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I found this infographic that seems to say that oxygen is the most abundant element on the surface of the Moon. Is this really the case? If so, under what form is this oxygen?

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    $\begingroup$ I would guess that ESA is trustworthy, but maybe someone can answer in more detail. $\endgroup$ – Jonas Feb 11 at 11:05
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    $\begingroup$ Oxygen is the most abundant element in the rocks of the Earth's crust, so that would make sense. en.wikipedia.org/wiki/Abundance_of_elements_in_Earth%27s_crust $\endgroup$ – Peter Erwin Feb 11 at 11:25
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    $\begingroup$ This ultimately boils down to some basic nuclear physics. Nuclei are more stable if their neutron number N and proton number Z are close to certain magic numbers, which are 2, 8, 20, 28, 50, 82, and 126. This is why big bang nucleosynthesis produced a lot of helium (N=2, Z=2), and stellar nucleosynthesis tends to produce a lot of oxygen-16 (N=8 and Z=8). en.wikipedia.org/wiki/… You can talk about specific reactions in stars, but the rates of those reactions are ultimately determined mainly by these facts about binding energies. $\endgroup$ – Ben Crowell Feb 12 at 0:53
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    $\begingroup$ @BenCrowell what you say is only partly true. Nitrogen is the 5th most abundant element by number and of course helium is much more abundant than oxygen, but there is hardly any on the Moon. Carbon is common in in the universe but underrepresented by a factor of I think 10 in the Earth/Moon. The binding energy per nucleon of iron is the largest, but it is not the most abundant. You can't just invert a binding energy table to estimate the relative abundances on the Moon. $\endgroup$ – ProfRob Feb 12 at 7:49
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    $\begingroup$ @StianYttervik it's not that the moon is covered with rocks. The moon is rocks. $\endgroup$ – Gimelist Feb 13 at 11:24
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Yes, that's correct; it's also true for the Earth's crust. The reason is that "rocks" are typically made up of components containing combinations of silicon or one or more metals (e.g., magnesium, aluminum, iron) and oxygen, such as silica ($\mathrm{SiO}_{2}$); alumina ($\mathrm{Al}_{2}\mathrm{O}_{3}$); lime ($\mathrm{CaO}$); iron oxide ($\mathrm{FeO}$); and magnesium oxide ($\mathrm{MgO}$).

Examples of common lunar minerals formed from these components includes plagioclase feldspars (mixtures of NaAlSi$_{3}$O$_{8}$ and CaAl$_{2}$Si$_{2}$O$_{8}$), pyroxene (typically XYSi$_{2}$O$_{6}$, where X and Y are metals such as calcium, sodium, iron, magnesium, and aluminum), and olivine (made up of Mg$_{2}$SiO$_{4}$ and Fe$_{2}$SiO$_{4}$), along with oxide minerals like ilmenite (FeTiO$_{3}$). (Source)

Since in all these cases you have between one and two oxygen atoms for every non-oxygen atom, you end up with oxygen as the most abundant single element.

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    $\begingroup$ Silica is not the most common mineral in the Moon's surface because (1) silica is not a mineral, (2) even if it was a mineral in the form of quartz, there's hardly an quartz on the moon's surface. Instead, silica is the most abundant chemical component on the Moon, which is a completely different thing. $\endgroup$ – Gimelist Feb 13 at 11:23
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    $\begingroup$ @Gimelist "In mineralogy, silica (silicon dioxide) SiO2 is usually considered a silicate mineral." That's what it says on Wikipedia but if you know better do feel free to correct it. $\endgroup$ – Brian Z Feb 14 at 14:52
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    $\begingroup$ @BrianZ this is correct, but it has nothing to do with the answer or my comment. There is no significant amount of silica/silicon-dioxide/etc on the Moon. All of the silica is bound up as a chemical component in minerals like feldspars. Silica, per se, is not a mineral on the moon. It's like saying that "dihydrogen gas" is the most abundant molecule in our bodies because there's plenty of it in organic matter. Or, that graphite is the most abundant mineral in the human body because organic matter has carbon in it. $\endgroup$ – Gimelist Feb 14 at 21:45
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    $\begingroup$ @Gimelist You stated "silica is not a mineral" but otherwise, point taken. $\endgroup$ – Brian Z Feb 14 at 22:02
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    $\begingroup$ @Gemilist I've updated my answer to more properly distinguish between components and minerals; hopefully it's an improvement. $\endgroup$ – Peter Erwin Feb 15 at 12:19
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Note this fact is unsurprising. Oxygen is the third most abundant element in the solar system (by mass and by number) after hydrogen and helium.

Planets/moons with the size and escape velocities of the Earth and the Moon are unable to hang onto much in the way of helium- or hydrogen-rich compounds at the equilibrium temperatures at 1 au from the Sun (in fact the presence of quite a lot of water on the Earth is still something of a mystery). On the contrary, even oxygen gas can be retained by the Earth, but more importantly, oxygen is so chemically reactive that it can bond with lots of other things, including other, heavier, but still abundant, elements like silicon, magnesium and iron, to form... rocks.

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    $\begingroup$ Is oxygen mostly produced in the CNO cycle? I get the impression from Wikipedia that not much O-16 is produced in carbon burning (because C-12 + C-12 -> O-16 + He-4 is endothermic), or by the carbon alpha process (since alpha process reactions have a low rate because they emit a gamma). $\endgroup$ – PM 2Ring Feb 11 at 20:17
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    $\begingroup$ Wikipedia's article on $^{16}O$ says "Most $^{16}O$ is synthesized at the end of the helium fusion process in stars; the triple-alpha process creates $^{12}C$, which captures an additional $^{4}He$ to make $^{16}O$. The neon-burning process creates additional $^{16}O$." $\endgroup$ – Ross Presser Feb 11 at 20:49
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    $\begingroup$ Note that Earth would have only a miniscule amount of free oxygen, if any, if it wasn't being continuously renewed by life forms - mostly photosynthetic plants. And if I'm not mistaken, most if not all of the helium is the product of alpha decay of radioactive elements. $\endgroup$ – jamesqf Feb 12 at 0:44
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    $\begingroup$ @jamesqf correct: en.wikipedia.org/wiki/Great_Oxidation_Event $\endgroup$ – RonJohn Feb 12 at 2:03
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    $\begingroup$ @jamesqf Mostly, but a significant percentage of Earth's helium is primordial. See physics.stackexchange.com/q/548056/123208 & physics.stackexchange.com/q/551119/123208 I find that rather surprising because I expect Earth lost a lot of primordial He during formation, due to all the heat produced. There isn't a consensus on whether the Earth was ever totally molten, but the heat of formation was certainly substantial. $\endgroup$ – PM 2Ring Feb 12 at 7:33

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