Britannica Sources used in this answer

Why can hydrogen and helium escape more easily from Earth than heavier gases?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

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.

Hydrogen and helium escape from Earth’s atmosphere more easily than heavier gases because of their low molecular mass and the resulting high velocities they attain.12

The speed at which a gas molecule moves is proportional to the square root of the ratio of its absolute temperature to its molecular mass ( $T / M$ ^{1/2}). Because hydrogen and helium have very low molecular weights (2 and 4, respectively), they move at higher average speeds than heavier molecules at the same temperature.3

To escape a planet’s gravitational field, a particle must reach escape velocity—for Earth, this is 11.3 km per second. In the exosphere, where molecular collisions are rare, these light, fast-moving atoms can reach velocities that allow them to overcome Earth’s gravity and escape into outer space. Heavier gases, conversely, have lower average velocities and are generally unable to reach the speed required to escape the planet's gravitational pull.3142