The sky is blue. Now there’s something we can all agree on (although you’re welcome to poke your head outside and check). But there was a time when that wasn’t true.

The sky gets its color from the way sunlight interacts with the gas molecules that make up our atmosphere. But the atmosphere has changed a lot over the last 4.5 billion years. In fact, in the earliest stages of Earth’s history, there was no real atmosphere at all — no barrier between the ground and the infinite reaches of space. Like the rest of us, our planet began its life stark naked.

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It was also piping hot. As the Earth cooled, molten rock bubbling up from the interior carried gasses like carbon dioxide and methane to the surface. The gasses were then released, but Earth’s gravity kept them close to the surface, ultimately forming that gassy shroud we call our atmosphere.

That early atmosphere was dense with methane, and when methane builds up in the atmosphere, sunlight can cause it to break apart and take on new forms — and new colors. As a result, the early Earth may have sometimes been covered in what we would have seen as waves of dense orange haze.

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At some unknown moment during Earth’s first billion years, life emerged, and scientists believe that some early organisms may have emitted methane as a waste product, making the orange hazes even thicker.

But there was another ancient, microscopic life form that would eventually make the sky look more like the one we see today. Cyanobacteria were the first organisms known to have used the sun’s energy to combine carbon dioxide and water into sugar molecules. Called photosynthesis, this process releases oxygen into the atmosphere and makes life possible for the rest of us. It also changes the balance of gasses in the atmosphere, ultimately changing the way it interacts with light.

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Today, our atmosphere is made up of about 78% nitrogen, 21% oxygen, and 1% argon, with other gasses like carbon dioxide, methane, nitrous oxide, and ozone accounting for only a tiny fraction of a percent. Sunlight is a combination of all colors, and the longer wavelengths that represent colors like red and yellow reach the ground without a problem. But the shorter wavelengths that look blue to our eyes bounce off the molecules of nitrogen and oxygen in our atmosphere. As these beams of light are scattered around the sky, they create the familiar blue color that reminds us to breathe deep and be grateful.

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