Comets' High Deuterium-to-Hydrogen Ratios Explained

In summary, the conversation discusses the possibility of comets being enriched in deuterium (D) compared to hydrogen (H) and the potential reasons for this. Some theories suggest that solar flares could generate neutrons, which could then bombard and convert H to D on the surface of comets during close approaches to the sun. However, it is also suggested that the high D/H ratios in comets could simply be due to the ablation process of water ice on the comet's surface, which is subject to kinetic and thermodynamic isotope effects that favor D enrichment. It is debated whether more exotic explanations, such as solar flares, are necessary to explain the high D/H ratios in comets.
  • #1
TEFLing
237
22
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

:)
 
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  • #2
TEFLing said:
[ ... ] And, the sun generates flares, which generate neutrons. [ ... ]
Are you sure?
 
  • #3
http://solarsystem.nasa.gov/news/display.cfm?News_ID=33075
 
  • #6
TEFLing said:
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.

The text indicates that the neutrons don't make it as far as Earth
 
  • #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 .
 
  • #8
TEFLing said:
http://www.nasa.gov/content/goddard...-window-into-high-energy-processes-on-the-sun

Neutrons are generated during solar flares, with energies of order 100 MeV, and associated with gamma rays and high energy collisions. All of that sounds to me like fusion.
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 Sun was intensely active from late October to the beginning of November 2003. A series of 11 X class solar flares occurred in NOAA regions 484, 486 and 488. Unique among this series of flares were those occurring on November 2 and 4 in which solar neutrons were observed by the ground based neutron monitors located at Mt. Chacaltaya, Bolivia and Haleakala, Hawaii, respectively. In these flares, intense emission of hard X-rays and γ-rays have been observed by the satellites. It seems that production of solar neutrons coincided with the production of the hard electromagnetic radiations of these two flares.
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.
 
  • #9
That is very interesting

Perhaps solar flares generate NEUTRINOS ? If so, then that could imply nuclear processes.
 
  • #10
Astronuc said:
...

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

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
 
  • #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
 
  • #12
TEFLing said:
Comets have high Deuterium to Hydrogen ratios.
TEFLing said:
Comets periodically plunge close to the sun.
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?
 
  • #13
Bystander said:
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?

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 ?
 
  • #14
TEFLing said:
If comets have nearly no escape velocity, what would hold back D, and keep it on the comet, relative to H ?
Heavier molecule, lower vapor pressure, lower evaporation rate.
 
  • #15
Bystander said:
Heavier molecule, lower vapor pressure, lower evaporation rate.
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?
 
  • #16
TEFLing said:
Yet, how can you have any kind of PRESSURE... in the vacuum of SPACE ?
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.
 
  • #17
Bystander said:
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.

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?
 
  • #18
TEFLing said:
Pressure implies thermodynamic equilibrium, yes?
No.
TEFLing said:
It would not bounce back to exert pressure on the surface of the comet, no?
Hence, "ablation" of cometary material.
 
  • #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?
 
  • #20
TEFLing said:
Is the material swept out by the solar wind?
That's the question I keep asking in hopes someone has tripped over any papers discussing the existence/absence of such effects.
 
  • #21
Bystander said:
That's the question I keep asking in hopes someone has tripped over any papers discussing the existence/absence of such effects.
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
http://arxiv.org/pdf/1306.6623.pdf

LONG-TERM EVOLUTION OF PHOTOEVAPORATING
PROTOPLANETARY DISKS

Jaehan Bae, Lee Hartmann, Zhaohuan Zhu, Charles Gammie
http://arxiv.org/pdf/1307.2585.pdf

My cursory search turned just this one paper mentioning the latter without much detail:
http://arxiv.org/ftp/astro-ph/papers/0602/0602232.pdf
Solar primordial gases and volatile elements were separated from the terrestrial planets soon after planet formation, presumably early during some solar super-luminous event, such as the T-Tauri phase mass-ejections, presumably associated with the thermonuclear ignition of the Sun (Herbig, 1962; Joy, 1945; Lada, 1985; Lehmann et al., 1995).
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.
 
  • #22
Bystander said:
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?
Bystander said:
That's the question I keep asking in hopes someone has tripped over any papers discussing the existence/absence of such effects.

I tripped over this.
http://pubs.acs.org/doi/abs/10.1021/jp312816k
 
  • #25
Bystander said:
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?

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.
 
  • #26
bitznbitez said:
Have you seen this being discussed as an explanation for the isotope variance of comets vs water on Earth ?
Not as an explanation of variation of D/H ratios for bodies in the solar system, but in reference to evaporation as studied in the laboratory. There's no reason to expect behavior of water to be a function of its location in the solar system. Hence the inquiry regarding papers people may have tripped over that deal with expected or observed isotope ratios.
 
  • #27
@Bystander can I ask for a clarification? I feel like I completely missed the point at least once before.
Are you looking for a basic process of D/H variation in water ice, or are you looking for one that may account for some divergence from the predictions of the basic process?
 
  • #28
Bandersnatch said:
I feel like I completely missed the point at least once before.
I'm not certain that I have a point. There are a couple isotope threads sputtering along right now, this one, one on D/H and 15N/14N ratios, https://www.physicsforums.com/threa...-between-comet-67p-and-Earth's-oceans.787158/ , and maybe another one rattling around somewhere. The geochemist in me is curious about the possibility of isotope evidence for a differentiation gradient of heavier to lighter materials at larger radii of the original accretion disc by radiation pressure, solar wind, or other mechanisms. There may be none, there may have been and it's been washed out by the kinetics of evaporative ablations of volatiles, or there may be something yet to be found in distributions of heavier nuclei. Or the original disc may have been "extra chunky" and nothing so subtle had a chance to occur.
 

1. What is the significance of high deuterium-to-hydrogen ratios in comets?

High deuterium-to-hydrogen ratios in comets provide important insights into the origins and evolution of our solar system. Deuterium is a heavier form of hydrogen that is believed to have been present in the early stages of the universe. By studying the ratio of deuterium to hydrogen in comets, scientists can gain a better understanding of the conditions in the early solar system and the processes that formed our planets.

2. How do scientists explain the high deuterium-to-hydrogen ratios in comets?

Scientists have proposed several explanations for the high deuterium-to-hydrogen ratios observed in comets. One theory suggests that comets formed in the outer regions of the solar system, where deuterium was more abundant, and were then later brought into the inner solar system by gravitational interactions. Another theory suggests that the high ratios are a result of comets being formed from material that has undergone chemical processing in the cold, outer regions of the solar system.

3. What do the high deuterium-to-hydrogen ratios in comets tell us about the formation of our solar system?

The high deuterium-to-hydrogen ratios in comets provide evidence that the outer regions of the solar system are more primitive and have undergone less chemical processing than the inner regions. This supports the theory that the planets in our solar system formed from the accumulation of material in the inner regions, while comets and other objects in the outer regions remained relatively unchanged.

4. How do scientists measure the deuterium-to-hydrogen ratio in comets?

Scientists can measure the deuterium-to-hydrogen ratio in comets by analyzing the chemical composition of their icy nuclei. This is typically done by studying the spectra of light emitted by the comet's coma, which is a cloud of gas and dust surrounding the nucleus. By comparing the ratios of different elements, including deuterium and hydrogen, scientists can determine the overall deuterium-to-hydrogen ratio in the comet.

5. What other insights can be gained from studying the high deuterium-to-hydrogen ratios in comets?

Studying the high deuterium-to-hydrogen ratios in comets can also provide insights into the formation and evolution of other planetary systems. By comparing the ratios in comets from different solar systems, scientists can learn more about the diversity of conditions and processes that lead to the formation of planets. Additionally, the study of deuterium can also help scientists understand the role of water in the origins of life on Earth and other planets.

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