The gas mixture that forms the earth's atmosphere is known as air. The main components are 78.08% nitrogen, 20.95% oxygen, 0.93% argon and 0.04% carbondioxide. Air also contains water vapour, the amount depending on temperature and relative humidity.
Air supports life on earth and remains in place thanks to gravity. Atmosperic pressure is caused by the gravitational attraction of the planet on the gases above the surface, and depends on
- mass of the planet
- radius of the surface
- composition and vertical distribution of the atmosphere.
The earth's atmosopheric pressure is approximately 1 atm (= 101.3 kPa) and can vary with wind speed and temperature variations and changes in the air composition.
Mars has an atmospheric pressure of only 0.006 atm, because its atmosphere is very thin.
Venus has an atmospheric pressure of approx. 92 atm, its atmosphere being much thicker that earth's.
Another important part of our atmosphere is the ozone layer, which is located in the stratosphere in a region between 10 and 50 km above the earth. The ozone layer filters more than 99% of the UV light from the sun. Without an ozone layer, all UV light from the sun would strike the surface and effectively oxidize any organic matter. So no life would be possible. Such as on Mars, that has no ozone layer.
