The mesosphere, (36k jpeg) a layer extending from approximately 50 km to 80 km, is characterized by decreasing temperatures, which reach 190-180 K at an altitude of 80 km. In this region, concentrations of ozone and water vapor are negligible. Hence the temperature is lower than that of the troposphere or stratosphere. With increasing distance from Earth's surface the chemical composition of air becomes strongly dependent on altitude and the atmosphere becomes enriched with lighter gases. At very high altitudes, the residual gases begin to stratify according to molecular mass, because of gravitational separation. .
Thermosphere .
The thermosphere (39k jpeg) is located above the mesosphere and is separated from it by the mesopause transition layer. The temperature in the thermosphere generally increases with altitude up to 1000-1500 K. This increase in temperature is due to the absorption of intense solar radiation by the limited amount of remaining molecular oxygen. At an altitude of 100-200 km, the major atmospheric components are still nitrogen and oxygen. At this extreme altitude gas molecules are widely separated. .
Exosphere .
The exosphere (41k jpeg) is the most distant atmospheric region from Earth's surface. The upper boundary of the layer extends to heights of perhaps 960 to 1000 km and is relatively undefined. The exosphere is a transitional zone between Earth's atmosphere and interplanetary space. .
Present Day Atmosphere.
The atmosphere we breathe is a relatively stable mixture of several hundred types of gases from different origins. This gaseous envelope surrounds the planet and revolves with it. It has a mass of about 5.15 x 10E15 tons held to the planet by gravitational attraction. The proportions of gases, excluding water vapor, are nearly uniform up to approximately 80 kilometers (km) above Earth's surface. The major components of this region, by volume, are oxygen (21%), nitrogen (78%), and argon (0.
