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The oxygen cycle demonstrates how free oxygen is made available in each of these regions, as well as how it is used. The oxygen cycle is the biogeochemical cycle of oxygen atoms between different oxidation states in ions, oxides, and molecules through redox reactions within and between the spheres/reservoirs of the planet Earth. [1]
Oxygen is the third most abundant chemical element in the universe, after hydrogen and helium. [68] About 0.9% of the Sun's mass is oxygen. [19] Oxygen constitutes 49.2% of the Earth's crust by mass [69] as part of oxide compounds such as silicon dioxide and is the most abundant element by mass in the Earth's crust.
Artist's rendition of the Earth's plasma fountain, showing oxygen, helium, and hydrogen ions that gush into space from regions near the Earth's poles. The faint yellow area shown above the north pole represents gas lost from Earth into space; the green area is the aurora borealis , where plasma energy pours back into the atmosphere.
Solid oxygen forms at normal atmospheric pressure at a temperature below 54.36 K (−218.79 °C, −361.82 °F). Solid oxygen O 2 , like liquid oxygen , is a clear substance with a light sky-blue color caused by absorption in the red part of the visible light spectrum.
Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a biological membrane that forms the boundary between an organism and its extracellular environment.
Liquid oxygen has a clear cyan color and is strongly paramagnetic: it can be suspended between the poles of a powerful horseshoe magnet. [2] Liquid oxygen has a density of 1.141 kg/L (1.141 g/ml), slightly denser than liquid water, and is cryogenic with a freezing point of 54.36 K (−218.79 °C; −361.82 °F) and a boiling point of 90.19 K (−182.96 °C; −297.33 °F) at 1 bar (14.5 psi).
Free oxygen did not exist in the atmosphere until about 2.4 billion years ago during the Great Oxygenation Event and its appearance is indicated by the end of banded iron formations (which signals the depletion of substrates that can react with oxygen to produce ferric deposits) during the early Proterozoic eon.
Dark oxygen production refers to the generation of molecular oxygen (O 2) through processes that do not involve light-dependent oxygenic photosynthesis.The name therefore uses a different sense of 'dark' than that used in the phrase "biological dark matter" (for example) which indicates obscurity to scientific assessment rather than the photometric meaning.