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The opinion of astronomers is that the planets were born from whirlpools of gas and dust, generally constituted by the various elements present, in proportions corresponding to their cosmic abundance. About 90 percent of the atoms were hydrogen and another 9 percent was helium. The rest included all other elements, mainly neon, oxygen, carbon, nitrogen, carbon, sulfur, silicon, magnesium, iron and aluminum.
The solid Earth globe itself was born from a rocky mixture of silicates and sulphides of magnesium, iron and aluminum, whose molecules were firmly held together by chemical forces. The excess iron slowly sank through the rock and formed an incandescent metal core.
During this process of agglomeration, the solid matter of the Earth trapped a series of gaseous materials and retained them in the openings that remained between the solid particles or by weak chemical bonds. These gases would surely contain atoms of helium, neon and argon, which were not combined with anything; and hydrogen atoms, which either combined with each other in pairs to form hydrogen molecules (H2), or they were combined with other atoms: with oxygen to form water (H2O), with nitrogen to form ammonia (NH3) or with carbon to form methane (CH4).
As the material of this budding planet was pounding, the oppressive effect of pressure and even more violent volcanic action were expelling gases. Hydrogen molecules and helium and neon atoms, being too light to be retained, quickly escaped.
The Earth's atmosphere was constituted by what remained: water vapor, ammonia, methane and some argon. Most of the water vapor, but not all, condensed and formed an ocean.
Such is, at present, the kind of atmosphere that some planets such as Jupiter and Saturn possess, which, however, are large enough to retain hydrogen, helium and neon.
For its part, the atmosphere of the inner planets began to evolve chemically. The ultraviolet rays of the nearby Sun broke the water vapor molecules into hydrogen and oxygen. Hydrogen escaped, but oxygen accumulated and combined with ammonia and methane. With the first it formed nitrogen and water; with the second, carbon dioxide and water.
Gradually, the atmosphere of the inner planets went from being a mixture of ammonia and methane to a mixture of nitrogen and carbon dioxide. Mars and Venus today have atmospheres composed of nitrogen and carbon dioxide, while the Earth must have had a similar billions of years ago, when life began to emerge.
That atmosphere is also stable. Once formed, the further action of ultraviolet rays on water vapor causes free oxygen to accumulate (molecules formed by two oxygen atoms, OR2). An even more intense ultraviolet action transforms that oxygen into ozone (with three oxygen atoms per molecule, OR3). Ozone absorbs ultraviolet radiation and acts as a barrier. The ultraviolet radiation that manages to cross the ozone layer in the high atmosphere and break down the water molecules below is very scarce, which stops the chemical evolution of the atmosphere ..., at least until something new appears.
Well, something new appeared on Earth. It was the development of a group of life forms capable of using visible light to break the water molecules. As the ozone layer does not intercept visible light, that process (photosynthesis) could continue indefinitely. Through photosynthesis carbon dioxide was consumed and oxygen was released.
Thus, 500 million years ago, the atmosphere began to become a mixture of nitrogen and oxygen, which is what exists today.
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