What Is Earth's Atmosphere Made Of?
QUICK ANSWER
Earth's atmosphere is 78 percent nitrogen, 21 percent oxygen, and about 1 percent other gases (mostly argon, with trace amounts of carbon dioxide, water vapor, and other gases). Oxygen wasn't always this abundant; it's a product of billions of years of photosynthetic life slowly transforming the atmosphere.
Earth's atmosphere is mostly nitrogen, not oxygen, which surprises a lot of people. Oxygen gets all the attention because we breathe it, but it's only the second most abundant gas. The rest of the atmosphere, and how it got that way, is worth knowing.
What is Earth's atmosphere mostly made of?
According to NASA, Earth's atmosphere is composed of about 78 percent nitrogen (N2), 21 percent oxygen (O2), and just under 1 percent argon. The remaining traces include carbon dioxide (about 0.04 percent), water vapor (varies by location and weather), neon, helium, methane, krypton, hydrogen, and a few other gases. The composition is remarkably stable, having stayed roughly the same for hundreds of millions of years, though carbon dioxide levels have been changing significantly in recent centuries.
Why is there so much nitrogen?
Nitrogen is unusually stable and doesn't react easily with most other elements. When Earth's early atmosphere was being formed by volcanic outgassing, nitrogen was released along with other gases. While reactive gases like hydrogen and helium escaped into space, and gases like ammonia and methane were broken down by sunlight, nitrogen simply accumulated and stayed. There's also a biological nitrogen cycle that recycles nitrogen between the atmosphere and living organisms, but the bulk of the atmospheric nitrogen has been there essentially since Earth formed.
Where did Earth's oxygen come from?
Almost entirely from life. Earth's early atmosphere had almost no free oxygen. Starting about 2.4 billion years ago, photosynthetic organisms (initially cyanobacteria) began releasing oxygen as a waste product of converting sunlight and CO2 into energy. Over hundreds of millions of years, this process gradually built up the atmospheric oxygen we depend on. The shift was so dramatic it's called the Great Oxidation Event. Without photosynthesis, Earth's atmosphere would be very different and animals as we know them couldn't exist.
What are the layers of Earth's atmosphere?
Earth's atmosphere has five main layers, defined by temperature changes with altitude. The troposphere (0 to 7 miles up) contains most of the weather and most of the air. The stratosphere (7 to 31 miles) holds the ozone layer and is where commercial jets fly. The mesosphere (31 to 53 miles) is where meteors burn up. The thermosphere (53 to 600 miles) is where the auroras occur. The exosphere (600+ miles) gradually fades into space. Most of the atmosphere's mass is concentrated in the lowest layer.
Earth's atmosphere is mostly nitrogen with a substantial oxygen component, structured in five layers stacked above the surface. The oxygen is the product of billions of years of life slowly transforming the planet. The atmosphere we breathe is, in a real sense, a biological byproduct, not a default condition of the planet.
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