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D/H ratios of comets

  1. Jan 6, 2015 #1
    Comets have high Deuterium to Hydrogen ratios. They are enriched with extra neutrons.

    Comets periodically plunge close to the sun. And, the sun generates flares, which generate neutrons.

    Separately, I wonder if those neutrons come from fusion of solar corona gas, trapped on magnetic field lines, during the intense heat and energy of flares. If so, then maybe flares are vaguely like a Tokomak fusion reactor, with hot gas trapped on field lines, and compressed to fusion densities during the magnetic reconnection events that trigger flares ?

    But, my main question is, could solar flare neutrons blast comets, on close approach, and so gradually but inexorably convert H to D ? Could comets be neutron enriched by neutron bombardment, vaguely like the linings of nuclear reactors ?

    The earth and moon, being farther away from the sun, receive fewer neutrons from the sun. And Jupiter and Saturn fewer still. Indeed, the earth and moon are deficient in D compared to comets, and the gas giant planets even more so.

    Thanks in advance

  2. jcsd
  3. Jan 6, 2015 #2

    Doug Huffman

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    Are you sure?
  4. Jan 6, 2015 #3
  5. Jan 6, 2015 #4
  6. Jan 6, 2015 #5
  7. Jan 7, 2015 #6


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    The text indicates that the neutrons dont make it as far as Earth
  8. Jan 7, 2015 #7
    Yes, that is my point.

    Neutrons do make it as far as Mercury. And, comets come that close to the Sun. Whereas, the Earth, Moon, and Jupiter do not.

    So comets could be enriched in neutrons, more than the Earth, Moon, and Jupiter .
  9. Jan 7, 2015 #8


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    I don't see a reference to 100 MeV with respect to neutrons on that NASA page.

    There is a statement: "This indicated that the neutrons were most likely produced by accelerated flare particles striking the lower solar atmosphere, releasing neutrons as a result of high-energy collisions." There is not statement about what reactions, but it seems to infer some spallation reaction, e.g., (p,n). Otherwise it could be photo-dissociation of deuterons. It would be useful to have a ratio of solar neutrons to solar protons, as well as there energy spectra.

    The most energetic fusion reaction d + 6Li -> 2 α produces 22.4 MeV. So 100 MeV particles must come from electromagnetic acceleration.

    The article "Solar neutron events in association with large solar flares in November 2003" by K. Watanabe at al is interesting.
    The neutrons were observed at ground stations, although those observations were made up in the mountains. It would be good to know if the neutrons were produced in the solar atmosphere or by solar protons interacting with the earth's atmosphere.
  10. Jan 7, 2015 #9
    That is very interesting

    Perhaps solar flares generate NEUTRINOS ? If so, then that could imply nuclear processes.
  11. Jan 8, 2015 #10
    I intuit that charged plasma particles, entrained on field lines, get flung as if with a slingshot, when field lines rapidly realign after a reconnection event

    Please permit me to try a calculation...

    Even hot corona particles have energies <1Kev... So 100Mev particles get almost all of their energy from the flare


    E ~= W = FD

    F ~= evB

    (100e6 V)e ~= evBD

    D ~= 1e3-4 km for a flare field line loop (e6.5)
    B ~= 0.1-0.4 T in an active region (e0.5)

    1e8 ~= v (1e7)
    v ~=10

    Alternatively accelerating a particle to 1e8 eV over 1e7 m requires an effective voltage drop of 10V/m

    I think that there is an order of magnitude calculation in the numbers somewhere

    Free food for thought
  12. Jan 9, 2015 #11
    I want to try one more order of magnitude calculation

    Maxwell states

    Curl E = dB/dt

    E/L ~ B/t

    L = length scale ~ 1e7.5 m
    t = time scale ~ 1e2.5 s
    B ~ 1e0.5 T

    E ~ 1e5.5 V/m

    100 MV across 1e4.5 m ~ 30km


    Changing B => helical E...
    When you watch videos of flares you seem to see plasma circulating around field lines
    Perhaps the magnetic reconnection drives plasma around in circular loops up to 100 MeV

    At those energies the accelerated particles smash into other corona particles, and break apart helium nuclei, releasing sprays of debris including neutrons??

    Someone else told me helium nuclei are apparently ripped apart in flares
  13. Jan 9, 2015 #12


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    And undergo ablation of water ice. The ablation process is subject to both kinetic and thermodynamic isotope effects which tend toward D enrichment of the residual ice. Is it necessary to seek more exotic explanations?
  14. Jan 9, 2015 #13
    What I read seemingly implied that comets' high D/H ratios were unexplained

    If comets have nearly no escape velocity, what would hold back D, and keep it on the comet, relative to H ?

    Can you please explain your comments ?
  15. Jan 10, 2015 #14


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    Heavier molecule, lower vapor pressure, lower evaporation rate.
  16. Jan 10, 2015 #15
    That certainly sounds very reasonable

    Yet, how can you have any kind of PRESSURE... in the vacuum of SPACE ?

    Could you name one or some of the relevant equations? The Maxwellian distribution?
  17. Jan 10, 2015 #16


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    Something magical about "the vacuum of SPACE?" Ice at ~4 K is certainly going to have a non-zero vapor pressure --- small, but non-zero.
  18. Jan 10, 2015 #17
    Pressure implies thermodynamic equilibrium, yes?

    In space, if a water molecule broke free into the vacuum, it would simply freely travel away ... It would not bounce back to exert pressure on the surface of the comet, no?
  19. Jan 10, 2015 #18


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    Hence, "ablation" of cometary material.
  20. Jan 10, 2015 #19
    If all comets are ablating material...

    Then wouldn't that material reside in the planetary plane...

    And so often be reabsorbed onto comets?

    Is the material swept out by the solar wind?
  21. Jan 10, 2015 #20


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    That's the question I keep asking in hopes someone has tripped over any papers discussing the existence/absence of such effects.
  22. Jan 10, 2015 #21


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    There are two such processes I have seen mentioned. Photoevaporation by UV radiation being the most widely discussed in recent literature, and massive solar wind outflow associated with T-Tauri phase (when the protostar ignites).
    There's quite a few papers on arxiv discussing the former. For example:

    Photoevaporation of Circumstellar Disks Revisited: The Dust-Free Case
    Kei E. I. Tanaka, Taishi Nakamoto, Kazuyuki Omukai


    Jaehan Bae, Lee Hartmann, Zhaohuan Zhu, Charles Gammie

    My cursory search turned just this one paper mentioning the latter without much detail:
    The sources cited look somewhat dated, though. This idea might have fallen out of fashion. I wasn't able to find non-paywalled copies of those articles to peruse.
  23. Jan 14, 2015 #22


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    I tripped over this.
  24. Jan 14, 2015 #23


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  25. Jan 14, 2015 #24


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  26. Jan 27, 2015 #25


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    Have you seen this being discussed as an explanation for the isotope variance of comets vs water on earth ? If so I would be interested in reading more on this.

    I suppose if this was the cause, and if we had a reasonable basis for what the original water isotopes were then wouldn't it be potentially possible to infer the approximate % vaporization of the original volume. Probably not as the variance wouldn't be uniform throughout the comet. I haven't thought on that for longer than this reply.
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