The atmosphere

There are also two layers on top of Earth's rocky crust: the hydrosphere and the atmosphere.

The atmosphere is very thin. If planet Earth is thought of as an apple, the thickness of the atmosphere can be compared to that of the apple's peel. 

The atmosphere also contains various smaller layers.

However, the borders between these layers are not clear. Instead, they change into one another gradually. 

At the altitude of 100 km, we can already consider that we are in space. At this altitude, the sky will already be black. 
 
The Sun provides the Earth with plenty of radiation. This radiation comes in different wavelenghts, some of which are more beneficial for living organisms than others.

Light and heat radiation are both beneficial of organisms. They make life on the planet possible. 

Other forms of radiation are harmful. For example, the Sun's ultraviolet radiation can cause damage to living organisms. 

The planet's ozone layer (O3) protects living organisms from harmful ultraviolet radiation.

The atmosphere is held in place by Earth's gravity. If the planet would not have gravity, the gases of the atmosphere would escape into space.

The Moon has a low gravity, which is why it cannot support an atmosphere.