Briefly Describe Earth's Atmosphere

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The atmosphere is the air that is wrapped all around a planet. Not all planets have atmospheres. In order to have an atmosphere, the planet has to have enough gravity to hold on to light atoms like hydrogen and helium and keep them from floating away into space. That means that the planet has to have a lot of mass. Because the force of gravity is stronger near the planet and gets weaker as you get further away, the atmosphere is thicker close to the ground and gradually gets thinner as you go further out into space. There is no sharp edge to the atmosphere. Earth, our own planet, has an atmosphere. When the Earth first formed, about four and a half billion years ago, Earth's atmosphere was almost entirely made of hydrogen and helium atoms, because they were the lightest atoms and floated to the top. But the Earth was still so hot, and the Sun heated the Earth so much, that most of the hydrogen and helium atoms ended up drifting off into space. Soon after that, about 4.4 billion years ago, the Earth cooled down a lot. But there were still a lot of volcanoes that shot out steam, carbon dioxide, and ammonia. This created a new atmosphere made mostly of carbon dioxide and water, with some nitrogen. Then about three billion years ago, early prokaryotic cells, one of the earliest forms of life on Earth, began to use photosynthesis to get food for themselves. They made their food out of what was available - sunlight, carbon dioxide, and water. And they excreted (pooped out) what they didn't need - mainly oxygen. At first most of these oxygen atoms bonded with other atoms to form molecules, like this iron that has turned red by combining with oxygen. After almost a billion years of millions of cells shooting out oxygen, everything that oxygen could join with had enough oxygen, and the leftovers began to pile up in the

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