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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 air is less heavy there. There are two parts 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 exopause because it is in between the exosphere and the thermosphere.