History of Oxygen in Earth’s Atmosphere
Oxygen isn’t the most abundant gas in Earth’s atmosphere composition. Based on the relative volumes of the gases in Earth’s atmosphere, nitrogen is 3 times greater than oxygen.
Because the troposphere is the lowest atmosphere layer, it contains 75 percent of the atmosphere’s mass.
From largest to smallest, Earth’s atmosphere composition contains nitrogen, oxygen, argon, CO2 and trace gases.
But it never used to be like this in the past. Over time, Earth’s oxygen levels have changed significantly with varying levels of hydrogen, helium, carbon dioxide, nitrogen and oxygen.
Hydrogen and Helium in the Hadean Eon
Earth’s early atmosphere was enriched with hydrogen and helium gases. But over time, Earth lost these gases because it wasn’t large enough to hold onto them.
Atmospheric composition is related to escape velocity. The escape velocity of Earth is the speed at which a free object must travel to escape into space from a planet’s gravitational pull.
To this day, Earth loses about 3 kg of hydrogen every second. But Jupiter, Saturn, Uranus and Neptune hold on tight to these gases because they are much bigger in size. In fact, their atmospheres are mostly these gases.
Carbon Dioxide in the Archean Eon
After hydrogen and helium atoms escaped the atmosphere in the Hadean Eon, the atmosphere mostly consisted of methane, ammonia, water vapor and nitrogen.
CO2 played a dominant role early in Earth’s history. Earth was uninhabitable for life at this time because the atmosphere was without oxygen.
The only life forms that could exist were anaerobic cyanobacteria (blue-green algae). It turns out that these microorganisms laid the foundation for enriching the atmosphere with oxygen.
The Great Oxygenation Event
The Great Oxygenation Event marks a time in our geologic history when free oxygen filled the atmosphere. The key to their existence was that they lived without oxygen.
Over time, these cyanobacteria released oxygen as waste. The generated so much oxygen, that it kept building up in the oceans. Eventually, oxygen filled the atmosphere and started an oxygen crisis of their own.
And after oxygen filled the air, it created a habitable planet. Since early Earth, oxygen levels have changed significantly. For example, free oxygen levels peaked just before the era of dinosaurs.
Earth’s Current Atmosphere Composition
Oxygen plays a pivotal role for aerobic life such as humans. But oxygen isn’t the most abundant gas in Earth’s atmosphere composition.
Based on the relative volumes of the gases in Earth’s atmosphere, nitrogen is actually more than 3 times more abundant than oxygen.
We commonly refer to nitrogen (N2) as an inert gas. But it forms nitrogen sulfide with sulfur, nitrogen dioxide with oxygen, ammonia with hydrogen such as in the nitrogen cycle.
While 78.1% of the atmosphere consists of nitrogen, 20.9% is oxygen. So the bulk of the atmosphere’s composition are these two gases. And since the Paleozoic Era, their relative compositions have see-sawed back and forth ever so slightly.
The next most abundant gas is argon which is an inert gas. Argon doesn’t bond or do much in the atmosphere. This is why the argon cycle doesn’t exist in nature.