I'm curious about the "dirty snowball" hypotheses for comets. In particular, I have two questions:

(1) What evidence do we have in support?

(2) What evidence do we have against it?

Leo

In what sense? If I'm not mistaken the term is used to distinguish two ideas about the composition of comets, namely (simplified) whether there is more ice or more rock. Or do you mean something else?

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You might want to read up on the results from the StarDust mission

It was the first sample return mission for comets.

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Well, crudely, we have observations of dust and water in comet comae and tails. How much more detail do you want?

Is "dirty snowball" only a hypotheses? I thought it was taken for granted now.

Dust, yes. Water, no. Only the OH radical, which can be formed by other processes than photolysis. In fact, there is no experimental confirmation that photolysis can be as fast as would be necessary to account for the presence of OH and nearly total absence of water in the tails and comae.

Leo

I think the Giotto spacecraft can tell a lot about comets .
http://giotto.esa.int/science-e/www/...objectid=14610

Every single mission we sent to a comet didn't find any significant amount of water. Only one found meager traces. No observation of disintegrating comets, including Shoemaker-Levy, showed any water in the remains.

In fact, there is not a single experimental evidence in favor. I'd expect that, in order to be taken for granted, at least one would have to exist. But no.

Leo

Yeah, and did it find any water or a hint of water?

Leo

Ok... I am not an expert in comets. I've just heard experts in comets always talking about water production rates of various comets that they study, so I had assumed they had good reason to expect that there is water in comets.

This link may provide some information ...
http://spaceguard.rm.iasf.cnr.it/tum...ng/giotto4.htm
Other sources which were "hot" at the time of the encounter mention a composition of the tail of 80% water .

Well, there may be some production in the comet tails, due to recombination of the oxygen ions from the comet with the protons from the solar wind. That's what probably produces OH and may, with a lower chance of happening, some H2O in small amounts. But note that it says nothing about water ice being a part of a comet body.

Leo

Is this a thinly disguised promotion of an 'Electric Comet' theory?

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It certainly seems ATM since deep impact did show lots of H2O, IIRC. But I haven't kept track of all the resulting research so I really can't say for sure. Quoting wiki seems insufficient

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It's disguised?

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I think it's "guised" now
I wonder how OH- radicals may exist in space if they were'nt part of H2O ?
Maybe they could be have been part of C2H50H , but production of this stuff is strictly regulated , at least here on Earth ....

Silly Nonsense.

1. Bonev, et al., 2008
2. Woodward, et al., 2007
3. Feaga, et al., 2007
4. Sunshine, et al., 2007
5. Bonev, et al., 2007
6. Barber, et al., 2006
7. Schulz, et al., 2006
8. dello Russo, et al., 2005
9. Bensch, et al., 2004
10. dello Russo, et al., 2004
11. dello Russo, et al., 2002
12. Hjalmarson, et al., 2002
13. Harris, et al., 2002
14. Chiu, et al., 2001
15. Mäkinen, et al., 2001a
16. Mäkinen, et al., 2001b
17. Neufeld, et al., 2000
18. dello Russo, et al., 2000
19. Bertaux, et al., 1999
20. Lellouch, et al., 1998
21. Bockelee-Morvan, et al., 1998
22. Cosmovici, et al., 1998
23. Meier, et al., 1998
24. Combi, et al., 1998
25. Davies, et al., 1997
26. Bjoraker, et al., 1996
27. Mumma, et al., 1996
28. Sprague, et al., 1996

And so on & etc.

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Which one of them shows evidence of water ice on a comet's body? Sorry, but I only had time to glance through the first few and didn't find any indication. As I mentioned before, without your prompting, there are some traces of H2O besides much mroe abundant OH in the spectra. But that doesn't mean the water comes from the comet. Nor are the amounts sufficient to claim the comets are made mainly of ice.

Anyhow, thanks for pointing out what indirect evidence does exist.

Leo

On the list above: 4, 7, 20 & 25, and in addition see Sunshine, et al., 2006 & Bregman, et al., 1988.

Yes it does. The idea that oxygen from the comet can recombine with solar wind protons does not make physical sense, because you would not get neutral H₂O, you would get O₂⁺ ions.

Besides, look at Bensch, et al., 2004 (number 9 in my list above). They derive a water production rate of 4x10²⁸ molecules per second. So you have to slam 8x10²⁸ protons into the O (in order to get the 2 on H₂O), if we assume that you can even make H₂O that way. With a typical solar wind particle density about 10 per cm³, you need 80% of the protons in a volume of about 10²⁸ cm³ to combine with the cometary oxygen. That volume is a box about 21,380 km on a side, which certainly dwarfs the nucleus of a comet and the inner coma of same. And since the solar wind moves about 800 km/sec at its fastest, you just don't have enough solar wind protons physically present where the comet is to pull this trick off.

And besides again, there is no reasonable source for all that oxygen. If the oxygen does not come from the comet as water, where does it come from? You can't sublimate minerals, you have to have ice. The oxygen won't all come out as OH, because OH won't hang around inside the comet without binding to something else. So you are just replacing one "problem" (where does the water come from?) with another, much tougher problem (where does all the oxygen come from?).

On the other hand, we do see a substantial number of H₂O⁺ ions entrained in the solar wind. And that is exactly what we expect to see when neutral water comes from the comet and picks up a solar wind proton (i.e., Coates, 2004 and Introduction to Comets, Brandt & Chapman, Cambridge University Press, 2004 (2nd ed.)).

Yes they are. Why do you think there should be more? We already know the the comet surfaces are certainly not dominated by water, they are too dark too warm and reflect radar inconsistent with water. We don't know how thick the outer layers are. So how do you know that more water should escape from the comet, even if it is made mainly of water?

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I will ask Tim's question directly. Where does the loose oxygen come from? Does the collision cross section of O allow reaction with a fast moving H+?

Come to think of it, I can answer part of that. Oxygen reacts with Hydrogen to form water to fill Oxygen's valence shell. The electrons of H atoms are needed for this. A lone proton should get booted right out if it trys to grab one of the oxygen atom's electrons.

Basically, the only process that could go from oxygen supplied by the comet needs something to kick off an oxygen atom, then it needs to grab an electron, then it needs to grab a proton, then it needs to not find anything else before it gets another electron, then it needs to encounter another proton. All this in vaccum. Not likely.

korjik, everybody knows that those oxygens are created by electric discharge machining (EDM)

no no no, you know that there are also electrons in the solar wind, and everything recombines to create H2O

Okay, I give up!
This comes back to a question that I have posed several times here on the board, mainly in the ATM part. Can any of the "water does not come from a comet" supporters please give me the model and equations for the processes taking place.

1. How is the oxygen coming from the nuleus?
2. What is the cross section and "recombination" rate for the process:
O^{plus or minus} + H⁺ (few 100 km/s) + e^- -> H₂O⁺

None of the (electric) comet guys have ever given me an answer.

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Answers are a mainstream thing.

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Thanks. I'll take a look at those and will comment on them later, as I'm short on time right now. It's 3am here.

Huh? Proton is an ion of hydrogen. What you're saying is that, if ions of oxygen recombined with ions of hydrogen, then O2+ ions would result. What happened to the H part? Where has it gone? You don't make any sense.

I'm not theorizing idly. As I said, I'm short on time right now, so instead of hunting for a gazillion of links, I'll give you this example:

P. Chaizy et. al. Negative ions in the coma of comet Halley.
http://www.nature.com/nature/journal.../349393a0.html

Note the O- and OH- ions in the paper. Note also the argument that the observed densities of negative ions imply that "in similar neutral gas and dust environments farther away from the Sun [...], negative ions should also be present." I would go as far as to say that the comet is negatively charged with respect to the solar wind, which may explain the observed densities.

A plenty of negative ions have been detected by the spacecraft every time one approached a comet, including negative ions of oxygen and OH. It is also well known that the solar wind slows down significantly near the coma, which is not surprising considering that the solar wind protons have a positive charge and therefore are attracted to the predominantly negative ions from the comet. Your "volume argument" breaks apart because the protons and the ions of oxygen do not just randomly pass each other--they are attracted to each other. Nor is there any need to pick additional electrons, because the ions of oxygen already have extra electrons to recombine with the protons.

There are enough negative ions of oxygen in the coma to produce the observed OH and H2O. Yet you feel compelled to go into the arguments about the water mysteriously leaving the comet's body from its deeper layers while leaving the obviously non-water upper layers apparently intact. While it is not impossible, it's highly unlikely. Consider:

(1) Jets have been observed originating from the dark side of the comet. In fact, most of them come from the dark side. If the comet were melting, they'd be coming mostly from the sunward side.

(2) Comets are known to flare well beyond the orbit of Saturn. If the sunlight could melt water at that distance, the moons of Saturn would have been hapitable.

(3) If water jets erupted from some deeper layers, then the nozzles would have had to be finely machined to account for the jets that are strongly collimated over the millions of miles. Neutral gas in vacuum does not behave that way. However, the interaction of the positively charged solar wind with the negative ion jets would explain the collimation perfectly.

Leo

Definitely "electric comet" move to ATM.

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“A good scientist has freed himself of concepts and keeps his mind open to what is”
道德經, 二十七 (dào dé jīng, 27)

1. Whether or not we know or may hypothesize this or that mechanism of oxygen coming from the nucleus, the hard fact is that it does come (see the numerous articles about the negative ions of oxygen). Experiment trumps any reasoning.

2. For example:

M. Summers, D. Siskind. Surface Recombination of O and H2 on Meteoric Dust as a Source of Mesospheric Water Vapor.
http://www.agu.org/pubs/crossref/199...GL900430.shtml

Leo

You are the first one using the "electric comet" expression here.

Leo

I am saying that the "ion of hydrogen", which is in fact a naked proton, will not bind chemically with anything at all, ever; protons don't do that. If you have atomic oxygen (O instead of H₂O), then the oxygen atoms will quickly bind to form molecular oxygen, O₂. A solar wind proton encountering neural O₂ will knock an electron loose, resulting in O₂⁺ ions. Likewise, a proton encountering neutral H₂O will kick out an electron and result in an H₂O⁺ ion. The only way you can get the "ion of hydrogen" to bind is for it to grab an electron and become neutral hydrogen, H. You might be able to get the H to bind with OH to form H₂O, if the energetics work out. I am not well enough versed in the relevant chemistry to tell whether or not it makes sense. But even if it does, there is still the problem of where the OH comes from, since we know it can't come from the comet (it is chemically active and will not remain OH inside the comet over any long period of time; the OH we observe comes from photodissociation of H₂O).

Is any of this relevant to the initial question: Where does the H₂O come from? And you ignore the equally numerous positive ions (i.e, Saxena & Misra, 1993), so it is highly unlikely that the comet has any significant net charge.

All wrong. To start with, the protons & ions will not be attracted to each other at all if they are farther apart than about 1 Debye length, which is about 10 meters in the solar wind plasma. The Debye length is proportional to the ratio (electron temperature)/(proton number density). Since the electrons slow down near the comet, and the plasma is more dense near the comet, then the Debye length in the plasma near the comet must be somewhat smaller than the solar wind value of ~10 meters. So, no they will not significantly attract each other. You still have the fundamental problem that there are nowhere near enough protons near the comet to make all the water we see, or to do all the other stuff these protons are supposed to do.

Why unlikely? After all, we can clearly see the jets coming through holes in the outer layers, so why should the outer layer not remain intact?

There are jets on the dark side, but in fact most jets do not come from the dark side, you are wrong on that point (i.e., Sekanina, et al., 2004). Remember, comets do rotate. So if a jet starts out as expected on the sunlight side, and rotates into the dark side, it can continue to jet, depending on the thermal inertia of the comet nucleus, compared to the rotation period of the comet. Sometimes the dark side jets will shut off if they get too cold.

Cometary activity at large distances from the sun is common. The cause is a legitimate question. But do keep in mind that not all comet jets are water (i.e, Farnham, et al., 2007). Cometary activity at large distances can be explained by the sublimation of ices other than water, such as CO or CN (i.e., Mazotta, et al., 2007; Korsun, Ivanova & Afanasiev, 2006), and the observed water production rate falls off as expected, with decreasing temperature at increasing distance (i.e, Gehrz, et al., 2005).

Actually, neutral gas in a vacuum does behave that way, because its flow is ballistic, although I will disagree with the comment that jets are "strongly" collimated over millions of miles. Indeed, even "collimated" is probably too strong a word all by itself. But the stream remains intact for long distances, and may extend to millions of miles for all I know.

As for the interaction between the jets and the solar wind, you are part right. It is the solar wind which controls the flow of a comet's ion tail, but the fan shaped dust tail is too massive, and is controlled by gravity (dust particles in comet tails tend to follow Keplerian orbits if I am not mistaken) and radiation pressure from sunlight.

So the unanswered questions remain unanswered, if we are going to buy the claim that the water observed does not come from comets:

1. Where does all the Oxygen come from?
2. How do you get a large enough number of protons in the small volume?

And in addition, the counter evidence, already presented, for the presence of water ice grains in comets, which would naturally explain how the water gets out of the comet.

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Negative ions are nothing special! I discovered them e.g. around Europa (Cl^-) and basically negative ions can maintain their charge for a longer time only when the extra electron fills up the electron shell. I do not see what the problem is here, except that it is a heck of a lot easier to create negative ions from dissociation (for example H₂O + hν -> H⁺ + OH^-). It is very much harder to use any kind of other process on the surface of the comet to create negative ions.

The process that you are linking to here is in the Earth's mesosphere, approximately 70 km above the surface. How will you translate this effect, where dust gathers the compounds and acts as a catalyst, to a comet? And this works with atoms and molecules, not with ions. (pdf of the paper available from me).

And by the way, your theory here:

does not make sense. The solar wind speed is indeed reduced by the bow shock around a comet. Your idea, with a negatively charged nucleus (however it becomes that, who knows) and the positive protons would speed up the particles, as they attract each other. And what happens with the electrons? And this is just a very naive view of what would happen.

Definitely "electric comet" move to ATM.

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There is actually a plenty of neutral hydrogen in the coma. But see below about Weibel instability.

I don't claim that the comet must have a significant net charge (despite what some people here say I do). But it may have a non-zero potential difference with its immediate plasma environment.

In plasma with anisotropic distribution of velocities, Weibel instability is highly likely to develop. It creates a lattice of non-neutral electric current filaments, of alternating charge and direction (both the negative filaments and the positive filaments stream in the same direction). It is well known that the width of the filaments is on the order of the Debye length. This mixes the positive ions and negative ions extremely well. Upon saturation of the instability, the protons and the negative ions will attract each other and recombine.

Where do the other ions, such as CO you mentioned earlier, come from?

If water indeed hides inside the comet's nucleus, then a disintegrating comet ought to exhibit lots of it. Perhaps, you'd claim that a comet disintegrates when its water is gone. But some comets' disintegration has been attributed to other reasons. Where's the water, then?

Leo

Are you sure about this? There have been no papers published of any observation of the Weibel instability in cometary tails. Now how can that be?

From Treumann & Baumjohann (chapter 5.1: Weibel Instability)

Now, how exactly does this fit with your idea in a cometary tail? I think you mean the XXXX instability (Your homework to find out the correct term, I am sure you have Peratt's book, although that does not imply I believe that the XXXX istability is actually appearing in a cometary tail.)

So, again, it sounds scientific, but if you really look at what is said, it is garbage.

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善數, 不用籌策 (shàn shù, bù yòng chóu cè)
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“A good scientist has freed himself of concepts and keeps his mind open to what is”
道德經, 二十七 (dào dé jīng, 27)