the upper boundary of the atmosphere). The exosphere is the region far from the thermosphere. It extends from the exobase, which is located at the top of the thermosphere at an altitude of about 700 km above sea level. The exosphere merges with the nakedness of outer space, where there is no atmosphere. This layer is mainly composed of tremendously low densities of hydrogen, helium and numerous heavier molecules which are nitrogen, oxygen and carbon dioxide closer to the exobase. These molecules are so distant that they can move hundreds of kilometers without colliding with each other. Therefore, the exosphere no longer behaves like a gas and particles constantly escape into space. Thermosphere The thermosphere is the second highest layer of the Earth's atmosphere. It extends from the mesopause at an altitude of about 80 km to the thermopause at an altitude of 500–1000 km. The height of the thermopause varies significantly due to changes in solar movement. Since the thermopause is located at the lower edge of the exosphere, it is also called