Science Forums 2

[Pyrotex]

A few thoughts on CharlieO's other questions about origin of Earth's iron core.

There is, indeed, a LOT of iron in our Solar System. Aside from H, He, C, O, Cl, Ar, Si, Na, K, Mg, and possibly a few other elements, iron (Fe) is the most common element around. And it is the MOST COMMON "heavy" element by far. H,He, O, Cl and Ar are gases -- Na, K, and Mg are very light metals, and C and Si are light "transition" elements.

Iron is common in the universe because it is the lightest element that is ENDOTHERMIC under Fusion. That means, all other lighter elements can be fused with H or He and RELEASE energy. If you attempt to fuse Fe with H or He, it ABSORBS energy. All elements heavier than Fe are also Endothermic under Fusion. Why do we care? The lighter elements are created in the cores of ordinary stars as they age, until finally they produce iron. If the star is massive enough, the core will become denser and denser as the Fusion fuels become exhausted, and the pressure will force iron to fuse. This absorbs so much energy that the core collapses, triggering a supernova, which blasts all the outer shells of elements (including iron) out into the surrounding space. Look at a picture of the Crab Nebula.

This is where all the common elements come from. Including iron.

The innermost core of the supernova releases so much energy that the elements heavier than Fe are produced, but only in trace amounts. That's where gold comes from.

When a Sun like ours condenses from primordial gas and dust, there will almost always be a residual spin. As it condenses smaller, the spin increases until the star suffers instabilities at its equator. An equatorial ring of matter forms and cools. Compounds containing heavy metals, like iron, or the metals themselves, will be the first to become solids. We know this because a significant portion of meteorites that crash to Earth contain 95% pure iron/nickel alloy. (I have one.)

When the Sun-like star finally achieves full Fusion at its core, there is a "fusion-flash" that occurs while the star is achieving equilibrium. This flash heats up and drives off the equatorial ring. But selectively. Solids, being denser, will be slower to heat up, and harder to drive off. The proportion of iron, and indeed, of all solid compounds, many of which contain C and Si, will be highly increased in the region nearest the star, and the lighter stuff, especially H, He, and a portion of the C and Si, will be selectively driven further outward until it, too, can cool enough to form molecules, compounds and dust.

So, we have a plausible mechanism whereby the elements and compounds are separated by mass. Very much like a mass spectrometer in a laboratory. Only it's done by the fusion-flash, instead of with a magnetic field. Near the Sun, iron and its compounds can become a significant percentage of all matter -- say 10% to 20%. Far from the Sun, H and He remain the dominant components. In the region in between, you have iron making up only 1% or 2% of the matter and H and He still very significant.

This results in a relatively smooth transition of planet types. Iron core planets near the star, H/He gas giants far from the star. Transition planets in between.

How do the iron planets form? The secret is water. Water, in both liquid and solid forms, makes for an adequate "glue" to bind small dust particles together. Water in vapor form can absorb to the surfaces of particles, helping to cool the particles, and itself. As more and more particles clump, the average size of dust particles increases. This will occur mostly in a zone around the star where water can be in its liquid state, at least part of the time, under the right conditions -- like in the core of a clump of dust particles.

Particles and clumps smash into each other, and break up into finer dust. The finer dust reclumps eventually and smashes again. But this process slowly but surely evens out all the dust orbits, making them almost circular and all in the same plane. By then, some clumps have gotten large enough to hold together by their own gravity.

When a clump is large enough, a planetisimal, its gravity gives it a spherical shape. Its core comes under tremendous pressure. This once again acts like a mass spectrometer! And the various compounds at the core again differentiate. Lighter stuff perculates up. Denser stuff sinks down. The core heats up. At temperatures above 2000 F, compounds of iron give up their C, O, H, Si, S and other lighter elements, leaving only elemental iron. Iron is denser stuff. It sinks down.

Similar processes are going on in the regions of the gas giants and transition planets. Only those planetisimals not only scoop up dust as they whirl around, they also scoop up the lighter gases, H and He. At first, this is easy because they are still so cold that the gas pressure of even He isn't enough to escape a planet with, say, Earth's gravity. They grow fast, becoming gas giants like Jupiter, with a tiny iron/rock core. By "rock", we mean any substance (not the pure metals) that tends to be a solid at temperatures above the freezing point of water and below, say 1000 F. For example, Silicon Dioxide.

By the time the fusion-flash is over, and the sun warms up to its full equilibrium state, most of the gas giants will have enough gravity to hang on to their H and He, even as they warm up a bit.

So, that's where all the iron came from. Even though it is only .0001% of all atoms in the universe, there are (at least) two mechanisms or processes around the newly formed star that tended to act as a mass spectrometer, or a mass differentiator, separating out the heavy stuff from the light. Iron in close orbit to the sun tended to stay there after the fusion-flash, whereas the lighter stuff tended to be driven further away. And the cores of planetisimals, if their temperature got high enough, tended to melt and accumulate iron, and drive the lighter stuff upwards towards the surface.

The Universe is full of wonderful and totally natural processes that separate, differentiate, purify and accumulate matter. Heads up!