Iron cores of super-Earths

[baric]

I was reading an article about super-Earths in the latest Scientific American, and there was one paragraph that I didn't quite follow:

But at the pressures that exist in a large planet's core, iron can solidify even at temperatures as high as 10,000 Kelvins, according to recent theoretical calculations.These high temperatures are probably exceeded only when the planets are very young. But a little cooling would be sufficient for the cores of super-Earths to solidify.Thus, a typical super-Earth may have a completely solid iron core and no global magnetic field.

ok, I understand that the greater pressures of a super-Earth would cause an iron core to solidify at temperatures where it would remain liquid on Earth.

But what I don't get is how this translates into a completely solid iron core. Wouldn't the pressure on the core decrease further away from the center, getting to a spot on the Fe state diagram to provide for a liquid Fe layer?

Fe state diagrams are not easy to come by, for obvious reasons, but this paper takes a good stab at it (figure 2 on pg. 2): http://www.gps.caltech.edu/~sue/TJA_...eismo_2069.pdf

This all assumes, of course, that the core has not cooled so sufficiently that Fe is solid even at low pressures, rendering the discussion moot. Since Earth still has enough heat for a liquid layer after 4B+ years, I think a super-Earth would be at least as warm.

The only way I can envision the scenario described in the article is if the non-Fe mass is so great that it alone can generate the pressures needed to solidify even the outermost region of the iron core regardless of temperature. But, in that case, it seems like the remaining layers of the planet would be extremely iron-deficient as iron sank to the core due to earlier convective processes.

[jlhredshift]

I was reading an article about super-Earths in the latest Scientific American, and there was one paragraph that I didn't quite follow:



ok, I understand that the greater pressures of a super-Earth would cause an iron core to solidify at temperatures where it would remain liquid on Earth.

But what I don't get is how this translates into a completely solid iron core. Wouldn't the pressure on the core decrease further away from the center, getting to a spot on the Fe state diagram to provide for a liquid Fe layer?

Fe state diagrams are not easy to come by, for obvious reasons, but this paper takes a good stab at it (figure 2 on pg. 2): http://www.gps.caltech.edu/~sue/TJA_...eismo_2069.pdf

This all assumes, of course, that the core has not cooled so sufficiently that Fe is solid even at low pressures, rendering the discussion moot. Since Earth still has enough heat for a liquid layer after 4B+ years, I think a super-Earth would be at least as warm.

The only way I can envision the scenario described in the article is if the non-Fe mass is so great that it alone can generate the pressures needed to solidify even the outermost region of the iron core regardless of temperature. But, in that case, it seems like the remaining layers of the planet would be extremely iron-deficient as iron sank to the core due to earlier convective processes.

I think the best way to think of it is to allow a gray area, a transition zone. Also, it is unlikely that it would be pure iron, it would be some alloy mix and the mix may not necessarily be uniform. Another thought, when iron solidifies it contracts and the angular momentum would have to be conserved; not sure on the effect this would have on the generation or not of a magnetic field.

[George]

Could it have been liquid first before cooling to solid?

[dgavin]

This is a very subjective article IMHO.

Core size and Planet mass is going to have a lot to do with onthe core being molten or solid. While a small iron core under high pressure might solidify completley, larger cores might have centers that are solid, while the outer portions are still liquid. This could infact lead to generation of stronger magnetic fields then a pure liquid core, via induction effects from the luquid into the solid parts of the core.

A super earth would also retain more heat over time, so it's not past reasoning that the core of a 4.5 bill year old super earth could still have a core over 10,000K temps.

Not to mention that super earths might have more radiogentic heating going on the in thier mantle and core as well.

There are a lot of variables that could effect a core's solidity, so indicating that super earths may not have strong magnetic fields due to inactive cores, seems a bit premature.

[baric]

This could in fact lead to generation of stronger magnetic fields then a pure liquid core, via induction effects from the liquid into the solid parts of the core.

These were my thoughts exactly! Well, at least they were before reading that article and then getting very confused :P

I can certainly envision a larger pressure-related core, but the temperatures seem so high that it's hard to visualize the lack of a liquid iron layer at the outer areas of the core. To me, the scenario pointed towards a stronger magnetic field.

[jlhredshift]

These were my thoughts exactly! Well, at least they were before reading that article and then getting very confused :P

I can certainly envision a larger pressure-related core, but the temperatures seem so high that it's hard to visualize the lack of a liquid iron layer at the outer areas of the core. To me, the scenario pointed towards a stronger magnetic field.

I concur, and the liquid part is the gray area I was referring to.

[korjik]

The assumption in the article is that the pressure is high enough that the core is solid until you get out of the core, leaving you in the silicate mantle before pressures drop enough to allow liquid iron. If that leaves you with a big area of liquid mantle, that still isnt going to generate a magnetic field, cause it is not metallic liquid circulating around.

One note to everyone: When astronomers/geologists/planetary scientists in general say that the core is 'iron', what they are really saying is 'a nickel-iron mixture close to the composition of metallic meteors'. They shorten that to 'iron' cause it is alot easier to write and say.

[baric]

The assumption in the article is that the pressure is high enough that the core is solid until you get out of the core, leaving you in the silicate mantle before pressures drop enough to allow liquid iron. If that leaves you with a big area of liquid mantle, that still isnt going to generate a magnetic field, cause it is not metallic liquid circulating around.

Yes, I acknowledged that possibility in my OP. But considering that we don't have those pressures in the Earth's core (maybe very near the center?), I wonder how much more massive a planet would have to be to generate that much pressure outside of the core.

Even so, do you agree that we would expect to see an iron-deficient crust in that scenario due to the increased convection?

[jlhredshift]

Yes, I acknowledged that possibility in my OP. But considering that we don't have those pressures in the Earth's core (maybe very near the center?), I wonder how much more massive a planet would have to be to generate that much pressure outside of the core.

Even so, do you agree that we would expect to see an iron-deficient crust in that scenario due to the increased convection?

Absolutely, our crust is the "slag" from differentiation.

[baric]

Absolutely, our crust is the "slag" from differentiation.

So.... zipping off on a tangent, I wonder what effect this iron-deficiency would have on biological evolution given its importance in hemoglobin on this planet. Could it be that super-Earths are relegated to be nothing more than huge reservoirs of unicellular life, or can trace amounts of Fe support the oxygen-transport mechanisms in larger life forms? Now we need a biologist in this thread!

[jlhredshift]

So.... zipping off on a tangent, I wonder what effect this iron-deficiency would have on biological evolution given its importance in hemoglobin on this planet. Could it be that super-Earths are relegated to be nothing more than huge reservoirs of unicellular life, or can trace amounts of Fe support the oxygen-transport mechanisms in larger life forms? Now we need a biologist in this thread!

There could be iron input from meteorites.

[korjik]

Yes, I acknowledged that possibility in my OP. But considering that we don't have those pressures in the Earth's core (maybe very near the center?), I wonder how much more massive a planet would have to be to generate that much pressure outside of the core.

Even so, do you agree that we would expect to see an iron-deficient crust in that scenario due to the increased convection?

To some degree we do have temps like that in the Earth's core. The inner core is solid due to pressure, not temperature. You also dont have to crystalize the entire core to interrupt the magnetic field generation. If there just isnt enough metallic liquid depth to get a good circulation going, then you arent going to get much field.

It would have to be a pretty big planet tho.

I dont really know enough geology to say, but it wouldnt suprise me if increased convection left more iron in the crust, not less.

[baric]

There could be iron input from meteorites.

ooh, good point. You sound like a panfermia proponent.

I dont really know enough geology to say, but it wouldnt suprise me if increased convection left more iron in the crust, not less.

huh. I always presumed that increased convection would allow the iron to more easily sink to the core over time.

[dgavin]

The assumption in the article is that the pressure is high enough that the core is solid until you get out of the core, leaving you in the silicate mantle before pressures drop enough to allow liquid iron. If that leaves you with a big area of liquid mantle, that still isnt going to generate a magnetic field, cause it is not metallic liquid circulating around.

Thats a big assumption, if you have a super earth with a Iron core thats 75% of it's total mass (using mercury as a model), again thier resoning doesnlt account for it. In that cases like this, there well might be a solid inner core surrounded by a molten outer core.

Or even more trippy a large enough super earth may have a Solid inner core, fluid middle core, and a semi/solid outer core shell.

There are also the other variables mentioned before that could allow super earths to have fluid or partial fluidic cores even at the pressures they based thier papers on.

Also that as a core cools and solidifies(crystalizes), like with earths core, it pushes the heat out of the solid into the remaining fluid around it. I didn;t see core crystalization take into account in thier article either.

Basically the article didn't take into acount many of the variables in core sizes, radiogenic heating, crystalization heat trasfer, post formation impact reheating, etc etc etc.

[korjik]

ooh, good point. You sound like a panfermia proponent.



huh. I always presumed that increased convection would allow the iron to more easily sink to the core over time.

Gravity will take care of the seperation. Convection actually gives a possibility of some enriched material travelling against gravity back to the surface.

[korjik]

Thats a big assumption, if you have a super earth with a Iron core thats 75% of it's total mass (using mercury as a model), again thier resoning doesnlt account for it. In that cases like this, there well might be a solid inner core surrounded by a molten outer core.

Or even more trippy a large enough super earth may have a Solid inner core, fluid middle core, and a semi/solid outer core shell.

There are also the other variables mentioned before that could allow super earths to have fluid or partial fluidic cores even at the pressures they based thier papers on.

Also that as a core cools and solidifies(crystalizes), like with earths core, it pushes the heat out of the solid into the remaining fluid around it. I didn;t see core crystalization take into account in thier article either.

Basically the article didn't take into acount many of the variables in core sizes, radiogenic heating, crystalization heat trasfer, post formation impact reheating, etc etc etc.

A super-earth of the same overall density as Mercury has less Iron, not more. Thing is, this article is pretty much all guesswork. With there being two possible methods of creation for a super-earth (cthonian and terran) which are wildly different in end composition of the planet involved, any thought of what is happening on the planet is just guesswork. For super-earths, our current understanding is roughly analogous to our understanding of the moon and the planets in 1955

[kaikki]

Hi people,
I have a question about exactly the same paragraph.

But at the pressures that exist in a large planet's core, iron can solidify even at temperatures as high as 10,000 Kelvins, according to recent theoretical calculations.These high temperatures are probably exceeded only when the planets are very young. But a little cooling would be sufficient for the cores of super-Earths to solidify.Thus, a typical super-Earth may have a completely solid iron core and no global magnetic field.

Does anyone know what this "recent theoretical calculations" refers to? I think this paragraph comes from a paper that was just published in the past few months, as my supervisor told me. In this paper the cooling rate of liquid cores are estimated and conclusion is reached that the liquid state of cores (and therefore the geodynamo) is short-lived. But I can't find this paper even after much effort. Maybe someone can give me more clue?

Thank you very much in advance!
Warm regards,
kaikki

[Trakar]

With the larger mass of iron, are they presuming the same ratio of radioactives, if so, why?

[willstaruss22]

bigger iron core

[Cosmologist]

A super-earth of the same overall density as Mercury has less Iron, not more. Thing is, this article is pretty much all guesswork. With there being two possible methods of creation for a super-earth (cthonian and terran) which are wildly different in end composition of the planet involved, any thought of what is happening on the planet is just guesswork. For super-earths, our current understanding is roughly analogous to our understanding of the moon and the planets in 1955

Are you discounting the georeactor hypothesis?

http://www.nuclearplanet.com