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The exosphere is the highest and top layer of the Earth's atmosphere. It starts at 500 kilometers high and marks the edge of space. There are very few molecules in this layer. This means that it has the lightest atmospheric gases such as hydrogen, helium, carbon dioxide and atomic oxygen near the base of exosphere. The molecules collide so rarely, that the idea of atmospheric pressure has little meaning. There are two borders to the exosphere; the lower boundary and the upper boundary. The lower boundary can be known as the exobase. It can also be known as the thermopause because it is in between the exosphere and the thermosphere. The molecules are moving very fast, so that the temperature in this zone is estimated to be thousands of degrees celsius.