The outermost exosphere interfaces outer space, theoretically halfway to the Moon (190,000 km). The exosphere is mostly dispersed hydrogen and helium, along with stray molecules of carbon dioxide and atomic oxygen toward the lower boundary, which ranges from 200–500 km, depending upon the solar wind.
The thermosphere begins 80 km above the Earth’s surface. At such high altitude, residual gases sort into strata according to molecular mass.
Solar radiation excites the residual oxygen ever more as altitude climbs, heating the sparse gas in the upper thermosphere to 2,800 K, depending upon the flux of solar activity. Yet a thermometer would measure the thermosphere below freezing, because it is so near vacuum.
Above 160 km, gas density is so low that molecular interactions are too infrequent to carry sound (the anacoustic zone).
The thermosphere is vital to life on Earth. Conditions there determine the net loss of hydrogen: the hotter, the greater the loss. Since water is the atmospheric source of hydrogen to the thermosphere, losing hydrogen means planetary water loss.
The dynamics of the lower thermosphere are driven by the atmospheric tide, analogous to the ocean tides. Variations in diurnal heating swagger the atmospheric tide. The tide rides below 120 km; above that, gas concentration is too thin to support coherent fluid flow.
At 53–85 km, the mesosphere is atmospheric drama incarnate. Most meteors burn up there. Fierce sunlight cracks air molecules like a squirrel cracks nuts. Water molecules violently split into hydroxyl radicals (HO) and hydrogen atoms (H). CO2 cleaves into CO and a lonely O.