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Oxygen is present as compounds in the atmosphere in trace quantities in the form of carbon dioxide (CO 2) and oxides of nitrogen (NO x). The Earth's crustal rock is composed in large part of oxides of silicon (silica SiO 2, found in granite and sand), aluminium (aluminium oxide Al 2 O 3, in bauxite and corundum), iron (iron (III) oxide Fe 2 O
The first atmosphere, during the Early Earth's Hadean eon, consisted of gases in the solar nebula, primarily hydrogen, and probably simple hydrides such as those now found in the gas giants (Jupiter and Saturn), notably water vapor, methane and ammonia.
Trace gases are gases that are present in small amounts within an environment such as a planet's atmosphere.Trace gases in Earth's atmosphere are gases other than nitrogen (78.1%), oxygen (20.9%), and argon (0.934%) which, in combination, make up 99.934% of its atmosphere (not including water vapor).
Water is present as vapor in: Atmosphere of the Sun: in detectable trace amounts [166] Atmosphere of Mercury: 3.4%, and large amounts of water in Mercury's exosphere [167] Atmosphere of Venus: 0.002% [168] Earth's atmosphere: ≈0.40% over full atmosphere, typically 1–4% at surface; as well as that of the Moon in trace amounts [169]
Stages 4 and 5 (0.85 Ga–present): O 2 sinks filled, the gas accumulates. [1] Although oxygen is the most abundant element in Earth's crust, due to its high reactivity it mostly exists in compound forms such as water, carbon dioxide, iron oxides and silicates.
The transition of Earth's atmosphere from an anoxic prebiotic reducing atmosphere high in methane and hydrogen sulfide to an oxidative atmosphere of which free nitrogen and oxygen make up 99% of the mole fractions, had led to major climate changes and caused numerous icehouse phenomena and global glaciations.
About 99% of the Earth's atmosphere is composed of two species of diatomic molecules: nitrogen (78%) and oxygen (21%). The natural abundance of hydrogen (H 2 ) in the Earth's atmosphere is only of the order of parts per million, but H 2 is the most abundant diatomic molecule in the universe.
Both elements may be replaced by isotopes, for example in the doubly labeled water isotopologue D 2 18 O. Altogether, there are 9 different stable water isotopologues, [2] and 9 radioactive isotopologues involving tritium, [3] for a total of 18. However only certain ratios are possible in mixture, due to prevalent hydrogen swapping.