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PetTastic
2015-Jun-04, 11:34 AM
Where does the carbon dioxide in comets come from?
Carbon dioxide looks very spars in the ISM because it reacts with hydrogen.

Did comets start out as ice rich organics that later got oxidised to CO2 wihen the water ice lost hydrogen or was is somehow produced in the proto-planetary disk?

tusenfem
2015-Jun-04, 03:31 PM
i think it is usually assumed that these gases condensed in the protoplanetary nebula outside the snow boundary.
this paper by michael a'hearn might help (https://www.google.at/url?sa=t&source=web&rct=j&ei=IW5wVfrFNMW0UdyRgKAH&url=http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120008332.pdf&ved=0CCwQFjAF&usg=AFQjCNElCnRukLpLaTwwtbJr4C3tq1FReQ&sig2=F5QI2z1VDOTYT6QrJq_m7Q)

Amber Robot
2015-Jun-04, 05:21 PM
I think, and this may be what the A'Hearn paper is saying, that despite CO2 not forming in the gas phase in the ISM it can form on the surfaces of grains as an ice after CO freezes out.

EigenState
2015-Jun-04, 05:41 PM
Greetings,

Laboratory experiments show efficient formation of CO2 via CO + OH on surfaces consistent with our understanding of the composition of grains. Google Scholar gives many citations when searching "CO2 surface formation".

The UMIST Database for Astrochemistry (http://udfa.ajmarkwick.net/index.php) does not list gas phase reactions between CO2 and hydrogen atoms. The gas phase rate coefficients for destruction of CO2 via reactions with H+, H2+, and H3+ are all of the order of 10-9. Such ion-molecule reactions are substantially more efficient than neutral-neutral reactions because the ion and neutral are attracted by charge-dipole (induced dipole) interactions.

Best regards,
ES

publiusr
2015-Jun-06, 07:16 PM
And here I thought multi-instrumentalist Michael O' Hearn was finding new work...

PetTastic
2015-Jun-06, 09:11 PM
Reading through the above and a few papers about Hubble and other observations it seems very difficult to rule out many possibilities of what was going on going on when the comets formed.

How far does light from the Sun/star penetrate in the plane of the disk if it is still rich with CO and therefore dust.
Some bits I was reading on the latest ideas on planet formation suggest even where the Earth formed was a very cold dense gas env, with almost no light from the Sun or solar wind until most of the disk had been condensed out into planets or blown away.
So the frost-lines inside the disk were very must closer to the star than in space above and the below the disk that had modern values.

How well accepted are these models of the protoplanetary disk being highly ordered with most of the material orbiting in rings?
Collision speeds down by a factor of 5 compared to older models that had red-hot proto-planets crashing together at high speed from random orbits.
Reduce the energy of impacts by a factor of 25 and the Earth forms as an ice-ball with ice sploshing down onto the surface through a thick CO hydrogen atmosphere. CO could have been converted into CO2 even before it arrived on our planet.

There is far more CO coming from comets than I had expected.
I think it is highly plausible that CO ice is being oxidized to CO2 by OH ions.
However, it it seems unclear if this happened at the time the comets were formed or slowly in the billions of years since.

If the protoplanetary disk is the very cold sheltered dense gas environment then it is possible that even small icy bodies could develop a small CO CO2 atmosphere that would reduce hydrogen levels at the surface greatly accelerating the conversion process by keeping out hydrogen.

Then there is the problem that comets should be tar-balls most of the carbon in space that is not in the form or CO is in organics and dense forms of carbon.
Does the CO come from organics being oxidised by OH ions, and not from CO that condensed in the disk?

EigenState
2015-Jun-06, 10:46 PM
Greetings,


I think it is highly plausible that CO ice is being oxidized to CO2 by OH ions.

The surface chemistry is a neutral-neutral reaction: OH is a radical, not an ion. The interaction with the grain surface is most likely physisorption rather than chemisorption. As stated earlier, this phenomenon has been demonstrated within the laboratory.

The gas phase reaction CO + OH \to C{O}_{ 2} + H is not efficient: the rate coefficient is of the order of 10-13.

Best regards,
ES

PetTastic
2015-Jun-07, 08:42 AM
Greetings,



The surface chemistry is a neutral-neutral reaction: OH is a radical, not an ion. The interaction with the grain surface is most likely physisorption rather than chemisorption. As stated earlier, this phenomenon has been demonstrated within the laboratory.

The gas phase reaction CO + OH \to C{O}_{ 2} + H is not efficient: the rate coefficient is of the order of 10-13.

Best regards,
ES

I did mean ions. At temperatures they are assuming in the disk with sun/star light not penetrating OH is relatively stable molecule not a radical as we would consider it in room temperature chemistry. The activation energy for it to do anything is not there.

I was reading stuff modelling high energy (x-ray/gamma-ray) photochemistry in very low temp environments. This is slightly different from collision based reactions in the ISM.
The chemical abundances deep inside ice or cryogenic micro near-liquid atmospheres look very different to astrochemistry in the ISM.
OH- (hydroxide) is far more common and hydrogen is often heavily depleted compared to the UMIST database.


This type of chemistry is more about what rebuilds first when molecules get obliterated by a gamma-ray, in dark cryogenic but highly radioactive environments.

EigenState
2015-Jun-07, 04:45 PM
Greetings,


I did mean ions. At temperatures they are assuming in the disk with sun/star light not penetrating OH is relatively stable molecule not a radical as we would consider it in room temperature chemistry. The activation energy for it to do anything is not there.

Who is "they"? OH is a molecule; that molecule is a radical because it has total electron spin angular momentum of 1/2.


I was reading stuff modelling high energy (x-ray/gamma-ray) photochemistry in very low temp environments. This is slightly different from collision based reactions in the ISM.
The chemical abundances deep inside ice or cryogenic micro near-liquid atmospheres look very different to astrochemistry in the ISM.
OH- (hydroxide) is far more common and hydrogen is often heavily depleted compared to the UMIST database.

References might be nice. And of course it is different chemistry. Yet if you are discussing cometary formation and properties within the solar system, one might ask what are the sources of the proposed high energy photons in sufficient densities to drive the proposed chemistry?

What is the existing evidence characterizing the chemical abundances "deep inside ice or cryogenic micro near-liquid atmospheres"?


This type of chemistry is more about what rebuilds first when molecules get obliterated by a gamma-ray, in dark cryogenic but highly radioactive environments.

Again, what is the source of the required density of high energy photons?

Both the gas phase chemistry and the surface chemistry are well studied. I do not believe that can be said for your conjecture.

Best regards,
ES

PetTastic
2015-Jun-08, 04:25 PM
Again, what is the source of the required density of high energy photons?

Both the gas phase chemistry and the surface chemistry are well studied. I do not believe that can be said for your conjecture.

Four and a half billion years ago almost everything in the solar system was highly radioactive fresh from the supernova or star.
The material the comets was made from was the source of the radiation.

Then the comet's ice has been exposed to 4.6 billion years of secondary radiation from cosmic ray hits.
Then there is high positrons annihilating on the surface etc, etc.

Since the 1970s there has been loads of stuff around on the subject. The voyager probes stirred up loads of interest when Europa's ice was discovered inside Jupiter's radiation belts.

The problem is with surface chemistry and gas chemistry models is chemistry is very slow below 68 Kelvin required to freeze out CO.
We know very little CO2 has been detect in the gas of protoplanetary disk.
How fast do comets form? How long is surface CO exposed? How thick are surface conditions, a few nanometres?
We know there is CO2 in comets now.

The conversion of CO to CO2 on the surface or inside ice is also interesting when applied to interstellar ice, because of the difference in partial pressure they exert. Making it more likely for larger objects to grow and less likely to evaporate over billions of years.

EigenState
2015-Jun-08, 05:35 PM
Greetings,

You posed a question within that component of the forum dedicated to mainstream answers. You received mainstream answers. Now you opt to argue with those mainstream answers with pure speculations unsupported by provided references--just assertions.

Why is this not ATM material? Consider that question to be pure rhetorical.


Four and a half billion years ago almost everything in the solar system was highly radioactive fresh from the supernova or star.
The material the comets was made from was the source of the radiation.

Then the comet's ice has been exposed to 4.6 billion years of secondary radiation from cosmic ray hits.
Then there is high positrons annihilating on the surface etc, etc.

Since the 1970s there has been loads of stuff around on the subject. The voyager probes stirred up loads of interest when Europa's ice was discovered inside Jupiter's radiation belts.

The problem is with surface chemistry and gas chemistry models is chemistry is very slow below 68 Kelvin required to freeze out CO.
We know very little CO2 has been detect in the gas of protoplanetary disk.
How fast do comets form? How long is surface CO exposed? How thick are surface conditions, a few nanometres?
We know there is CO2 in comets now.

The conversion of CO to CO2 on the surface or inside ice is also interesting when applied to interstellar ice, because of the difference in partial pressure they exert. Making it more likely for larger objects to grow and less likely to evaporate over billions of years.

Best regards,
ES

PetTastic
2015-Jun-10, 09:30 AM
Greetings,

You posed a question within that component of the forum dedicated to mainstream answers. You received mainstream answers. Now you opt to argue with those mainstream answers with pure speculations unsupported by provided references--just assertions.

Why is this not ATM material? Consider that question to be pure rhetorical.



Best regards,
ES

What part was ATM?

Solar system highly radioactive 4.6 billion years ago?
Billions of years of exposure to space radiation?
Surface chemistry very slow at low temp?


If you consider dust surface chemistry as exposure to the ISM then your UMIST database shows shows that CO2 is destroyed far faster than it is created.

Observational evidence shows CO2 exists in dust but very little exists in the grain mantles.
However, CO2 is produced in the lab by exposing interstellar ice analogues to radiation with hydrogen produced at by-product.
This is an old but commonly reference paper.
http://adsabs.harvard.edu/abs/1991MNRAS.252...63W

EigenState
2015-Jun-10, 01:01 PM
Greetings,



This is an old but commonly reference paper.
http://adsabs.harvard.edu/abs/1991MNRAS.252...63W

Commonly referenced paper?! 15 citations within 23 years (http://adsabs.harvard.edu/cgi-bin/nph-ref_history?refs=CITATIONS&bibcode=1991MNRAS.252...63W) is commonly referenced?

Experimental study of CO2 formation by surface reactions of non-energetic OH radicals with CO molecules (http://iopscience.iop.org/2041-8205/712/2/L174): Yasuhiro Oba et al. 2010 ApJ 712 L174.

CO2 formation in quiescent clouds: an experimental study of the CO+ OH pathway (http://iopscience.iop.org/0004-637X/735/2/121): J. A. Noble et al. 2011 ApJ 735 121.

More to the point: You launched this thread by posing a question yet you already had what you consider the answer. That is intellectually disingenuous.

Best regards,
ES

Swift
2015-Jun-10, 03:25 PM
Greetings,

You posed a question within that component of the forum dedicated to mainstream answers. You received mainstream answers. Now you opt to argue with those mainstream answers with pure speculations unsupported by provided references--just assertions.

Why is this not ATM material? Consider that question to be pure rhetorical.

EigenState, please don't ask such questions. If you think material is inappropriate for Q&A, then Report it, don't try to moderate it yourself.

In any case, PetTastic, you do not seem to be asking a simple question (at least past your first post), but seem to be looking for an extended discussion.

This thread is moved from Q&A to Astronomy. If it gets too non-mainstream it may be moved to ATM.

Hornblower
2015-Jun-11, 03:11 AM
Four and a half billion years ago almost everything in the solar system was highly radioactive fresh from the supernova or star.
The material the comets was made from was the source of the radiation.

Then the comet's ice has been exposed to 4.6 billion years of secondary radiation from cosmic ray hits.
Then there is high positrons annihilating on the surface etc, etc.

Since the 1970s there has been loads of stuff around on the subject. The voyager probes stirred up loads of interest when Europa's ice was discovered inside Jupiter's radiation belts.

The problem is with surface chemistry and gas chemistry models is chemistry is very slow below 68 Kelvin required to freeze out CO.
We know very little CO2 has been detect in the gas of protoplanetary disk.
How fast do comets form? How long is surface CO exposed? How thick are surface conditions, a few nanometres?
We know there is CO2 in comets now.

The conversion of CO to CO2 on the surface or inside ice is also interesting when applied to interstellar ice, because of the difference in partial pressure they exert. Making it more likely for larger objects to grow and less likely to evaporate over billions of years.

My bold. What do you mean by the supernova? As I think I understand it, the primordial nebula that forms Population I stars typically is laced with ejecta from multiple supernovae and evolving red giants, and could have been that way for a long time before whatever event it was that induced collapse and star formation. Except for uranium, thorium and potassium-40, the radioisotopes ejected from dying stars have very short half-lives.