# Why carbon is considered volatile?

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

• True, elemental carbon is refractory, but common carbon compounds like $\rm CO_2$ and $\rm CH_4$ are volatile. – PM 2Ring Nov 6 '20 at 13:55
• 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 6 '20 at 15:46
• 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 6 '20 at 16:23
• @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 6 '20 at 16:59
• That's why it is strange why on the moon it is not the same... – Rahlis Alexander Nov 6 '20 at 18:01

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.

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.

• CO₂ is not organic. Why don’t just say “compounds of carbon”? – Incnis Mrsi Dec 22 '20 at 18:01