67P, why erosion from the neck?

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Re: 67P, why erosion from the neck?

Unread postby Maol » Fri Dec 12, 2014 5:49 pm

It may not be erosion from the neck, it could be accretion to the poles.


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Re: 67P, why erosion from the neck?

Unread postby viscount aero » Fri Dec 12, 2014 5:50 pm

Maol wrote:It may not be erosion from the neck, it could be accretion to the poles.


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Excellent brainstorming idea :idea:

I could be a confluence of this and corrosion. The corrosive particles adhere to the poles.
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Re: 67P, why erosion from the neck?

Unread postby Frantic » Tue Dec 16, 2014 2:28 pm

When olivine is high in Iron content we have the following :

Olivine + water + carbonic acid → serpentine + magnetite + magnesite + silica

When high in Mg content :

Olivine + water + carbonic acid → serpentine + magnetite + methane

Then :

serpentine + carbon dioxide → talc + magnesite + water

2 Mg3Si2O5(OH)4 + 3CO2 → Mg3Si4O10(OH)2 + 3 MgCO3 + 3 H2O

Iron oxides flakes will not stop continuing corrosion. However talc will. Given an electrolytic environment high in Carbon, oxidation of serpentinite can create talc. If this forms a layer it could protect the surface from further erosion. While the higher iron concentrations will produce methane gas expelling the talc from the surface and continue corroding. The rocks/boulders may be carbonate minerals or rock salts. The iron in the olivine is oxidized to Fe2O3. The magnesium in olivine is converted to talc and carbonate rock, and there still remains olivine, serpentinite, brucite and magnetite.

Don't know if any of that helps, but I think an electrolytic environment is a very good model of the comets we have investigated so far. And talc provides corrosion resistance that may help explain the erosion at specifically the neck area. It is a very complex system of reactions occurring and not an iceball heating up. I think it must have originated as a electric/magma discharge or collision event.
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Re: 67P, why erosion from the neck?

Unread postby viscount aero » Tue Dec 16, 2014 2:48 pm

Frantic wrote:When olivine is high in Iron content we have the following :

Olivine + water + carbonic acid → serpentine + magnetite + magnesite + silica

When high in Mg content :

Olivine + water + carbonic acid → serpentine + magnetite + methane

Then :

serpentine + carbon dioxide → talc + magnesite + water

2 Mg3Si2O5(OH)4 + 3CO2 → Mg3Si4O10(OH)2 + 3 MgCO3 + 3 H2O

Iron oxides flakes will not stop continuing corrosion. However talc will. Given an electrolytic environment high in Carbon, oxidation of serpentinite can create talc. If this forms a layer it could protect the surface from further erosion. While the higher iron concentrations will produce methane gas expelling the talc from the surface and continue corroding. The rocks/boulders may be carbonate minerals or rock salts. The iron in the olivine is oxidized to Fe2O3. The magnesium in olivine is converted to talc and carbonate rock, and there still remains olivine, serpentinite, brucite and magnetite.

Don't know if any of that helps, but I think an electrolytic environment is a very good model of the comets we have investigated so far. And talc provides corrosion resistance that may help explain the erosion at specifically the neck area. It is a very complex system of reactions occurring and not an iceball heating up. I think it must have originated as a electric/magma discharge or collision event.


Excellent work, Frantic.

I wonder if talc is included in the dust tail? If H20 is part of the coma, as is methane, would not the talc be streaming off the comet, too (and also settling back down to the cometary surface as it is part of the flurry of coma volatiles)?
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Re: 67P, why erosion from the neck?

Unread postby Lloyd » Tue Dec 16, 2014 7:11 pm

V.A.: I wonder if talc is included in the dust tail? If H20 is part of the coma, as is methane, would not the talc be streaming off the comet, too (and also settling back down to the cometary surface as it is part of the flurry of coma volatiles)?

I think Charles figured that comets are protected from the solar wind by the coma, which I think he says has an electric double layer.
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Re: 67P, why erosion from the neck?

Unread postby Frantic » Tue Dec 16, 2014 10:03 pm

http://en.wikipedia.org/wiki/Debye%E2%80%93H%C3%BCckel_theory

Debye–Hückel theory is an adjustment to account for the reality of electrolytic solutions with their ionic nature. Activity coefficients effectively represent concentrations. Where the comet is convex (the neck) it has the most surface exposed to itself. If there are surface corrosion reactions, we will have much higher ionic activity in this convex area, which equates to higher activity (or concentration), and therefore more reactions. The electrolytic environment has bounds limited by the ability of a charged entity to attract another. I don't know where the bounds would be, but the convex area would be most likely to fall within those bounds. There would be a larger build up of gases and ions in this area around the neck than else where. A charge differential between the nucleus and coma creates jets?

As there would be far more activity at the neck, there should be, I think, less activity on the rounded lobes.

Notice the directions of the jets in the ESA images that are over exposed. Hard to tell, but none of the jets are perpendicular to the neck, and there are clearly multiples jets. I don't think the center of the neck is eroding, but rather the cliff faces on each side of the neck.

If the jets are ionic could there be enough there to generate a small secondary magnetic field?
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Re: 67P, why erosion from the neck?

Unread postby viscount aero » Wed Dec 17, 2014 1:03 am

Frantic wrote:http://en.wikipedia.org/wiki/Debye%E2%80%93H%C3%BCckel_theory

Debye–Hückel theory is an adjustment to account for the reality of electrolytic solutions with their ionic nature. Activity coefficients effectively represent concentrations. Where the comet is convex (the neck) it has the most surface exposed to itself. If there are surface corrosion reactions, we will have much higher ionic activity in this convex area, which equates to higher activity (or concentration), and therefore more reactions. The electrolytic environment has bounds limited by the ability of a charged entity to attract another. I don't know where the bounds would be, but the convex area would be most likely to fall within those bounds. There would be a larger build up of gases and ions in this area around the neck than else where. A charge differential between the nucleus and coma creates jets?

As there would be far more activity at the neck, there should be, I think, less activity on the rounded lobes.

Notice the directions of the jets in the ESA images that are over exposed. Hard to tell, but none of the jets are perpendicular to the neck, and there are clearly multiples jets. I don't think the center of the neck is eroding, but rather the cliff faces on each side of the neck.

If the jets are ionic could there be enough there to generate a small secondary magnetic field?


Frantic, it's concave at the neck, not convex. The neck is a cavity :)

Great points to consider by the way.
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Re: 67P, why erosion from the neck?

Unread postby viscount aero » Wed Dec 17, 2014 1:05 am

Lloyd wrote:
V.A.: I wonder if talc is included in the dust tail? If H20 is part of the coma, as is methane, would not the talc be streaming off the comet, too (and also settling back down to the cometary surface as it is part of the flurry of coma volatiles)?

I think Charles figured that comets are protected from the solar wind by the coma, which I think he says has an electric double layer.


Ok but could the comet be corroding away--eating itself--from within its own coma? If the coma creates an atmospheric envelope (which can grow to planetary or stellar size), can not this envelope be the comet's own death chamber? Processes within the coma--corrosion reactions-- could be hostile to the comets own body.
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Re: 67P, why erosion from the neck?

Unread postby willendure » Wed Dec 17, 2014 1:41 am

Maol wrote:It may not be erosion from the neck, it could be accretion to the poles.
Image


It could well be. The only reason I initially suggested that it is eroding from the neck, is because jets appear to be coming out of the neck, and I am making a guess that that means material is being eroded from the neck. That and the fact that other lobed comets are known.

It might be more correct to ask, what could be different between the neck and the lobes in terms of chemical/mechanical/electrical processes that may cause comets to take a lobed peanut shape?
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Re: 67P, why erosion from the neck?

Unread postby viscount aero » Wed Dec 17, 2014 1:47 am

willendure wrote:
Maol wrote:It may not be erosion from the neck, it could be accretion to the poles.
Image


It could well be. The only reason I initially suggested that it is eroding from the neck, is because jets appear to be coming out of the neck, and I am making a guess that that means material is being eroded from the neck. That and the fact that other lobed comets are known.

It might be more correct to ask, what could be different between the neck and the lobes in terms of chemical/mechanical/electrical processes that may cause comets to take a lobed peanut shape?


Crevice corrosion possibly.
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Re: 67P, why erosion from the neck?

Unread postby Frantic » Wed Dec 17, 2014 3:23 pm

I think the area within the neck cavity may act as a small ionic atmosphere where electro-chemical reactions take place. As the neck cavity increases in size, erosion may slow down as the ion concentrations decrease. It seems to be carving out a smoother round bar from the comet material, not actually eroding through the neck. The cavity has out jets, but something must also be depositing dust onto the neck, if the interactions are in the small ionic atmosphere around the neck, it can expel its materials both away from and onto the neck.

Charles was describing a charged sheath of positive charged plasma around the comet that eventually recombines with the coma and the ions react in the coma. I don't think it is a sustained double layer, Charles? If it is, the double layer effectively prevents ions from attracting each other and no electro-chemical reactions can occur.
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Re: 67P, why erosion from the neck?

Unread postby viscount aero » Wed Dec 17, 2014 5:33 pm

Frantic wrote:I think the area within the neck cavity may act as a small ionic atmosphere where electro-chemical reactions take place. As the neck cavity increases in size, erosion may slow down as the ion concentrations decrease. It seems to be carving out a smoother round bar from the comet material, not actually eroding through the neck. The cavity has out jets, but something must also be depositing dust onto the neck, if the interactions are in the small ionic atmosphere around the neck, it can expel its materials both away from and onto the neck.

Charles was describing a charged sheath of positive charged plasma around the comet that eventually recombines with the coma and the ions react in the coma. I don't think it is a sustained double layer, Charles? If it is, the double layer effectively prevents ions from attracting each other and no electro-chemical reactions can occur.


Yes good point.
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Re: 67P, why erosion from the neck?

Unread postby Lloyd » Wed Dec 17, 2014 10:38 pm

I think CC says the DL is sustained by the solar wind. Maybe it's time to pm him.
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Re: 67P, why erosion from the neck?

Unread postby Frantic » Thu Dec 18, 2014 11:56 am

Charles Chandler wrote:Once the comet has passed, the electrons and +ions will eventually recombine, while the chemical composition might have changed a bit from the IPM, with the ionization and recombination possibly forming new compounds. But at least some of what glows in the coma didn't come from the comet -- it was just matter that got ionized by friction when the IPM slammed into the detached bow shock in front of the comet, and the subsequent charge recombination causes it to glow.


I was looking at his statement above. Which is where I got that his idea is a double layer develops and dissipates depending on the charge density of the IPM in relation to the comet as opposed to a double layer that requires pressure to maintain like his star model. I sent him a PM in case someone hadn't already.
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Re: 67P, why erosion from the neck?

Unread postby CharlesChandler » Thu Dec 18, 2014 4:11 pm

Hey Folks,

In my comet model, there are a total of 4 layers of alternating charge (2 positive and 2 negative). The charge separation mechanism is frictional charging from the comet moving through the interplanetary medium (IPM). When the comet is going against the solar wind, the charging is the most robust. As is always the case when a solid object moves through a gas/plasma, a boundary layer of gas/plasma builds up on the leading side of the object. If the velocity of the object is supersonic, this transforms into a detached bow shock, and the boundary layer gets bigger with increasing velocity, contrary to the subsonic regime, in which the boundary layer gets compressed with increasing velocity. The reason for the detached bow shock is that as the comet moves through the IPM, neutral atoms in the IPM impinging on the boundary layer have their electrons stripped off, and the atomic nuclei get embedded in the boundary layer deeper than the electrons, due to the greater mass of the +ions. This creates a halo around the comet, with a layer of stripped electrons left around the outside, and a layer of +ions on the inside, up against the comet. Electrostatic repulsion between the +ions is what bloats the boundary layer out into the detached bow shock. The other 2 layers of charge are inside the comet itself. The +ions in the comet's atmosphere induce a negative charge on its surface, and that induces a positive charge in its core.

So that's the model from the cometary frame of reference. Now let's look at it from the frame of reference of the IPM. As the comet plows through the solar wind, charges in the IPM are separated by that friction on the leading side of the comet. After the the comet has passed, those charges can recombine. This gives the appearance of a tail following the comet wherever it goes, and suggests that the tail (i.e., the coma) is comprised of particles that were eroded from the comet. But that would be a truly enormous amount of erosion going on, and it's a bit hard to believe that there would be much left of the comet after just one trip through the inner solar system. Then it becomes impossible to explain regularly recurring events such as Halley's Comet, whose orbital period should change dramatically with the mass loss, since lighter objects have lower terminal velocities, encountering more friction per mass than large objects. But if the coma is actually just particles in the IPM that are recombining after the comet has passed, there isn't any mass loss. It's like watching a motorboat zip across a lake. If we think that the wake is comprised of particles stripped off of the motorboat, and if we estimate the amount of mass involved, we'd be hard-pressed to explain why there would be anything left of the motorboat by the time it got to the far shore. But if the wake is just the effect that the motorboat had on the medium, there isn't any mass loss. Also note that in the case of a supersonic detached bow shock, the friction actually goes down instead of up with increasing velocity. Below the speed of sound, the drag force increases with the cube of the velocity, but above the speed of sound, the drag force actually relaxes. This is because the object gets surrounded by a teardrop-shaped envelope of gas/plasma that eliminates the vacuum on the leeward side of the object, which is the primary retarding force.

If comets are surrounded by plasma sheaths that insulate them from the solar wind, it becomes possible to understand how dust could accumulate on their surfaces. Dust doesn't generally last very long in high winds, and one would think that supersonic winds would sweep the surface quite clean. But if the impinging winds never actually get close to the surface, dust can accumulate. We can also understand why jets emanating from the interior of the comet don't show the effects of the supersonic speed. If you rigged up something to release a gas from the nose cone of a commercial airliner, in flight at 400 mph you'd see the gas getting swept back by the onslaught of air through which the plane is flying. You wouldn't see the gas expand away from the plane unaffected by the ambient atmosphere. Thus the behavior of cometary jets proves that they are expanding into a sheath that surrounds the comet, which insulates it from the solar wind.

So why do comets sometimes break up, especially when they get near to large objects, such as planets, or the Sun? This is typically attributed to gravitational forces. But the effect of gravity should be extremely slight compared to stresses from the drag force. Analogously, anything not bolted firmly to an airplane has a tendency to get torn off at flight speed, not because of differential gravity, but because of the drag force incurred by the airplane moving rapidly through the atmosphere. OK, so what if there isn't any drag force, because the supersonic object is encased in a protective sheath? Then the forces in that sheath are the ones to look at. And if the interior of the comet is positively charged, the Coulomb force will be pressing outward, so that's the likeliest force responsible for cometary break-ups. It's also the force that would drive jets emanating from the surface. If there is internal pressure due to the Coulomb force, but if the comet's crust is still strong enough to hold the whole thing together, the pressure will be relieved by jets. Once away from the surface, the positively charged jets will be attracted to the negatively charged outer layer of the detached bow shock. So the jets will expand conically away from the comet, and will be made visible from charge recombination.

Now -- back to the original topic -- what is the nature of the corrosion on the comet's surface? I don't know, and I'm going to defer to Viscount Aero (et al.)'s analysis of the chemistry. If the surface was positively charged, I'd say that it was just the positive charge that was liberating atoms by removing the covalent bonding. So whichever chemicals had the lowest ionization potentials would be the first to go, leaving stronger bonds in place, producing selective corrosion, and possible pitting of the surface. If the surface is negatively charged, it wouldn't seem to have this problem. But then again, when exposed to the unfiltered light from the Sun, photo-ionization might make the day side of the comet positively charged, with the night side picking up an equal-but-opposite negative charge.

Why don't comets get eroded spherically, as we otherwise would expect, and hence the topic of this thread? I don't know. ;)
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