Earth's atmosphere, which is around 300 miles thick, is mainly made of nitrogen and oxygen, although there are many other trace substances in its composition. Trace substances make up only 1 percent of the bulk of the atmosphere while nitrogen (78 percent), oxygen (21 percent) and argon (1 percent) make up the rest.
The minority 1 percent contains a variety of things, including minute amounts of neon, carbon dioxide, helium, krypton, methane, nitrous oxide, hydrogen, iodine, ozone, xenon, ammonia and carbon monoxide. At lower altitudes, water vapor may also be present.
The early atmosphere of the Earth was not as hospitable as the current atmosphere. In fact, it was about 100 times more dense around 4.4 billion years ago when the earthy first cooled and was much more abundant in ammonia and carbon dioxide than it is today. Earth is the only planet known to sustain life since the atmosphere protects the planet from radiation and heat from the sun.
The atmosphere is divided into five layers, including the troposphere, stratosphere, mesosphere, thermosphere and exosphere. The layer closest to Earth is the troposphere. It's around four to 12 miles thick. Scientists know the least about the mesosphere since weather balloons and jets can't reach its height (31 to 53 miles above the Earth) and satellites orbit too high above it.