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From Citizendium, the Citizens' Compendium

The heterosphere is the upper portion of the Earth's atmosphere, in which gases are fractionated according to their molecular weight. It lies above the homosphere, where chemical constituents are well mixed. On average the heterosphere begins at an altitude of about 80 km.

Within the heterosphere, the thermosphere extends from the bottom of the heterosphere to an altitude of 500 km or more and the exosphere lies beyond the thermosphere.