Gravity pulls atmospheric gas molecules toward Earth, preventing most of them from escaping into space. Their continuous thermal motion keeps them moving, but gravity concentrates most atmospheric mass near Earth's surface.
Level: HL only: Subtopic 6.1 Introduction to the atmosphere.
The Mechanism
Every gas molecule with mass experiences gravitational attraction toward Earth's centre. The force on a molecule can be represented by:
Here, is the gravitational constant, is Earth's mass, is the molecule's mass, and is its distance from Earth's centre. As altitude increases, increases and gravitational force decreases.
Gas molecules also possess kinetic energy, so they move rapidly and randomly in all directions. Most do not move fast enough to overcome Earth's gravity and reach escape velocity. Gravity therefore retains the atmosphere while molecular motion spreads it above the surface.
The weight of overlying air produces atmospheric pressure. Near sea level, a location has a large column of air above it, so pressure and density are relatively high. At higher altitudes, fewer molecules remain above that location, so both pressure and density decrease.
For example, atmospheric pressure at the summit of Mount Everest is much lower than at sea level. Climbers therefore receive fewer oxygen molecules with each breath, even though oxygen still forms approximately the same proportion of the surrounding air.
A common misconception is that the atmosphere forms a fixed shell with a sharp outer edge. In reality, atmospheric density decreases gradually with altitude, and very light gases such as hydrogen can sometimes escape into space.
Exam Technique
For an IB ESS explain question, link each step clearly: gravity attracts molecules toward Earth, molecular motion opposes complete collapse, and the balance creates decreasing pressure and density with altitude. Do not simply state that "gravity keeps air down"; explain the mechanism and its observable consequence.