Atmospheric Composition Criterion
1.Using chemical composition as a criterion, the atmosphere divides into two broad regions, the heterosphere (80 to 480 km altitude) and the homosphere (Earth's surface to 80 km altitude). As you read, note that we follow the same path that incoming solar radiation travels through the atmosphere to Earth's surface.
2.The heterosphere is the outer atmosphere in terms of composition. It begins at about 80 km (50 mi) altitude and extends outward to the transition to the exosphere and interplanetary space.The International Space Station and most Space Shuttle missions orbit in the upper heterosphere.As the prefix hetero - implies, this region is not unifom - its gases are not evenly mixed.
3.This distribution is quite different from the nicely blended gases we breathe near Earth's surface, in the homosphere. Gases in the heterosphere occur in distinct layers sorted by gravity according to their atomic weight, with the lightest elements (hydrogen and helium) at the margins of outer space and the heavier elements (oxygen and nitrogen) dominant in the lower heterosphere.