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Title: How did Earth's atmosphere form?

Breathe!

Fill your hungry lungs with life-giving air. Wake up those sleepy brain cells!

This is the only planet we know of where you can do this simple act.

Other planets and moons in our solar system have atmospheres, but none of them could support life as we know it. They are either too dense (as on Venus) or not dense enough (as on Mars), and none of them have much oxygen, the precious gas that we Earth animals need every minute.

So how did Earth get to be so special?

First, what are the ingredients in Earth’s atmosphere?

Pie chart shows proportions of atmospheric gases.
Note that this pie does not account for any water in the air. As you can see, air is mostly nitrogen (78%), with oxygen a distant runner up (21%). Argon and some other gases make up another small amount (about 1%). And carbon dioxide is only a very tiny slice (.0385% or only about 385 parts per million parts of air). But these proportions were quite likely different in Earth’s much younger days.

Some scientists describe three stages in the evolution of Earth’s atmosphere as it is today.

Earth's three atmospheres. Three images show atmospheres of just-formed Earth, young Earth and current Earth. Explanation is in text below.

  1. Earth’s original atmosphere was probably just hydrogen and helium, because these were the main gases in the dusty, gassy disk around the Sun from which the planets formed. The Earth and its atmosphere were very hot. Molecules of hydrogen and helium move really fast, especially when warm. Actually, they moved so fast they eventually all escaped Earth's gravity and drifted off into space.

  2. Earth’s “second atmosphere” came from Earth itself. There were lots of volcanoes, many more than today, because Earth’s crust was still forming. The volcanoes released

    1. steam (H2O, with two hydrogen atoms and one oxygen atom),

    2. carbon dioxide (CO2, with one carbon atoms and two oxygen atoms), and

    3. ammonia (NH3, with one nitrogen atom and three hydrogen atoms).

  3. Much of the CO2 dissolved into the oceans. Eventually, a simple form of bacteria developed that could live on energy from the Sun and carbon dioxide in the water, producing oxygen as a waste product. Thus, oxygen began to build up in the atmosphere, while the carbon dioxide levels continued to drop. Meanwhile, the ammonia molecules in the atmosphere were broken apart by sunlight, leaving nitrogen and hydrogen. The hydrogen, being the lightest element, rose to the top of the atmosphere and much of it eventually drifted off into space.

Now we have Earth’s “third atmosphere,” the one we all know and love—an atmosphere containing enough oxygen for animals, including ourselves, to evolve.

So plants and some bacteria use carbon dioxide and give off oxygen, and animals use oxygen and give off carbon-dioxide—how convenient! The atmosphere upon which life depends was created by life itself.
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