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PetTastic
2013-Sep-02, 10:01 AM
If you assume that metal meteorites we find on earth were originaly formed in a larger parent body.
That the parent body was large enough to heat-up to the melting point of iron, and also had stronge enough gravity to make the iron sink inwards.

I would expect the metorites we examin to have a definit 'up' direction corresponding to the parents gravity.

When the iron solidified I would hope to see evidence of mineral inclusions floating upwards, maybe being tear shaped or leaving trails in the iron.

Inside inclusions, I would hope to see gas bubbles near the top and denser material near the tail.
The only example I could find in my google image searches, also had shocked quarts orientated in the same direction. The inclusions could have been melted during an high-g impact.

What am I missing?

Cougar
2013-Sep-04, 06:04 PM
Melting due to radioactivity?

chornedsnorkack
2013-Sep-05, 06:17 AM
Melting due to radioactivity?

Not missed. Only large enough parent bodies can heat to melting point due to radioactivity - smaller ones will simply conduct the heat away to surface.

As for insufficient gravity to make the iron sink inwards, note that small gravity is not only found in small parent bodies. It also occurs near the centre of larger parent bodies.

What kind of traces will metal meteorites have from partial and incomplete melting? On heating, which part of meteorite material melts first - does iron melt first leaving solid rock grains to float upwards, or does rock melt first and drain out of still solid and unmolten iron grains?

PetTastic
2013-Sep-05, 10:06 AM
The melting point argument is interesting.
I did not consider the possibility that the nickel-iron was only soft, moving slowly, not fully melted, and the inclusions were still solid.

On the other hand we find meteorites families, each suspected of coming from different parent asteroids, you would expect some of them to have high temputature and stronger gravity. Even if some inclusions were still solid you would expect them to float with the heavy end down.

To be honest my main interest is linked to this thread.
http://cosmoquest.org/forum/showthread.php?145961-Radiation-induced-recrystallization-in-space
I was deliberately looking for problems with the current theories, in order to evaluate the possibility of an alternative means of meteor formation.
If the current theories have no issues, why look for an alternative.

Formation by cold welding and recrystallization would not leave any traces of an up or down, but would make radioisotope dating interesting.
Much of the evidence for the age of the solar system comes from meteorites.

korjik
2013-Sep-06, 04:32 AM
Any parent body large enough to differentiate is going to be big enough to cool slowly enough that any boyant motion of any inclusion will be vanishingly small when the iron solidifies.

chornedsnorkack
2013-Sep-06, 07:59 AM
Any parent body large enough to differentiate is going to be big enough to cool slowly enough that any boyant motion of any inclusion will be vanishingly small when the iron solidifies.

Is melting of iron necessary to segregate iron from rock?

PetTastic
2013-Sep-06, 10:33 AM
As the iron stiffened and the buoyant behavior is lost, how would evidence of previous buoyant behavior be lost?


Is melting of iron necessary to segregate iron from rock?
You might get a tiny bit of differentiation from a mixture of dust being vibrated by impacts, but you still need to melt the iron into meteoroids.

I have also come across another issue. Iron has a massive speed of sound and is dense, meaning it can transimit huge amounts of energy as sound waves.
(anti-tank weapon research)
The impact energy required to split open an iron asteroid is massive and would leave very little material orbiting in the asteroid belt.

How about this, as best case senario for the current theory.
For a differentiated body, peek gravity is inside the body but well outside the iron core, gravity at the centre of the body is zero.
Is it possible that core in the zero gravity area is molten but not heavily differentiated?
Later after cooling, asteroid has a weak core that does shatter when hit.
Sound waves transmitted by the iron blast of all the oriented materail from the outer layers to be lost.

The more I look into this the more probable it seems the Widmanstätten patterns are produced by later radiation-induced recrystallization, not slow cooling.

PetTastic
2013-Sep-06, 02:09 PM
Thinking about it, every time a squishy asteroid with a liquid iron core took a bump, it would squirt white-hot liquid iron out through the cracks.
This would quickly cool with no crystallisation.

AGN Fuel
2013-Sep-07, 02:08 AM
If you assume that metal meteorites we find on earth were originaly formed in a larger parent body. That the parent body was large enough to heat-up to the melting point of iron, and also had stronge enough gravity to make the iron sink inwards.

I would expect the metorites we examin to have a definit 'up' direction corresponding to the parents gravity. When the iron solidified I would hope to see evidence of mineral inclusions floating upwards, maybe being tear shaped or leaving trails in the iron.

Not really an area I know much about, but if differentiation had occurred in the parent body and this iron was subsequently freed due to some impact event to form the asteroid (for the iron to be released, the impact would need to be very significant), wouldn't the iron re-vapourise in that impact event and subsequently re-cool in a low-g environment, obliterating any such trails anyway?

I'm probably way off base here, but that was my original thought while reading the OP.

PetTastic
2013-Sep-09, 03:29 PM
It takes a lot of energy to vapourise iron. I think the asteroid is more likely to be destroyed.

I am assuming the accepted process goes something like the following. ( but I have my doubts)

These objects form in the cold of the early solar system, as a mixture of ice and dust.
Almost all the heating comes from internal radioactivity. This is a relatively small energy source and heating is slow. Any process that absorbs heat (melting or evaporating) will stop the temperature increasing until complete.

These are my guesses at the approximate temperatures stages the asteroid goes through as it heats up.

100k Methane ice starts to evaporate (object starts to look like a self heating comet)
200K Ammonia and carbon-dioxide ice evaporates (most of the carbon-14 heat source lost)
270K Water ice starts to melt and some evaporate (denser material start to sink into the ice)
370K Mixed material core now dry, outer layers still icy (water still percolating down and be expelled as steam)
400K Sulphur etc melt and start to flow down into voids in the core (outer surface now dry dust, a good insulator)
500K Water rich minerals start to decompose (more water vapour vented)
700K Carbonates start to decompose (carbon dioxide)
900K sulphur based compounds break down ( hydrogen-sulphide sulphur-dioxide)
1000K Hydrogen driven off from iron (hydrogen vented)
1200K Iron-nickel alloys could start forming
1800K Pure Iron and rock starts to melt, iron core forms surrounded molten rock( uranium etc sink into iron core)
2000K No more volatiles. (nothing more to vent or melt so temperature keeps rising)

At some point heat generated balances heat being lost, until insulation is lost or heat source decays away.
Asteroid needs to stay above 800K for several million years for Widmanstätten patterns to form.

Then the whole thing is smashed open by a massive impact, that breaks a near solid iron core.

That looks like a lot of geological activity that should leave traces in stony meteorites from the crust.
Has any evidence of this process been found?

AGN Fuel
2013-Sep-10, 03:37 AM
Sorry, my mistake. I meant re-melt, not re-vapourise. The impact energy required to shatter an asteroid to reveal an iron core is going to be significant.

Are you sure that radioactivity is the only form of heating to be considered? During the formation period, you would have had the energy of impacts as well as gravitational contraction. This may mean that your parent body was not formed cold and then heated, but rather formed hot, differentiated and then cooled.

Also, your melting/sublimation points appear to be based on one atmosphere pressure. That is not necessarily the case during the asteroid formation.

(Edited to note: I am just speculating here - this is in no way my area of expertise!)

PetTastic
2013-Sep-11, 05:01 AM
Sorry, my mistake. I meant re-melt, not re-vapourise. The impact energy required to shatter an asteroid to reveal an iron core is going to be significant.

Are you sure that radioactivity is the only form of heating to be considered? During the formation period, you would have had the energy of impacts as well as gravitational contraction. This may mean that your parent body was not formed cold and then heated, but rather formed hot, differentiated and then cooled.

Also, your melting/sublimation points appear to be based on one atmosphere pressure. That is not necessarily the case during the asteroid formation.

(Edited to note: I am just speculating here - this is in no way my area of expertise!)

I am not claiming expertise either. This is book research.
The temperature numbers are very approximate. I was just trying to get a sensible order of events by looking up minerals on Wikipedia.
I find it difficult to imagine an object massive enough to have the required gravity for impact heating, ever being broken up by an impact once cooled, and the Widmanstätten patterns observed in meteorites, are assumed to be caused by millions of years of slow cooling.

I think you are suggesting some form of impact welding. Two objects hitting fast enough for one to melt and stick. I assume this could happen, but I don't think that the results would look like the Widmanstätten patterns if you cut it open.

It is just that I am nearly 53 years old, and until now, I just accepted the standard version of events, not appreciating what it implies.
I can't see how a mixture of iron and rock can heat-up to the point it melts and starts to differentiate under gravity without other consequences.

An asteroid belt containing self-heating comets, some getting hot enough to have volcanic activity.
What would a 'sulphur comet' look like? What colours would a sulphur-dioxide hydrogen-sulphide comet tail glow in the UV light from the star?
Every time one of these hot objects took a hit it would spew lava out into space.

In the early stages, these objects would have very good conditions for life.

How does one of these objects get broken open without smashing all the inclusions to dust, iron is a superb transmitter of sound energy.

The problem is I don't see any evidence for it in meteorites. I can only see mineral inclusions embedded in an iron matrix, or a stony matrix…
Most of the material does not look volcanic in nature (some does). There are no signs of an orientation of material, or even signs of flow.

My best guess is these things formed cold, by a slow on-going process of cold welding, and radiation-induced recrystallization.
Radiation moving one atom at a time between touching crystals. Stronger crystals grow by destroying weaker crystals. (weak = impurities, stressed, or deformed)
Two compatible objects come together in space for long enough and crystals start to grow across the contact point, cold welding the two together. The differentiation comes from incompatible materials being shake free by impact vibration.

My problem is this version is a bit dull for my book, compared to volcanic explosions on asteroids.

Romanus
2013-Sep-11, 10:57 AM
Certain achondrites (HEDs, I'm looking at you) have evidence of fractionation and other features that indicate that they were derived at or near the surface of a massive body--Vesta, in this case. However, the size of meteorite parent bodies is poorly constrained beyond that, and an area of active research and debate. There are quite a few meteoriticists who speculate that many meteorite parent bodies were not that large at all, and were able to differentiate due to high radioactivity-induced heat fluxes even at sizes that would seem too small in today's Solar System.

PetTastic
2013-Sep-13, 04:00 PM
Many achondrite do look like erupted basalts we see on Earth that show little signs of orientation because they cooled quickly, and contain assorted debris from the eruption.

I expect meteorites we find have many origins, including the moons, Mars and larger asteroids.

Rewinding the exponential decay of isotopes a billion years or more, makes the solar system a very radioactive place, with plenty of heat being produced.

One thing I got wrong is, I was initially guessing for every asteroid that became hot enough to differentiate iron, there would be hundreds that only boiled off ices and carbon compounds.
However, every time material is boiled off the asteroid shrinks reducing its surface area, and therefor the heat lost through the surface.
At each stage of melting the rubble pile collapses becoming more compact, reducing the surface area again, while the heat source remains.
It looks like the once the process starts it is biased towards going all the way.

The more I research radiation induced recrystallization, the more likely it looks that any evidence of an original crystal structure in meteorites, was lost hundreds of millions of years ago.