SL108: Where are Heavy Elements Made?
David Noel
<davidn@aoi.com.au>
Ben Franklin Centre for Theoretical Research
PO Box 27, Subiaco, WA 6008, Australia.
Heavy Elements
Almost all of the 92 elements of the Periodic Table (Figure F1) can be found occurring naturally on Earth. Chemistry is the study of the interaction of these elements, and geochemistry is mostly concerned with how much of these elements exist in our planet.
Fig. SL108-F1. The Periodic Table of the Elements.
Where and how were all the elements, making up the rocks, seas, and atmosphere of the Earth, formed? This simple and rather fundamental question does not have a readily-available answer which makes sense. Here is what Google says.
"The elements composing Earth's rocks, oceans, and atmosphere were formed primarily within stars and through the Big Bang. The Big Bang created hydrogen, helium, and some lithium, while stars were responsible for fusing smaller atoms into larger ones through nuclear fusion. These heavier elements, including those found in Earth's composition, were then dispersed into space when larger stars exploded as supernovae."
So the conventional idea is that the material currently making up the Earth was formed in its current mix of elements in stars and supernovas. It was then released into space, and collected together (through gravitational aggregation) to form our planet (and other planets). This conventional idea is Completely Wrong, and when looked at closely, can be seen to lack logic and reason.
Fig. SL108-F2. Percentages of Elements in Earth's Crust. From [1].
Figure F2 shows the calculated percentages of elements making up the Earth's crust. Of course these do not include gaseous elements such as Hydrogen and Helium, which are known to make up the biggest fraction of star materials, but what about the other elements shown? If the gaseous elements are subtracted, is the mix of other elements in stars similar to that on Earth?
This question was answered about a hundred ears ago. Here is what it says in [3].
"The chemical composition of the stars. In the early days of astrophysics, scientists thought that the stars were probably similar to the Earth in chemical composition. In the nineteen-twenties, Cecilia Payne studied the spectra of stars, and devised a way to figure out the temperature and true chemical composition of stars. She concluded that the atmospheres of stars were NOT made up of the same mix of elements as the Earth, NOT wildly variable in composition, but in fact, almost entirely hydrogen, in almost all stars.
This was so surprising that scientists ignored or rejected the idea for several years. Eventually, after further study confirmed Payne's work, the astronomical community had to concede that the stars were, in fact, very different from the Earth. They appeared to be made up of 90% hydrogen (by number of atoms), 10% helium, and tiny traces of heavy elements (everything else).
So is there anything to suggest the mix of elements heavier than Hydrogen and Helium in stars is anything like the mix of the same elements on Earth, as in Figure F1? The answer is NO.
It is accepted that in normal stars, fusion of hydrogen into helium and heavier elements takes place, and that each step up to the next heaviest element represents an additional barrier. Theory says that star fusion cannot produce elements heavier than Iron (Fe), so where do the elements heavier than Fe in the Earth's Crust come from?
The only (rather feeble) answer to this, until now, has been that they are produced in supernovas, in a flux of neutrons streaming out from the exploding star. The likelihood of this happening, building up a nucleus such as Uranium (with about 238 nucleons) from lighter elements in an event lasting only a few days or weeks, is insignificant. And if our heavier elements came from supernovas, how and where did they all happen at the time Earth first formed, about 4.7 billion years ago?
There is more explanation of this in "XT804: Heavy Elements are made in Planets, not Stars" [A].
Current theories about what substances exist within the Earth (and how they got there)
Seismology (the study of earthquake waves) can tell us about divisions within the Earth, and about the state (liquid or solid) of material in different parts, but it cannot tell us anything about the chemical makeup of those parts.
No techniques or tests are currently known which can reveal the chemical makeup of deep Earth substances (neutrino diffraction analysis is a possibility, but at present that is just a concept). So at present we can only analyse samples dug up in deep drill holes, and the deepest of these to date goes down to only about 13 kilometres. Everything else is speculative.
Now a new model of the Earth's formation and and inner makeup has been developed, explained in "SL107: What's Inside the Earth and planets?" [B]. This article shows why earlier suggestions that a mix of Iron and Nickel within most of the planet is unlikely to be true, and makes the novel suggestion that the Earth grew up from a tiny ball of compressed neutrons, less than a kilometre across, formed at its initial aggregation from interstellar material.
This is a radically new model, but it does lead to a logical explanation of the Earth's structure, and so should be accepted in the absence of anything better. According to the model, at the outer boundary of the compressed-neutron centre (called the "Earth-Nucleus"), neutrons are decaying into protons and electrons, which recombine to form hydrogen atoms. A hydrogen atom is enormously greater in volume than the neutron from which it could have come (by a factor of 2.875 x 1014, almost 300 million million).
So in the early development of a planet, a thicker and thicker "atmosphere" of hydrogen will build up around the centre, and the pressure which will exist at the bottom of this atmosphere will eventually become great enough to cause some of the hydrogen to fuse into helium, and eventually into even heavier elements. This is the same as the process which is accepted as taking place in stars,
Moreover, at the active site (the "Element Kitchen" above the compressed-neutron Earth-Nucleus), elements are exposed to the adjacent neutrons, and can absorb some of these to build up into heavier and heavier nuclei -- bombardment with neutrons is the standard way to create heavy elements in the laboratory. Some elements will be built up to the point that their nucleus is so heavy as to be unstable, and they will break down, with the emission of more neutrons. The result is that the Element Kitchen is a roiling mass of transmuting elements, the final composition of which depends on the relative stability of the different nuclei. As Iron (Fe) is a particularly stable nucleus, the final mix will contain a good proportion of Iron.
Fig. SL108-F3. Relative stability of different nuclei. From [A].
Figure F3 shows how this stability varies with mass number of elements. Elements lighter than Iron can yield energy if they take part in fusion, elements heavier than iron can yield energy if they undergo fission.
Composition of the Planets
This model of the development of Earth fits quite well with what we know of the other planets in our solar system, if we assume they also started off with compressed-neutron centres which decayed into hydrogen and then led to solid envelopes from element kitchens enclosing these centres.
The four outer planets, the "gas giants" Jupiter, Saturn, Uranus, and Neptune, even till today retain very thick atmospheres, mostly consisting of hydrogen, with some helium. At the bottom of their atmospheres they are believed to hold solid cores, which actively produce heat, and could be compressed-neutron balls.
Mercury, closest to the Sun, has little atmosphere left. Mars, next out from the Sun after Earth, has a little atmosphere left. Venus is too close to the Sun and too hot to retain hydrogen or helium, buy has a dense atmosphere, mostly of relatively heavy carbon dioxide.
The MORB story
Figure F4 shows a conventional picture of how the Earth exists in concentric layers. The outermost layer, the Crust, is the part upon which we tread. and is easily sampled for chemical analysis. All the Crust is re-worked material -- sedimentary rocks fractionated by water (or occasionally by wind), and igneous rocks, partially or completely melted by earth movements and later solidified.
Fig. SL108-F4. Notional cross-section through the Earth. From [A].].
The layer immediately under the Crust is called the Mantle. Because the Crust is relatively thin (0-40 km) and is discontinuous. there are places on Earth (mostly on underwater ranges) where the Mantle is exposed, and can be sampled. The most prominent of these are along the Mid-Ocean Rifts which run down and across our major oceans.
These rifts are sites where new rock is being exposed, welling up from the Mantle at Earth-expansion lines. All this new rock is a type of Basalt, of fairly uniform chemical composition, which has gained the name MORB (Mid-Ocean Rift Basalt). The MORB is essentially the upper surface of the Mantle, and its uniformity implies that the Mantle contains rock of a single chemical makeup.
Fig. SL108-F5. Bulk composition of MORB glass. From [2].
Figure F5 is a table of the chemical composition of MORB. It uses the geochemical convention of presenting metals as their common oxides, rather than as elements as in Figure F2, but the two figures are essentially different views of a similar thing. This is as we might expect, re-working and fractionating a material usually won't affect its chemical composition much.
Where this may not hold is when some of the elements make highly soluble compounds, which may be leached out and lost during re-working, as with Sodium (Na) and Potassium (K). Something often neglected in considering composition of rocks is how much water they hold.
Water in rocks
Water can exist in rocks in two forms, either bonded with the rock material (called its hydrated form) or loose, between rock particles. Analyses of rocks usually ignore both forms, and the presence of water in deep rocks is not much thought about.
So it was something of a surprise when the Russian exploratory borehole which reached around 13 km deep found that the rocks were saturated with water at every depth. However, this might be expected with the present model, as hydrogen and oxygen would combine readily whenever quantities of both were available.
What's inside the Earth
If the Concore Model holds, the expectation is that at the very centre of our planet there is a tiny Earth-Nucleus consisting of compressed neutrons. Outside this, in the Core or Mesolayer, is a very active area (the Element Kitchen) where neutron decay and nuclear transmutation occur. Above this, and stretching all the way up to the top of the Mantle, are volumes of similar chemical composition subjected to mixing and melting as the planet contorts to accommodate the expansionary pressures from within, caused by the neutron/hydrogen mega-expansion.
If, in fact, the chemical compositions of the Core, the Mesolayer, and the Mantle are the same, this implies that the divisions between them found by seismological studies would have to be due to differences in state (phase changes and liquid/solid behaviour).
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AOI articles with relevant evidence
[A]. XT804: Heavy Elements are made in Planets, not Stars..
[B]. SL107: What's Inside the Earth and planets? .
[C]. XT807: The Concore Model of planet and star interiors.
References and links
[1]. Magma and How It Forms. https://openoregon.pressbooks.pub/earthscience/chapter/4-1-magma-and-how-it-forms/ .
[2] Oliver Thomas Lord. Bulk composition of the MORB glass. https://www.researchgate.net/publication/232840081 .
[3]. Michael Richmond. The Chemical Composition of Stars and the Universe. http://spiff.rit.edu/classes/phys240/lectures/elements/elements.html .
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SL108 Commenced writing 2025 May 6. First version 1.0 on Web 2025 May 15.
