The atmosphere as a membrane

The Earth has been compared to a single cell, and the atmosphere is the cell's membrane. The atmosphere serves to protect the delicate functions of life from the harsh radiation of the Sun. It filters out the harmful ultraviolet rays, while allowing visible light to shine through and nourish the Earth's vegetation. Most people look to the sky and are amazed by the wide open space that seems to go on forever. In reality, however, the atmosphere is very small compared to the rest of the planet. The Earth itself is nearly 13,000 km in diameter, but the atmosphere rises only a few tens of kilometers above the surface. If you were to compare the Earth to an ordinary apple, the atmosphere would be only as thick as the apple's skin.


Credit: NASA

For more info: http://images.jsc.nasa.gov/