The troposphere, stratosphere, mesosphere, thermosphere, and exosphere are the five layers which make up Earth's atmosphere. 75% of the gases in the atmosphere are located within the troposphere, the lowest layer. In all, the atmosphere is made up of about 78. 0% nitrogen, 20. 9% oxygen, and 0. 92% argon. In addition to the nitrogen, oxygen, and argon there are small amounts of other gases including CO2 and water vapor. Water vapor and CO2 allow the earth's atmosphere to catch and hold the Sun's energy through a phenomenon called the greenhouse effect. This allows Earth's surface to be warm enough to have liquid water and support life. In addition to storing heat, the atmosphere also protects living organisms by shielding the earth's surface from cosmic rays—which are often incorrectly thought to be deflected by the magnetic field. The magnetic field—created by the internal motions of the core—produces the magnetosphere which protects Earth's atmosphere from the solar wind. As the earth is 4. 5 billion years old, it would have lost its atmosphere by now if there were no protective magnetosphere.