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A (relatively) recent 'Sky & Telescope' magazine feature story mentioned how little carbon the Earth has, and how some scientists' models predict we should somehow have even less....

So I looked it up, and sure enough, our planet is almost half oxygen (which seems weird enough), and only about 1 in 625 atoms is carbon....

Is there a known reason(s) for this?

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  • $\begingroup$ Wikipedia's articles on elemental abundance doesn't really suggest almost half to me, though 30% is indeed a significant value (should be less by number of atoms given molecular mass?). And also seems to suggest some explanation. $\endgroup$ Jun 10, 2023 at 18:43
  • $\begingroup$ Also known as the 'missing carbon problem'. There are as many ideas where this might come from, as chondrules in a chondrite... $\endgroup$ Jun 12, 2023 at 17:09

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As the comments suggest, there probably is no one factor, but a big one seems to be the gas and ice giants took most of the carbon away.

In our Solar System as a whole, the primordial nebula had almost as much carbon as oxygen. However, during the accretion of the planets smaller objects, even including eventual neighborhood-clearing planets such as Earth, had only enough gravitational pull to retain nonvolatile substances. These include many oxides of elements such as alkali and alkaline earth metals, aluminum and silicon (lithophiles), not so many carbon-rich materials. Carbon remained largely as volatile compounds, mostly methane, and only the biggest objects at favorable positions were able to gather up these volatiles.

So a disproportionate amount of carbon ended up in the atmospheres of the giant planets instead of the solid structures of most everything else. Jupiter's atmosphere, for instance, has more carbon in the form of (predominantly) methane than oxygen in the form of (predominantly) water.

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  • $\begingroup$ That is a very strange idea - as the C/O of Jupiter is solar and given how dry Earth is, molecular (not atomic) selection effects must have been rather local, than global in the disc. $\endgroup$ Jun 21, 2023 at 11:33
  • $\begingroup$ The data from Wikipedia on Jupiter's atmosphere seems to indicate C/O there is greater than solar. $\endgroup$ Jun 21, 2023 at 14:57
  • $\begingroup$ Sorry, I misread the table. I find it however too simplistic a scenario to say "Jupiter is enriched in C/O, Earth is depleted, so it all must sit in Jupiter." Without a physical model that can test what C/O ratio you'd get from particular accretion scenarios, this is just numerology. $\endgroup$ Jun 22, 2023 at 8:15

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