### Is oxygen really the most abundant element on the surface of the Moon?

• 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?

I would guess that ESA is trustworthy, but maybe someone can answer in more detail.

Oxygen is the most abundant element in the rocks of the Earth's crust, so that would make sense. https://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth%27s_crust

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). https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements#Universe You can talk about specific reactions in stars, but the rates of those reactions are ultimately determined mainly by these facts about binding energies.

@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.

Oxygen is the highest occuring element in rocks. The moons surface is covered in tiny rocks.

@StianYttervik it's not that the moon is covered with rocks. The moon _is_ rocks.

@ProfRob: I agree with your comment. I wasn't making such a strong claim.

• 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.

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.

@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.

@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.

@Gimelist You stated "silica is not a mineral" but otherwise, point taken.

@Gemilist I've updated my answer to more properly distinguish between components and minerals; hopefully it's an improvement.