Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar
    Gold Member

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

    davenn

    User Avatar
    Science Advisor
    Gold Member

    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

    Astronuc

    User Avatar

    Staff: Mentor

    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

    So

    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

    OoM
    -------
    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

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    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

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    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

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    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

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    No.
    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

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    That's the question I keep asking in hopes someone has tripped over any papers discussing the existence/absence of such effects.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: D/H ratios of comets
  1. Sun-diving comets! (Replies: 4)

  2. Ison Comet (Replies: 27)

  3. Comet Velocity? (Replies: 1)

Loading...