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Io is the only body in the Solar System whose crust is dominated by sulfur and its compounds. Carbon and nitrogen are also very common in the universe but we don't see them or their compounds in Io's crust. Why not?

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Io is not dominated by sulfur. Io is mostly silicate rock and iron. Sulfur is a thin coating on the surface and sulfur dioxide makes up most of the atmosphere, but that's because sulfur dioxide is a volatile. In other words, it has a much lower melting and boiling point than silicate rock. So while the rock remains solid, the sulfur dioxide is gas.

Carbon is not common on Io's crust for the same reason that it is not common on Earth's crust. Carbon may be common in the universe, but it binds to oxygen to make a gas. When the solar system was forming most of the carbon was in the form of CO or CO2 and would not easily have coalesced into a solid body. Silicon is much less common than carbon, but Earth and Io are made of silicon because silicon makes solid stuff at the relevant temperatures.

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  • $\begingroup$ I would add that Carbon also readily bonds with hydrogen (CH4), so it's not just Oxygen, but the point is still correct. I'd also add something about it's Tidal-volcanism which is the mechanism that coats the planet with Sulfur and other lighter elements. en.wikipedia.org/wiki/Io_(moon)#Volcanism $\endgroup$ – userLTK Aug 5 '17 at 9:50
  • $\begingroup$ OK, thanks for the answer. But then why isn't all the sulfur locked up with oxygen? Shouldn't there be plenty of oxygen around for that? $\endgroup$ – MackTuesday Aug 5 '17 at 13:28
  • $\begingroup$ userLTK: For the purpose of my post, it IS basically just oxygen. Methane exists, but it is not an important molecule in a protoplanetary disk. Most of the carbon is in CO because carbon reacts more readily with oxygen than hydrogen and because every disk seems to have more O than C. So, to a first a approximation, every carbon atom finds an oxygen atom to bind with, and the remaining oxygen goes on to make other molecules. If I were to expand the list, I would mention molecules like HCN, DCN, or organics like HCOOH, CH3CHO, etc before I get to methane. arxiv.org/pdf/1310.3151.pdf $\endgroup$ – DanielC Aug 11 '17 at 3:08

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