sources of atmospheric nitrogen on Earth are volcanoes and bottom-dwelling denitrifying bacteria.
Venus used to be more like Earth, but as the Sun grew hotter, Venus’s oceans evaporated, temperatures spiraled upward, plate tectonics shut off, and massive amounts of carbon dioxide were dumped into the atmosphere. Venus still has a lot of nitrogen in its atmosphere, in fact it has four times as much as Earth’s atmosphere. That’s the product of a long history of volcanic activity. But it’s a much smaller percentage of the whole, because the atmosphere is now extremely dense and most of it is CO2.
The composition of an atmosphere depends on a couple of factors. The four most important are:
* The gravity of the planet.
* The temperature of the planet. Higher temperatures lead to higher speeds in the gas particles. Together with gravity, this affects what gases a planet can prevent from “rapidly” (in terms of the lifespan of a planet) escaping into space. The bigger and cooler the planet, the lighter the gases it can hold onto.
* The available elements. Some elements could form an atmosphere in principle, but are just too rare to actually appear in any large amount.
* Reactivity. Components of an atmosphere usually need to be very non-reactive, or they’ll react and leave the atmosphere. The exception to this is if they’re being constantly replenished by some process, as oxygen is by living things here on Earth (you may be surprised to know that Earth did not originally have oxygen in its atmosphere!)
If we apply these factors to Earth:
* Earth is small-medium sized and warm-ish as planets go.
Earth isn’t big or cool enough to hold onto hydrogen or helium. Hydrogen exists in an atmosphere mostly as dihydrogen (H2), with a weight of ~2, and helium exists as isolated atoms with a weight of ~4. Compare that to dioxygen (O2, weight ~32), dinitrogen (N2, weight ~28), or carbon dioxide (CO2, weight ~48) and you can see why they’d be hard to hold onto.
That’s why Earth did not end up as a gas giant, like Jupiter or Saturn did, or an ice giant (“ice” here is light molecules like methane and ammonia), like Uranus and Neptune did. Earth is, however, big enough to hold onto oxygen, carbon dioxide, nitrogen, and – critically for living things like you and me – water vapor.
* Nitrogen, oxygen, and carbon are very common elements. Heavier gases, like xenon, are rarer.
Xenon is in fact a small portion of Earth’s atmosphere, but it is overwhelmed by the far-more-common oxygen and nitrogen.
* Nitrogen is extremely unreactive.
That means that very few processes remove nitrogen from the atmosphere. Only a few living things (“nitrogen fixating” living things) can force nitrogen into reacting and forming biological molecules. All other living things get their nitrogen from those living things. You, for example, get most of your nitrogen from proteins, which are ultimately from the nitrogen in plants and thus from the nitrogen fixed by soil bacteria.
If not for living things, Earth’s atmosphere *wouldn’t* contain oxygen, because oxygen *is* reactive. (And as mentioned above, Earth’s original atmosphere did not contain oxygen.) It would react with rocks on the surface of the Earth and end up locked up in minerals. (And in fact, it does do so; almost all minerals on Earth’s surface are oxygen-containing in one form or another.) But living things provide an unusual geological process on Earth that sustains the reactive oxygen – but not at the same level of nitrogen.
The last mystery here is why we don’t have much carbon dioxide (a medium-weight, non-reactive molecule made of elements common on Earth), but living things solve that one too. Earth’s early atmosphere had plenty, but it was consumed by living things. Life has existed on Earth for so long that Earth’s own geology is inseparable from it, and living things and tectonic plates actually work together in a massive many-million-year cycle that shuffles carbon back into the Earth’s mantle rather than it building up in the air.
TLDR: Earth couldn’t gather hydrogen and helium because it’s too small and too warm. It started with nitrogen and carbon dioxide. Living things processed the carbon dioxide into our modern oxygen atmosphere and locked up much of Earth’s surface carbon in living or formerly-living things.
Nitrogen in the atmosphere exists as molecules consisting of two atoms connected with a triple bond. This bond is extremely strong, and for that reason atmospheric nitrogen is very hard to get to chemically react with anything, which would be necessary to remove it from the atmosphere. So once that nitrogen got into the atmosphere at an early point in Earth’s history, it then just stayed there and remains there to this day.
Living things (certain microorganisms) have the ability to extract nitrogen from the atmosphere, which is very important since building any organism requires nitrogen. But that nitrogen eventually goes back into air through the action of other microorganisms, so nitrogen content in the atmosphere remains the same.
The two major sources of atmospheric nitrogen on Earth are volcanoes and bottom-dwelling denitrifying bacteria.
Venus used to be more like Earth, but as the Sun grew hotter, Venus’s oceans evaporated, temperatures spiraled upward, plate tectonics shut off, and massive amounts of carbon dioxide were dumped into the atmosphere. Venus still has a lot of nitrogen in its atmosphere, in fact it has four times as much as Earth’s atmosphere. That’s the product of a long history of volcanic activity. But it’s a much smaller percentage of the whole, because the atmosphere is now extremely dense and most of it is CO2.
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