The exosphere is the highest and top layer of the Earth's atmosphere. It marks the edge of space. There are very few molecules in this layer. The lightest atmospheric gases such as hydrogen and helium exist throughout the exosphere. Heavier ones like carbon dioxide and atomic oxygen are near the base. 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 is about 500-800 km (300-500 miles) above the surface and is called the exobase. It is also called the thermopause because it is in between the exosphere and the thermosphere. The molecules are moving very fast, so the temperature in this zone is estimated to be thousands of degrees celsius.