<p>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.</p>
<p>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.</p>

CC BY-SA 4.0
2021-02-11T20:17:29.830Z
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).
PM 2Ring
CC BY-SA 4.0
2021-02-11T20:49:15.600Z
Wikipedia's [article on $^{16}O$](https://en.wikipedia.org/wiki/Oxygen-16) 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 $^{16}O$."
Ross Presser
CC BY-SA 4.0
2021-02-12T00:44:50.990Z
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.
jamesqf
CC BY-SA 4.0
2021-02-12T02:03:17.523Z
@jamesqf correct: https://en.wikipedia.org/wiki/Great_Oxidation_Event
RonJohn
CC BY-SA 4.0
2021-02-12T06:42:06.130Z
Thanks for that info, @Ross. That article doesn't have a relevant citation, but I found a good one in https://en.wikipedia.org/wiki/Isotopes_of_oxygen#cite_note-Meyer2005-14
PM 2Ring
CC BY-SA 4.0
2021-02-12T07:33:25.367Z
@jamesqf Mostly, but a significant percentage of Earth's helium is primordial. See https://physics.stackexchange.com/q/548056/123208 & https://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.
PM 2Ring
CC BY-SA 4.0
2021-02-11T19:48:00.260Z
2021-02-11T19:59:08.233Z
2021-02-11T19:59:08.233Z
ProfRob
<p>Yes, <a href="https://en.wikipedia.org/wiki/Geology_of_the_Moon" rel="nofollow noreferrer">that's correct</a>; it's also true for <a href="https://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth%27s_crust" rel="nofollow noreferrer">the Earth's crust</a>. The reason is that &quot;rocks&quot; are typically made up of components containing combinations of silicon or one or more metals (e.g., magnesium, aluminum, iron) <em>and oxygen</em>, such as <a href="https://en.wikipedia.org/wiki/Silicon_dioxide" rel="nofollow noreferrer">silica</a> (<span class="math-container">$\mathrm{SiO}_{2}$</span>); <a href="https://en.wikipedia.org/wiki/Aluminium_oxide" rel="nofollow noreferrer">alumina</a> (<span class="math-container">$\mathrm{Al}_{2}\mathrm{O}_{3}$</span>); <a href="https://en.wikipedia.org/wiki/Calcium_oxide" rel="nofollow noreferrer">lime</a> (<span class="math-container">$\mathrm{CaO}$</span>); <a href="https://en.wikipedia.org/wiki/Iron(II)_oxide" rel="nofollow noreferrer">iron oxide</a> (<span class="math-container">$\mathrm{FeO}$</span>); and <a href="https://en.wikipedia.org/wiki/Magnesium_oxide" rel="nofollow noreferrer">magnesium oxide</a> (<span class="math-container">$\mathrm{MgO}$</span>).</p>
<p>Examples of common lunar minerals formed from these components includes plagioclase feldspars (mixtures of NaAlSi<span class="math-container">$_{3}$</span>O<span class="math-container">$_{8}$</span> and CaAl<span class="math-container">$_{2}$</span>Si<span class="math-container">$_{2}$</span>O<span class="math-container">$_{8}$</span>), pyroxene (typically XYSi<span class="math-container">$_{2}$</span>O<span class="math-container">$_{6}$</span>, where X and Y are metals such as calcium, sodium, iron, magnesium, and aluminum), and olivine (made up of Mg<span class="math-container">$_{2}$</span>SiO<span class="math-container">$_{4}$</span> and Fe<span class="math-container">$_{2}$</span>SiO<span class="math-container">$_{4}$</span>), along with oxide minerals like ilmenite (FeTiO<span class="math-container">$_{3}$</span>). (<a href="https://www.lpi.usra.edu/publications/books/lunar_sourcebook/pdf/Chapter05.pdf" rel="nofollow noreferrer">Source</a>)</p>
<p>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.</p>

CC BY-SA 4.0
2021-02-13T11:23:11.923Z
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
CC BY-SA 4.0
2021-02-14T14:52:35.373Z
@Gimelist "In mineralogy, silica (silicon dioxide) SiO2 is usually considered a silicate mineral." That's what it says [on Wikipedia](http://en.wikipedia.org/wiki/Silicate_mineral) but if you know better do feel free to correct it.
Brian Z
CC BY-SA 4.0
2021-02-14T21:45:48.147Z
@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
CC BY-SA 4.0
2021-02-14T22:02:41.270Z
@Gimelist You stated "silica is not a mineral" but otherwise, point taken.
Brian Z
CC BY-SA 4.0
2021-02-15T12:19:05.253Z
@Gemilist I've updated my answer to more properly distinguish between components and minerals; hopefully it's an improvement.
Peter Erwin
CC BY-SA 4.0
2021-02-11T11:57:31.520Z
2021-02-15T21:14:45.370Z
2021-02-15T21:14:45.370Z
Peter Erwin
<p>I found <a href="https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2019/01/moon_facts_age_and_composition/19197687-7-eng-GB/Moon_facts_age_and_composition.png" rel="noreferrer">this infographic</a> 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?</p>

CC BY-SA 4.0
2021-02-11T11:05:43.587Z
I would guess that ESA is trustworthy, but maybe someone can answer in more detail.
jng224
CC BY-SA 4.0
2021-02-11T11:25:41.763Z
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
Peter Erwin
CC BY-SA 4.0
2021-02-12T07:49:06.757Z
@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.
ProfRob
CC BY-SA 4.0
2021-02-12T13:05:01.580Z
Oxygen is the highest occuring element in rocks. The moons surface is covered in tiny rocks.
Stian
CC BY-SA 4.0
2021-02-13T11:24:23.563Z
@StianYttervik it's not that the moon is covered with rocks. The moon _is_ rocks.
Gimelist
<the-moon><surface><elemental-abundances>
Is oxygen really the most abundant element on the surface of the Moon?
usernumber
# Is oxygen really the most abundant element on the surface of the Moon?

(Asked by: username_0 on 2021-02-11)

I found [this infographic](https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2019/01/moon_facts_age_and_composition/19197687-7-eng-GB/Moon_facts_age_and_composition.png) 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?

**Tags:** the-moon, surface, elemental-abundances

> **Comments:**
> **username_1**: I would guess that ESA is trustworthy, but maybe someone can answer in more detail.  
> **username_2**: 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  
> **username_3**: @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.  
> **username_4**: Oxygen is the highest occuring element in rocks. The moons surface is covered in tiny rocks.  
> **username_5**: @username_4Yttervik it's not that the moon is covered with rocks. The moon _is_ rocks.  

---

## 2 answers

### Answer by username_2 on 2021-02-11. Score: 60 | Accepted answer

Yes, [that's correct](https://en.wikipedia.org/wiki/Geology_of_the_Moon); it's also true for [the Earth's crust](https://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth%27s_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](https://en.wikipedia.org/wiki/Silicon_dioxide) ($\mathrm{SiO}\_{2}$); [alumina](https://en.wikipedia.org/wiki/Aluminium_oxide) ($\mathrm{Al}\_{2}\mathrm{O}\_{3}$); [lime](https://en.wikipedia.org/wiki/Calcium_oxide) ($\mathrm{CaO}$); [iron oxide](https://en.wikipedia.org/wiki/Iron(II)_oxide) ($\mathrm{FeO}$); and [magnesium oxide](https://en.wikipedia.org/wiki/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](https://www.lpi.usra.edu/publications/books/lunar_sourcebook/pdf/Chapter05.pdf))

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.

> **Comments:**
> **username_5**: 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.  
> **username_10**: @username_5 "In mineralogy, silica (silicon dioxide) SiO2 is usually considered a silicate mineral." That's what it says [on Wikipedia](http://en.wikipedia.org/wiki/Silicate_mineral) but if you know better do feel free to correct it.  
> **username_5**: @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.  
> **username_10**: @username_5 You stated "silica is not a mineral" but otherwise, point taken.  
> **username_2**: @Gemilist I've updated my answer to more properly distinguish between components and minerals; hopefully it's an improvement.  

---

### Answer by username_3 on 2021-02-11. Score: 23

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.

> **Comments:**
> **username_6**: 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).  
> **username_7**: Wikipedia's [article on $^{16}O$](https://en.wikipedia.org/wiki/Oxygen-16) 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 $^{16}O$."  
> **username_8**: 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.  
> **username_9**: @username_8 correct: https://en.wikipedia.org/wiki/Great_Oxidation_Event  
> **username_6**: Thanks for that info, @Ross. That article doesn't have a relevant citation, but I found a good one in https://en.wikipedia.org/wiki/Isotopes_of_oxygen#cite_note-Meyer2005-14  
> **username_6**: @username_8 Mostly, but a significant percentage of Earth's helium is primordial. See https://physics.stackexchange.com/q/548056/123208 & https://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.  

---

