Earth's atmosphere is a layer of gases that surrounds the Earth and is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet radiation, warming the surface through the greenhouse effect, and reducing temperature extremes between day and night. The atmosphere is 78% dinitrogen, 21% dioxygen, 0.9% argon, 0.04% carbon dioxide, 0.002% neon, 0.0005% helium, 0.0002% methane, 0.0001% krypton, and 0.00006% dihydrogen. It is layered from troposphere (0-7 mi), stratosphere (7-31 mi), mesosphere (31-50 mi), thermosphere (50-440 mi), and the exosphere (440-800 mi). The atmosphere was primarily methane and carbon dioxide until the Oxygen Catastrophe, which put large amounts of dioxygen into the atmosphere.
