Earth-Like Planets & Iron: Questions Answered

[Johninch]

I have 2 related questions about Earth-like planets and solar systems:

First question:

- I don’t understand why the Earth has so much iron.
- How much iron is blown off by the typical supernova in proportion to other metals?
- If the proto-solarsystem was a rotating gaseous disk, should most of the iron captured by the solar system have sunk into the sun?
- If so, what is all this iron doing in the Earth (proportionate to its mass)?
- If the outer planets are made mostly of lighter elements, why not the Earth too?

Related question:

- Does it matter for life that the Earth has a mainly iron core?
- Can habitable planets just as well produce a strong enough magnetic field with other liquid rotating metal cores?
- Is a non-iron metal core a likely scenario for habitable planets or is there something special about iron?

 

[mathman]

Partial answer: Most of the universe consists of Hydrogen and Helium. The effect of gravity is the main reason the outer planets (big) have held onto their lighter elements while the inner planets (small) have lost most of them. Iron is the end result of nuclear fusion processes in stars.

 

[Bandersnatch]

- How much iron is blown off by the typical supernova in proportion to other metals?

Depending on the supernova type and the meaning of metals. Metals in astronomy are all elements heavier than helium. By this definition, core-collapse supernova explosions (SNe II) eject only minor amounts of Nickel56 when compared to Oxygen, Carbon, Silicon and other "metals". Note that Nickel56 decays to Cobalt56 and then Iron56 with half-life of a few days, so it's equivalent to iron for the purposes of your question.
Still, by the everyday definition of metals, Nickel56 is the most abundant of all metals produced.
Here's the composition of core ejecta of SNe II stars:

(source: http://adsabs.harvard.edu/abs/1995ApJS..101..181W)
Everything below about 1.5 interior mass remains as the neuton core, hence the cut-off. You can see that the outer core is mostly He and H, the mid-inner range is dominated by oxygen, and the innermost is almost pure Nickel56.

It's supernova type Ia explosions (SNe Ia - the exploding white dwarf companions in binary systems), however, that produce the most iron:

(source: http://www.int.washington.edu/talks/WorkShops/int_04_2/People/Truran_J/truransne1a.pdf)
The ejecta are dominated by Nickel56, 58 and Iron54, 56.
All graphs show only the most abundant elements. See the source material for details on the less-abundant elements.

Iron is thus the most abundant of heavy metallic elements in any interstellar gas cloud.

- If the proto-solarsystem was a rotating gaseous disk, should most of the iron captured by the solar system have sunk into the sun?

No, it shouldn't. As long as the elements of the cloud are in orbits, there is no reason for such differentiation. It's the same reason why the "heavier" planets like the outer gas giants don't fall into the sun. It's only when the elements begin to clump together as the turbulences in the cloud develop and nucleate proto-planets and proto-stars, when the varying masses of the material begin to drive radial differentiation.

- If so, what is all this iron doing in the Earth (proportionate to its mass)?
- If the outer planets are made mostly of lighter elements, why not the Earth too?

Allow me to expand on mathman's answer a bit. The initial composition of the protostellar disc can be thought of as uniform across the board.
As the cloud evolves and bodies nucleate, the two factors driving the elemental differentiation are the abundance of heavy material available for accretion and the intensity of solar radiation once the star ignites.
With more material to accumulate, heavier masses can be reached by the protoplanets, which allows them to hold onto more and more light elements.
As the star ignites, it begins to deplete light molecules from its neighbourhood, with efficiency depending on distance. Close enough, and the radiation and solar wind can strip the gaseous atmospheres from the rocky cores of planets.
The interplay between star-driven depletion and planetary mass-driven accumulation is what creates the different types of compositions.

- Does it matter for life that the Earth has a mainly iron core?
- Can habitable planets just as well produce a strong enough magnetic field with other liquid rotating metal cores?
- Is a non-iron metal core a likely scenario for habitable planets or is there something special about iron?

I don't know my magnetism enough to answer the first two (I guess it could work for any electrically-conducting molten core), but the special thing about iron is that it's the most abundant. I'm not aware of a process during the star system formation that could possibly drive concetrations of iron down in favour of other, rarer metals. There are some such processes with respect to asteroids as they break apart and reform, but these are small, local differences in composition. To build a planet you need a lot of material, which statistically will retain the average composition.

 

[Bystander]

It's only when the elements begin to clump together as the turbulences in the cloud develop and nucleate proto-planets and proto-stars, when the varying masses of the material begin to drive radial differentiation.

For those of us from the shallow end of the gene pool who only got to the phys. chem. and geochem. of planetary formation before things got too deep, does solar wind produce any significant radial differentiation? (Wanting to look at some of this vis a vis the D/H discussion.)

 

[snorkack]

One special thing about iron is its abundance - it is easier to produce than nickel (Ni-56 can be produced by simple alpha process, and decays to Fe, while the more stable Ni isotopes like Ni-62 take time to produce, which is not available) and it is the endpoint of alpha process, so in the harsh conditions of explosions the alpha process mostly goes to the end, leaving less intermediate products like magnesium or calcium. The other special thing is that iron is chemically inactive, compared to other metals. Among the other common metals in Sun - Mg, Ni, Ca, Al, Na, Cr, Mn, K, Co, Ti - only Ni and Co are less active than Fe, and sink into the core. The other metals, to recapitulate, Mg, Ca, Al, Na, Cr, Mn, Ti - are chemically active, react with oxygen forming oxides and then silicates, which are less dense than metal Fe, and therefore remain outside the core. Also these silicates are electric insulators.

 

[Johninch]

Thankyou Bandersnatch for your detailed explanations. Prior to posting I got some of my information from the following site:
http://scienceline.ucsb.edu/getkey.php?key=24

Iron is thus the most abundant of heavy metallic elements in any interstellar gas cloud.

UCSB: “The reason why silicon is the most common element within the rocky planets, is because it was the most common element (next to hydrogen) in the original nebula that condensed to form the Solar System. This is simply how our Solar System was at the start - it had a greater ratio of silicon to all the other elements except hydrogen. Perhaps other planetary systems around other stars may have more aluminum or iron than silicon. Ours just happened to be a silicon-dominant one.”


This supports my assumption that a supernova does not have to continue fusion till iron. Depending on its mass, the fusion process may stop at before iron and still explode. Is this correct? So the question here is, how common are supernovas which distribute predominantly other metals than iron? You say that iron is the most abundant element in galactic gas clouds.

Johninch said: If the proto-solarsystem was a rotating gaseous disk, should most of the iron captured by the solar system have sunk into the sun?

No, it shouldn't. As long as the elements of the cloud are in orbits, there is no reason for such differentiation. It's the same reason why the "heavier" planets like the outer gas giants don't fall into the sun. It's only when the elements begin to clump together as the turbulences in the cloud develop and nucleate proto-planets and proto-stars, when the varying masses of the material begin to drive radial differentiation.

UCSB: “heavier elements like iron & nickel were concentrated towards the inner part of the early Solar System when it was still condensing. This is due to gravitational forces - heavier atoms fall towards the centre of the spinning nebula, lighter gases drift towards the perimeter.”


This supports my assumption that a rotating nebula pulls the heavier elements to the center as the mass of the center increases. As you say, if the angular momentum of the particles in the rotating gas cloud is sufficient, their orbits will be maintained. So the question comes up, where does a galactic gas cloud get its rotation in the first place? Why doesn’t it just implode when the density increases?

Could we explain it like this, that the heavy metals in the inner-most part of the gas cloud gravitate into the sun, whereas the heavy metals starting in the outer-most part of the gas cloud gravitate towards the center but don't quite make it into the sun due to their angular momentum?

 

[Bandersnatch]

For those of us from the shallow end of the gene pool who only got to the phys. chem. and geochem. of planetary formation before things got too deep, does solar wind produce any significant radial differentiation? (Wanting to look at some of this vis a vis the D/H discussion.)

Just to torture the metaphor a bit more, you're asking for tips on swimming from somebody standing on the shore, whose knowledge about being in water derives completely from being occasionally splashed by actual swimmers.

Having included that disclaimer, the few models I've seen that try to explain D/H abundances in cometary material use reactions in protostellar disc driven by temperature differences (like deuterium sequestration in water from DH gas). I haven't seen any that suggest the star has any significant effect on already coalesced bodies. There was one paper I read that included a consideration of pollution of the surrounding ISM by flare-produced deuterium, which could also conceivably pollute local orbiting bodies. But it put some constraints on the process, which turned out rather insignificant, even for an M-class flaring dwarf star (http://arxiv.org/pdf/astro-ph/0307183v1.pdf paragraph #4).

You say that iron is the most abundant element in galactic gas clouds.

You misunderstood. I said iron is the most abundant of the heavy (over 30 atomic mass) metals in the cloud.
Look at the graphs - it's obvious that the most abundant metal ejected in SNe II is oxygen. SNe Ia, however, produce predominantly Fe and Ni, with some partially burned silicon and unburned carbon.

UCSB: “The reason why silicon is the most common element within the rocky planets, is because it was the most common element (next to hydrogen) in the original nebula that condensed to form the Solar System. This is simply how our Solar System was at the start - it had a greater ratio of silicon to all the other elements except hydrogen. Perhaps other planetary systems around other stars may have more aluminum or iron than silicon. Ours just happened to be a silicon-dominant one.”

I'm not sure about this claim. The metallicity of the protosolar nebula is thought to be refleced in the metallicity of the Sun's photosphere:

(taken from Lectures in Astrobiology t.2; Gargaud, Martin, Claeys)
Si is then the fifth metal by abundance after O, C, Fe and Ne.

This supports my assumption that a supernova does not have to continue fusion till iron. Depending on its mass, the fusion process may stop at before iron and still explode. Is this correct?

No, supernovas (type II) can't explode without first fusing an iron/nickel core, and the fusing of nickel in the ejected core material is inevitable after the explosion. Similar with SNe Ia - once the reaction starts it continues until iron/nickel is formed (but as shown on the graphs, not all carbon ends up fused).
But not every star ends up in a supernova. Low mass stars will eject their partially-enriched envelope without exploding.

UCSB: “heavier elements like iron & nickel were concentrated towards the inner part of the early Solar System when it was still condensing. This is due to gravitational forces - heavier atoms fall towards the centre of the spinning nebula, lighter gases drift towards the perimeter.”

I don't think that's how it works, but admiteddly my knowledge is patchy, so I shan't comment. Wait for somebody else's more educated input.

 

[snorkack]

(taken from Lectures in Astrobiology t.2; Gargaud, Martin, Claeys)
Si is then the fifth metal by abundance after O, C, Fe and Ne.

And is a "metal" only for astronomers. Back to chemistry - the metals are, in order of abundance, Fe, then Mg, Ni, Ca, Al. So the fifth metal in abundance is Al, not Fe.

Regarding that sawtooth pattern: note that odd elements also tend to have fewer stable isotopes. Does anyone have a distribution of isobars? For example, there is less Al in the world than either Mg or Si... because Al has a single stable isotope (27) while Mg and Si each have 3 (Mg has 24, 25 and 26, Si has 28, 29 and 30). What patterns would show if you look at the comparative abundance of all atoms by mass instead of charge?

 

[D H]

UCSB: “The reason why silicon is the most common element within the rocky planets, is because it was the most common element (next to hydrogen) in the original nebula that condensed to form the Solar System. This is simply how our Solar System was at the start - it had a greater ratio of silicon to all the other elements except hydrogen. Perhaps other planetary systems around other stars may have more aluminum or iron than silicon. Ours just happened to be a silicon-dominant one.”

I'm not sure about this claim.

Whether silicon or iron is more common depends on perspective. From the perspective of mass, iron is more abundant, by about 1.8:1. From the perspective of number of nuclei, it's silicon by about 1.1:1.

 

[Bystander]

Just to torture the metaphor a bit more, you're asking for tips on swimming from somebody standing on the shore,

Here now, don't go destroying my faith in you. Conflation of ideas from too many threads and the distraction in Current Events right at the moment. What had crossed my mind was a radial differentiation of atomic masses by solar wind when/while the planetary nebula(e) were still diffuse enough to suffer such effects leading to a relative enrichment of heavier elements in the more sunward accretion discs; anything like that crossed your path?

 

[Ken G]

A point that has not been mentioned is that one very important issue in the abundance of various metals in planets is their melting point, which is also related to how close they can be to the Sun and still stick together. You don't make planets by just gathering up everything that is there, if that's all that happened all planets would be hydrogen-rich gas giants. To get the process started, you need atoms and molecules to stick together and make dust. This is a way more complicated process than what I'm going to say, but to keep it simple, consider Mercury. It has a very high iron content in part because it is so close to the Sun that only iron, with its high melting point, is good at sticking together that close to the Sun. Where the Earth is, the temperatures are lower, and silicon is also good at sticking together. So you start out with dust grains and start sticking more stuff to them, and you build up a protoplanet a bit like a snowball builds when it rolls down a hill, but it is only picking up the stuff that can stick at that temperature. So a very short answer to your question is, the Earth has a lot of iron and silicon because the more abundant elements than those don't stick together at this distance from the Sun.

 

[snorkack]

A point that has not been mentioned is that one very important issue in the abundance of various metals in planets is their melting point, which is also related to how close they can be to the Sun and still stick together. You don't make planets by just gathering up everything that is there, if that's all that happened all planets would be hydrogen-rich gas giants.

Which does not mean that these hydrogen-rich gas giants are not formed exactly so.

But the terrestrial planets, asteroids and comets are formed from condensed matter. And there, the relevant point is boiling point, not melting point.

 

[Johninch]

So a very short answer to your question is, the Earth has a lot of iron and silicon because the more abundant elements than those don't stick together at this distance from the Sun.

But Venus has a very massive atmosphere (of CO2) compared to the Earth, and little magnetic field, which should have blown a lot of it away. On the other hand, Mars has a very thin atmosphere. So how does this fit your theory?

 

[Ken G]

But the terrestrial planets, asteroids and comets are formed from condensed matter. And there, the relevant point is boiling point, not melting point.

Actually, I don't think that's true. If materials are maintained in a liquid state, they tend to evaporate in the low density of space, which essentially means that their boiling point tends to be way lower than their melting point, somewhat counterintuitively given our experience in the dense atmosphere of Earth. So I believe it is indeed the melting point that matters for condensed matter in space.

 

[Ken G]

But Venus has a very massive atmosphere (of CO2) compared to the Earth, and little magnetic field, which should have blown a lot of it away. On the other hand, Mars has a very thin atmosphere. So how does this fit your theory?

The atmospheres of planets is an entirely different kettle of fish from the planetary composition. I'm talking about how you get a planet to start sticking together in the first place, not what happens way later once the planet is formed and has a large gravity. Retaining atmospheres relates to the large gravity of the already-formed planet.

In regard to your question, the standard answer, though it is probably highly oversimplified, is that Venus retained a thick CO2 atmosphere because it combined a strong gravity (which distinguishes it from Mars) with the absence of liquid water oceans (which distinguishes it from Earth). Liquid water oceans are good at dissolving CO2 and forming it into rocks, whereas Mars' weak gravity made it susceptible to escape of its atmosphere.

 

[D H]

It has a very high iron content in part because it is so close to the Sun that only iron, with its high melting point, is good at sticking together that close to the Sun.

That idea was tossed long ago form the simple reason that iron does not have a high melting point. Iron and silicon, along with a number of other moderately refractory elements, all have very similar condensation temperatures.

Up until 2011, there were three leading hypotheses regarding the Mercury's anomalously high iron content.

The dominant hypothesis regarding terrestrial planet formation says that dust combines to form pebbles and rocks, which then combine to form protoplanets, which in turn combine to form planetary embryos, and these finally combine to form planets. One of the hypotheses of Mercury's formation posits that the less dense dust, pebbles, and rocks would have been subject to greater drag per unit mass and thus would have preferentially fallen into the Sun. Another hypothesis suggested that high heat in the planetary nebular would have stripped Mercury of volatiles during later stages of the planet's formation. The third hypothesis is that a giant impact at the very end of the planet's formation stripped Mercury of most of its mantle and crust.

The MESSENGER mission to Mercury first orbited the planet in 2011. One of the key objectives of the mission was to look for signatures that would differentiate these hypothesis. The mission has achieved that goal. It has poked gaping holes in each of those hypotheses.

 

[snorkack]

Actually, I don't think that's true. If materials are maintained in a liquid state, they tend to evaporate in the low density of space, which essentially means that their boiling point tends to be way lower than their melting point, somewhat counterintuitively given our experience in the dense atmosphere of Earth. So I believe it is indeed the melting point that matters for condensed matter in space.

No, it is still boiling point that matters.
There are materials whose triple point is higher than even the high pressure of Earth atmosphere. Notably carbon dioxide. It does not melt on Earth - it sublimates at -78 degrees, remaining completely dry. In order to melt it, at temperature -57 degrees, a pressure of 5,2 bar is needed.

Of course there are many materials which do melt under Earth atmospheric pressure, yet evaporate rapidly under their melting point. Water freezes at 0 degrees by definition, and at 1,01325 bar boils at 100 degrees by definition, but ice and snow readily evaporate in dry air. In fact, ice cannot melt under pressure of 611 Pa (6 mbar), like on Mars, any more than carbon dioxide can melt on Earth.

Compare quicksilver. It is liquid always when water is. But it is much harder to make into anything else. At 1 bar, water boils at 100 degrees, but quicksilver can be heated to 358 degrees before it boils. Also, quicksilver is resistant to frost - water freezes at 0 degrees, but quicksilver at -39 degrees.

Quite naturally, as water and ice evaporate appreciably below their boiling point, quicksilver also evaporates below its boiling point - but since it is the boiling point that is so high, the vapour pressure of quicksilver at the same temperature is much lower than vapour pressure of water or ice.

The triple point vapour pressure of quicksilver at -39 degrees is 0,165 mPa, compared to the 611 Pa of ice. Ice even at -40 degrees has vapour pressure of about 13 Pa.

The pressure in space is so low that even quicksilver cannot melt. But at same pressure and temperature, ice should evaporate much faster than solid quicksilver - although quicksilver has a lower melting point, and because it is the boiling point of quicksilver which is higher than that of ice.

 

[D H]

No, it is still boiling point that matters.

As far as I can tell, it's neither. It's the "condensation temperature" that matters.

For one thing, the pressure in the protoplanetary disk was rather low, less than 1 pascal to a few tens of pascals. This is below the triple pressure of many pure substances.

For another, boiling and melting refer to pure substances. The stuff that formed the dust, the growing grains of dust, pebbles, etc. was a mix of many different compounds. How to represent the variations that arise from that? The condensation temperature of a pure substance (with some reference pressure) is the temperature at which that substance becomes solid. The condensation temperature of an element is the temperature at which half of the stuff typically formed from that element in space become solid. Note that condensation temperature has a reference pressure; for the outer solar system it's typically 1/10 pascal, for the inner solar system, 10 pascal.

 

[Ken G]

That idea was tossed long ago form the simple reason that iron does not have a high melting point. Iron and silicon, along with a number of other moderately refractory elements, all have very similar condensation temperatures.

That's why you'll see I lumped iron and silicon together in my answer (if you read on). I'm not interpreting the OP question as "why does the Earth have more iron than silicon," because it doesn't, I'm interpreting the question as "why is the composition of the Earth so much different from the composition of the solar nebula." Indeed, the OP does not mention silicon at all, but it does mention the composition of the gas giants. But I hear what you are saying-- at some level, melting points are something of a red herring, because all we need to know is that metals and refractories are not abundant but are the things that can stick together into large grains. The abundant stuff, hydrogen and helium, won't stick together into a planet without gravity, so you need something else to get that gravity in the first place.

The MESSENGER mission to Mercury first orbited the planet in 2011. One of the key objectives of the mission was to look for signatures that would differentiate these hypothesis. The mission has achieved that goal. It has poked gaping holes in each of those hypotheses.

That is an interesting and important point to make, but falls under the heading of what I meant when I said it is a "way more complicated process than I am going to say." Nevertheless, the simple answer to why the Earth has so much different composition than the solar nebula is that the most abundant things in the solar nebula won't stick together if you are as close to the Sun as the Earth is. I should add they also don't get pulled in by the Earth's gravity because they are moving too fast at that temperature.

 

[Ken G]

No, it is still boiling point that matters.

I'm not clear on what argument you are presenting. I'm saying that the melting point of iron is not very sensitive to pressure, but the boiling point is. Hence, in the low-pressure environment of the solar nebula, it is more logical to talk in terms of the melting point of iron. We know that iron does in fact melt, for we find metallic chondrules that were formed by shocks, but these must cool back down and solidify. I'm saying that molten grains of iron are not a suitable building block for planets, because they evaporate too easily (not as easily as a volatile, of course, but we need a long time to make planets). If you can't build planets from molten grains, you have to be above the melting point, which is not that sensitive to pressure, or even composition for that matter. As for "condensation temperature", you won't find that if you just look up the melting point of iron and silicon, so it can be viewed as a technical difference.

 

[dragoneyes001]

wouldn't the rather large amount of asteroids rich in Iron which have been bombarding the planet for a couple billion years be the driving force behind how much we have on Earth in our crust?

 

[Ken G]

wouldn't the rather large amount of asteroids rich in Iron which have been bombarding the planet for a couple billion years be the driving force behind how much we have on Earth in our crust?

That bombardment constitutes a tiny fraction of the Earth mass, probably even a tiny fraction of the crust mass-- though that's less clear.

 

[dragoneyes001]

That bombardment constitutes a tiny fraction of the Earth mass, probably even a tiny fraction of the crust mass-- though that's less clear.

I meant as a source for the ferrous metals we dig up on Earth not the entirety of the crust

 

[Ken G]

I meant as a source for the ferrous metals we dig up on Earth not the entirety of the crust

In that case, I don't really know. Sometimes we dig down pretty deep to find "veins" of iron ore, which certainly seems like a geological feature of the Earth more so than what has impacted onto it. But by the same token, I've heard that you can easily find iron filings in your gutters that are meteor dust, so I really couldn't say how much iron falls onto the surface of the Earth. Maybe it's more an issue of, if you want to mine iron, you want to find a natural deposit in which it is fairly concentrated. Anyone around here mine iron ore?

 

[jim mcnamara]

Geologically the banded iron formations (http://en.wikipedia.org/wiki/Banded_iron_formation) typified by the Northern Minnesota Iron Range, are the direct result of the oxygen catastrophe. http://en.wikipedia.org/wiki/Great_Oxygenation_Event. This occurred long after the Earth's iron core formed. See the late heavy bombardment model: http://en.wikipedia.org/wiki/Late_Heavy_Bombardment

All this means is: the then extant upper crust of the Earth eroded into seas, the dissolved iron precipitated out. Most of the iron in the crust is thought to originate from bombardment. Why?

Early iron present in upper layers migrated down to form the core - discussion: http://es.ucsc.edu/~fnimmo/website/treatise3.pdf The iron in the silicate crust arrived later on, oxygen much later.

So unless there is a way for iron to migrate up through the mantle it had to stayed in the crust and upper mantle from the beginning. Or if large amounts of iron moved down into the core, as the discussion points out, then there has to be secondary, and later, source than the iron present in planetismals that made up protoearth. It seems iron did indeed move down. So that leaves a late dose of iron as the likely source of iron in the crust.

From a geological point of view, Dragoneyes001 point is well taken. This is timing, years later than Earth formation, not overall abundance of atom species at time zero. IMO. I think the crust's iron content came down as a belated birthday present.

 

[Ken G]

That's interesting-- the iron didn't become iron oxide until life released the oxygen, and so original iron would have sunk to the core while the Earth was molten. Only iron that arrived after the crust solidified was available to be oxidized, and that must have come from the late heavy bombardment, when the Earth was about 0.5 billion years old. That's not deposition over a couple billion years of bombardment though, it's very early on-- but is still separate from the original formation because it had to happen after the crust solidified. Makes sense-- and does suggest that we have the late heavy bombardment to thank for the iron age!

 

[Amaterasu21]

Here now, don't go destroying my faith in you. Conflation of ideas from too many threads and the distraction in Current Events right at the moment. What had crossed my mind was a radial differentiation of atomic masses by solar wind when/while the planetary nebula(e) were still diffuse enough to suffer such effects leading to a relative enrichment of heavier elements in the more sunward accretion discs; anything like that crossed your path?

I'm not sure about the solar wind itself but I do know solar radiation played a role in radial differentiation of atomic masses (and not just through heating and vaporisation of volatile elements either).

The chondrites (primitive, undifferentiated meteorites) are classified into groups based on their volatile content, iron content and oxidation state and oxygen isotope ratios. The volatile content tells us roughly how far out in the solar nebula they formed (since obviously the proto-sun would have vaporised volatiles closer into it), and iron oxidation state fits in well with this (the equilibrium condensation model tells us that solids that condensed at high temperatures will have a lower oxidation state for iron/more reduced iron, since iron metal condenses at a higher temperature than olivines and pyroxenes (containing iron (II)), which condense at a higher temperature than magnetite (containing iron(III), and we find that in general the more oxidised the iron of a meteorite is, the more volatile elements it contains - two independent lines of evidence that some meteorites formed further out in the solar nebula than others.)

These chondrites also vary in oxygen isotope ratios - those that appeared to have formed further out have relatively more ¹⁶O (compared to the much rarer and heavier stable isotopes, ¹⁷O and ¹⁸O) than those further in. One possible explanation is the fact that a significant amount of the oxygen in the solar nebula was tied up in carbon monoxide, which is ubiquitous throughout the universe, and which had to be split to release the oxygen and "free" it to react and form the silicate phases we see in meteorites. Different wavelengths of solar UV radiation photodissociate CO molecules (i.e. split them up) containing different oxygen isotopes.

Since ¹⁶O is an incredibly abundant isotope (99.8% of the CO in the protoplanetary disc was ¹²C¹⁶O), the ¹⁶O dissociating wavelength did not penetrate any significant distance into the disk (as there was so much C¹⁶O to absorb it). This means that out where the inner planets formed, there were more of the ¹⁷O and ¹⁸O dissociating wavelengths, splitting more CO molecules containing these isotopes and "freeing the oxygen atoms" up to react and form more silicate minerals with an enrichment of these isotopes. Even farther out in the disk, however, even these wavelengths would have been absorbed, making meteorites that formed in the outer parts of the asteroid belt relatively poor in ¹⁷O and ¹⁸O relative to ¹⁶O compared to those meteorites that formed further in. This hypothesis is known as spectroscopic self-shielding, and implies that the solar system on average is richer in ¹⁶O than Earth and the other terrestrial planets (which got an enrichment in ¹⁷O and ¹⁸O due to their distance from the Sun - far out enough to not absorb much C¹⁶O-dissociating radiation, but to absorb a lot of C¹⁷O and C¹⁸O dissociating radiation.).

This was found to be true by the Genesis mission, which captured some of the solar wind and analysed it isotopically (assuming of course that the Sun's composition represents the average solar system composition, which is a fair assumption to make given that the Sun represents 99.86% of the solar system's mass!). The Sun is indeed enriched in ¹⁶O relative to the inner planets.

I know this has nothing to do with the solar wind but I bring it up as an example of how solar radiation can affect the radial composition of the solar nebula beyond simple heating!

Edit: Everyone talking about the late heavy bombardment as the source of iron in the crust since iron would have been dragged down into the core during differentiation - please remember the core contains metallic iron! Oxidised phases of iron in silicates are less dense and there'd be no problem with them remaining in the crust (or mantle and then being brought up to the crust during volcanism) from the Earth's formation. Take the ferromagnesian minerals that make up much of the oceanic crust, for example.

 

[Bystander]

further out have relatively more 16O (compared to the much rarer and heavier stable isotopes, 17O and 18O) than those further in.

spectroscopic self-shielding,

Oh, goody. More things to consider regarding isotope ratios. Thanks.

 

[dragoneyes001]

Edit: Everyone talking about the late heavy bombardment as the source of iron in the crust since iron would have been dragged down into the core during differentiation - please remember the core contains metallic iron! Oxidised phases of iron in silicates are less dense and there'd be no problem with them remaining in the crust (or mantle and then being brought up to the crust during volcanism) from the Earth's formation. Take the ferromagnesian minerals that make up much of the oceanic crust, for example.

wouldn't being in a molten state have allowed the oxygen to escape from the iron?

 

[Bystander]

wouldn't being in a molten state have allowed the oxygen to escape from the iron?

Molten what? Takes a long time to completely degas anything.

 

[dragoneyes001]

Molten what? Takes a long time to completely degas anything.

he mentioned volcanism bringing up the iron in silicates wouldn't that mean being within the magma and in a molten state?

 

[Bystander]

iron in silicates

There's a lot of chemistry going on in the crust. "Molten" does not necessarily mean elemental.

 

[dragoneyes001]

There's a lot of chemistry going on in the crust. "Molten" does not necessarily mean elemental.

I meant in the magma not the crust since it would need to be carried up during volcanic activity it'd need to be fluid plus the crust would have formed of the lightest elements in the original magma during the cooling of the outer surface. to be more precise i'd expect some of the silicates would have formed into the crust but not sure if the volume we see would be accountable by this including volcanic activity because I suspect a lot of the iron would be lost to the affect of being in a molten state for so long.

 

[Ken G]

Edit: Everyone talking about the late heavy bombardment as the source of iron in the crust since iron would have been dragged down into the core during differentiation - please remember the core contains metallic iron! Oxidised phases of iron in silicates are less dense and there'd be no problem with them remaining in the crust (or mantle and then being brought up to the crust during volcanism) from the Earth's formation. Take the ferromagnesian minerals that make up much of the oceanic crust, for example.

You make a lot of good and detailed points, but the article sited about the iron content in the crust was talking about iron ore veins that get mined for iron, so that is not elemental iron either-- and it did claim they would have been differentiated had they not appeared after the crust solidified. Perhaps they are not elemental, but they are iron oxides that are still denser than the average silicate. I don't know the role that vulcanism could have played, it does seem a bit odd to imagine this stuff has just sat there since late bombardment!

 

[Bystander]

I meant in the magma not the crust since it would need to be carried up during volcanic activity it'd need to be fluid plus the crust would have formed of the lightest elements in the original magma during the cooling of the outer surface.

Might be worth your while to go through Mason's Principles of Geochemistry, for an overview of what's suspected to have gone on based on what's suspected to have been the starting condition for planetary accretion and what are suspected to have been the operative differentiation processes.