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Carbon is considered volatile by planetary science, eg Moon lacks volatiles and thus lacks carbon. However volatiles are defined as "elements or substances with low boiling point", but Carbon boiling point is very high! Its sublimation point is 3900K, so it should be refractory and not volatile. I am confused why it is volatile with such high sublimation point. Regards, Alex

Rahlis Alexander
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    True, elemental carbon is refractory, but common carbon compounds like $\rm CO_2$ and $\rm CH_4$ are volatile. – PM 2Ring Nov 06 '20 at 13:55
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    Thanks, but why elementary Carbon is not present as Graphite (its most simple form in low T and Pressure) on the moon? Everywhere I read it is said that moon lacks Carbon. – Rahlis Alexander Nov 06 '20 at 15:46
  • I'm not certain (which is why I just left a brief comment, rather than writing a proper answer). I assume that much of the primordial elemental carbon reacts with hydrogen or oxygen in the protoplanetary disk. During the early stages, it can get rather warm in the disk, due to gravitational contraction (& collisions). – PM 2Ring Nov 06 '20 at 16:15
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    I thought so too, but the moon formation theory says it was a molten ball of elements in the beginning, and the volatiles escaped leaving only refractories, so I am thinking that probably H and O would escape first before the ball cools down enough to create compounds with Carbon, leaving Carbon alone to sink into the magma... – Rahlis Alexander Nov 06 '20 at 16:23
  • Before it was a molten ball, the matter was dust & gas (see https://en.wikipedia.org/wiki/Planetesimal) and I assume most of the carbon in the Solar System was already in the form of gaseous compounds well before large planetesimals formed, although there probably was a tiny percentage of graphite & diamond too. In the current era, on Earth, carbon dioxide is far more common than diamond. ;) When the Moon was molten, conditions were good for any remaining elemental carbon to react with hydrogen or oxygen. It takes extremely high temperature to split such compounds into elements. – PM 2Ring Nov 06 '20 at 16:42
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    @RahlisAlexander that magma is mostly composed of oxygen. Carbon will happily grab the oxygen away from silicon and many metals to form CO2. – Christopher James Huff Nov 06 '20 at 16:59
  • On Earth we have Carbon not only with H or O compounds, eg here : https://en.m.wikipedia.org/wiki/Geochemistry_of_carbon it says that in the mantle when oxygen fugacity is very low, carbon makes not only diamond, but also Carbides eg like Fe3C and Fe7C3, and it also creates other compounds eg with magnesium, Si, etc.. – Rahlis Alexander Nov 06 '20 at 18:00
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    That's why it is strange why on the moon it is not the same... – Rahlis Alexander Nov 06 '20 at 18:01
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    "when oxygen fugacity is very low". Why do you think this is the case on the moon? The moon is enriched with light elements, and about 40% of it is oxygen, compared to 30% for Earth. Your two example carbides are both compounds with iron...the moon is about 9% iron, compared to 36% for Earth...and note that this is by mass, there being 3.5 times as many oxygen atoms as iron atoms for the same mass. The moon has a great abundance of oxygen, why do you think it's strange that carbon would combine with it? – Christopher James Huff Nov 06 '20 at 18:20
  • "Why do you think this is the case on the moon? The moon is enriched with light elements", it contradicts what I have read in numerous papers, eg: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012GL051711 or please see "Earth's Moon's origin" chapter in https://en.wikipedia.org/wiki/K-U_ratio , and since Oxygen is considered Volatile I assumed that it is also very low on the Moon. – Rahlis Alexander Nov 06 '20 at 18:56
  • But, on the other hand i see that you are right, oxygen for some reason indeed takes up 40% of the moon, this is strange considering that moon is lowest on volatiles. – Rahlis Alexander Nov 06 '20 at 19:49
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    Elemental oxygen is volatile. Silicates and most metal oxides are very much refractory...boiling iron is barely hot enough to melt magnesium oxide. Rocky materials in general are about half oxygen by mass. – Christopher James Huff Nov 06 '20 at 19:49

2 Answers2

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Oxygen is the third most common element in the universe after hydrogen and helium. Elemental oxygen is very volatile, but it's also highly reactive and will combine with almost anything it encounters.

The resulting oxides can be volatile in the case of CO (boiling point 81.6 K) or carbon dioxide (sublimates at 194.7 K), or extremely refractory: silicon dioxide has a boiling point of 3220 K, making it less volatile than iron (boiling point 3134 K). Magnesium oxide doesn't even melt until it reaches 3125 K, just a little short of the boiling point of iron. Rocky materials are almost entirely composed of oxides and silicates, and overall are about half oxygen by mass.

Carbon will readily react at high temperatures with the oxygen in many oxides to produce CO or CO2 (this being the basis of many smelting processes on Earth). Since oxygen is so common, most of the elemental carbon that ends up accreting with rocky materials can be expected to end up getting oxidized.

The moon not only appears to have formed by a process that allowed most of its volatiles to escape, that process also enriched it with light elements. As a result, it has a small iron core, but lots of silicates and metal oxides for carbon to react with, forming volatile CO or CO2 in the process.

Christopher James Huff
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Elemental carbon would be refractory, but elemental carbon is highly reactive, particularly with oxygen. As a result, most of the carbon in a protoplanetary disc ends up in volatile molecules such as carbon monoxide, carbon dioxide, methane and other, organic, molecules.

ProfRob
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