The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space.The Kármán line, at 100 km (62 mi), or 1.57% of Earth's radius, is often used as the border between the atmosphere and outer space.The atmosphere of Earth is the layer of gases, commonly known as air, that surrounds the planet Earth and is retained by Earth's gravity.

Layers drawn to scale, objects within the layers are not to scale.

Aurorae shown here at the bottom of the thermosphere can actually form at any altitude in this atmospheric layer.

Air content and atmospheric pressure vary at different layers, and air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth's troposphere and in artificial atmospheres.

The atmosphere has a mass of about 5.15 three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface.

Thus, the exosphere no longer behaves like a gas, and the particles constantly escape into space.

These free-moving particles follow ballistic trajectories and may migrate in and out of the magnetosphere or the solar wind.

Because the thermopause lies at the lower boundary of the exosphere, it is also referred to as the exobase.

The lower part of the thermosphere, from 80 to 550 kilometres (50 to 342 mi) above Earth's surface, contains the ionosphere.

Excluding the exosphere, the atmosphere has four primary layers, which are the troposphere, stratosphere, mesosphere, and thermosphere.