Pictured left: Global images of Earth from Galileo: In each frame, the continent of Antarctica is visible at the bottom of the globe. South America may be seen in the first frame (top left), the great Pacific Ocean in the second (bottom left), India at the top and Australia to the right in the third (top right), and Africa in the fourth (bottom right).
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth creates pressure, absorbs most meteoroids and ultraviolet solar radiation, warms the surface through heat retention (greenhouse effect), and reduces temperature extremes between day and night (the diurnal temperature variation), maintaining conditions allowing life and liquid water to exist on the Earth's surface.
As of 2023, by mole fraction (i.e., by number of molecules), dry air contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere. Air composition, temperature, and atmospheric pressure vary with altitude. Within the atmosphere, air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth's troposphere.
Earth's early atmosphere consisted of gases in the solar nebula, primarily hydrogen. The atmosphere changed significantly over time, affected by many factors such as volcanism, life, and weathering. Recently, human activity has also contributed to atmospheric changes, such as global warming, ozone depletion and acid deposition. (Full article...) (Full article...)