Deep within a planet’s atmosphere, these high-speed
molecules will almost certainly collide with other molecules
before they have a chance to escape. During the collision process,
energy is exchanged. The high-speed molecule usually
emerges with a lower speed, and the slower one tends to move
faster. After a collision, then, both are likely to move with
speeds closer to the average. There are fewer surrounding
molecules near the top of the atmosphere, and a high-speed
molecule has a good chance of escaping before colliding with
another molecule if it is heading more or less upward. Less
massive molecules and atoms, such as hydrogen and helium,
move faster and are more quickly lost to space than more
massive molecules such as nitrogen or carbon dioxide. The
ability of planets to hold on to their atmospheres is explored
further in Math Tools 9.1 (page 262).