The upper region of Earth's atmosphere above about 85 kilometers where the composition of gases varies with altitude, unlike the lower homosphere.
From 'hetero-' (different) + 'sphere' (Greek 'sphaira,' meaning ball). Coined by atmospheric scientists in the 20th century to distinguish this zone from the compositionally uniform lower atmosphere.
The heterosphere is where atoms get so spread out that they separate by weight—heavier nitrogen sinks down, lighter hydrogen floats up, creating an invisible chemical sorting machine!
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