why doesn’t Earths atmosphere just float off into space?

Question

Why doesn’t the atmosphere (air we breathe) just float off into space?

Some objects in space have atmospheres like Earth and Jupiter. Others like asteroids and the moon don’t have an atmosphere.
Why doesn’t earth atmosphere just drift away into space?
What am I missing about the concept of atmosphere?

In: Planetary Science

Answer 1

Gravity holds our atmosphere in place. Gases, like anything else, have mass and are affected by gravity. The lighter gases, like H and He, do indeed drift off.