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Earliest Carbon - What Form

  1. Jan 27, 2014 #1
    Hi, I hope this question is appropriate for this forum - if not can anyone suggest a place to ask it?

    There is a lot of carbon based matter around, on an in the earth. But,as I understand it, other than gaseous forms all the other liquid and solid forms have been generated/created by living organisms. This includes oil, coal, graphite, chalk, limestone as well as biomass.
    Q. As earth formed in the early days of the solar system - where did all the carbon come from and in what form did it exist? If it was gaseous then what gas? and does that mean that earth formed with a ready made atmosphere?

    Just wondering
  2. jcsd
  3. Jan 27, 2014 #2


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    Plenty of carbon containing minerals - like carbonates - that can be produced in purely inorganic reactions.
  4. Jan 27, 2014 #3


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    The element Carbon is fused in stars from lighter elements. All the carbon on Earth was once a part of some star that exploded and seeded its neighbourhood with most of the (reasonably stable) elements in the periodic table.
    The atomic carbon in space can exist in a variet of molecular forms, all of which could be reasonably expected to end up on the primordial Earth:

    http://news.harvard.edu/gazette/1996/07.11/NewCarbonCompou.html [Broken]
    (although I don't know why they say ammonia contains carbon...)

    The outer gas giants and some of their satellites(most notably Titan) have atmospheres which include a few percent of methane:

    After coalescing into a protoplanet, the geology(high temperature and pressure conditions) would take care of creating the remainder of carbon chemistry we have on Earth.
    Last edited by a moderator: May 6, 2017
  5. Jan 27, 2014 #4
    Thanks folks - something for me to chew on.
  6. Feb 2, 2014 #5


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    Main sequence stars like the sun have some carbon (and oxygen and nitrogen), which enables the CNO-cycle to contribute to some of the fusion in the star.

    Carbon can be formed in the Triple Alpha Process in red giant and supergiant stars. If the central temperature of a star exceeds 100 million Kelvins, as may happen in the later phase of red giants and red supergiants, then helium can fuse to form beryllium and then carbon.


    Red giants and supergiants can seed a galaxy with carbon. There are other fusion reactions that can form light elements.

    Some fusion reactions for light elements. There are a number of other reactions the preclude these reactions, or rather compete with a different outcome, usually to lighter elements like He.

    d + 4He -> 6Li + 1.7 MeV

    d + 6Li -> 7Be + n + 3.4 MeV
    -> 7Li + p + 5.0 MeV​

    t + 6Li -> 7Li + d + 0.9 MeV
    -> 7Li + p + n - 1.2 MeV​

    3He + 4He -> 7Be + 1.5 MeV

    3He + 6Li -> 7Be + d + 0.1 MeV

    3He + 7Be -> 10C + 15.1 MeV
    10C + n -> 11C (neutron capture)

    4He + 7Li -> 11B + 8.5 MeV

    4He + 7Be -> 11C + 7.5 MeV
    11C + n -> 12C (neutron capture)

    4He + 11B -> 14C + p + 0.8 MeV
    -> 14N + n + 0.2 MeV​
    Last edited: Feb 2, 2014
  7. Feb 4, 2014 #6


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    The CNO cycle uses C in two of it's steps. http://en.wikipedia.org/wiki/CNO_cycle
    But the CNO cycle is circular so on average it makes as much as it consumes.

    Leakage of C from the CNO cycle can be a source of external C.
    Any leakage from the cycle can be made up by alternative synthesis paths that make either C, N or O.

    So 12C and 13C are both available from a stars normal activity.
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