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Why are hydrogen and helium more common in the highest atmospheric layers?

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Wispy cirrus clouds stretch across a bright blue sky above trees and a river at Pinawa Dam Provincial Park in Manitoba, Canada, with sunlight illuminating the scene. — Atmosphere The atmospheres of planets in the solar system are composed of various gases, particulates, and liquids. They are also dynamic places that redistribute heat and other forms of energy. On Earth, the atmosphere provides critical ingredients for living things. Here, feathery cirrus clouds drift across the deep blue sky over Pinawa Dam Provincial Park, near Pinawa, Manitoba, Canada.

In the lower atmosphere, known as the homosphere, turbulent mixing keeps the composition of gases relatively uniform regardless of height. However, above approximately 100 km, in a region called the heterosphere, this turbulence ceases, allowing atmospheric gases to separate based on their molecular mass.12

In this upper region, heavier gases like nitrogen and oxygen decrease rapidly in concentration with increasing altitude. Because hydrogen and helium are much lighter, they become the dominant constituents at these high altitudes. Consequently, while heavier elements are concentrated closer to the surface due to gravitational attraction, the lighter gases are able to persist and concentrate in the highest layers of the atmosphere.123