XT804: Heavy Elements are made in Planets, not Stars
"XT804: Heavy Elements are made in Planets, not Stars" is an extract from an article about the structure of the Earth. It gives evidence and logic supporting the view that heavy elements (those with atomic weights greater than that of Iron) are made within planets, at the margins of neutron-rich planetary cores.
Over millions of years, elements in this layer are subjected to continuing bombardment of neutrons from the core, which gradually build up the atomic weights of the affected elements. For more background and explanation, refer to the full article, Inside The Earth -- The Heartfire Model.
About heavy elements
We are familiar with the idea that a star gets its energy from combining hydrogen atoms into nuclei of heavier elements, such as helium, carbon, and oxygen.
Stars cannot use this process to make very heavy atoms. Different sorts of atoms contain different amounts of binding energy, and if you plot binding energy against atomic weight, you get a characteristic curve with a hump in the middle.
Figure ITE17. Fission and fusion can yield energy. From .
Normal star processes can only produce elements as heavy as iron (Fe), using their processes of fusing atoms together. Human activities can so far only mimic the simplest of these processes on a large scale, fusing hydrogen together into heavier products, as on the left of the curve. This is the source of the energy in hydrogen bombs, and the target (so far not achieved) of producing nuclear energy from hydrogen to supply power utilities.
We also know how to synthesize heavy elements on a laboratory scale, usually by bombarding nuclei of somewhat lighter elements with neutrons. In this way, tiny amounts of elements heavier than uranium, the heaviest natural element, can be made. All have very short half-lives, sometimes much less than a second.
Figure ITE18. Postulated 'fusion shells' in a massive, evolved star just prior to core collapse. Not to scale. From .
Figure ITE18 is a graphic of postulated 'fusion shells', produced in a star which has completed the process of generating elements up to iron (Fe). While only a graphic, and noted as 'not to scale', it does illustrate one of the fallacies often found in ideas about stellar and planetary objects -- that heavier elements will sink to the centre.
In fact, even assuming that parts of a 'fluid' will somehow separate out by density, lighter items in fluids move towards zones of lesser gravity. But since gravitational forces decrease towards the centre of a star (zero at the centre), lighter items should migrate towards the centre of a star, rather than the surface. Only pressures are higher towards the centre.
Present-day nuclear power stations get their energy from nuclear fission, using naturally-occurring elements on the right-hand side of the fusion/fission chart, such as uranium. These elements break down spontaneously, giving out energy. Each isotope has its own well-defined half-life, the time that half a given mass of the element breaks down.
The half-life of the most common isotope of uranium, U-238, is about 4500 million years -- very close to the believed age of the Earth itself. It is this isotope that is predominantly used in nuclear power stations.
So if the usual star processes can only make elements as heavy as iron, what is the source in the Universe of heavier elements, like uranium?
Making heavy elements -- Supernovas?
Where heavy elements are made is a question to which science has, till now, not provided a very satisfying answer. The usual explanation is that heavy elements are created during Supernova explosions. But if you google "Why do they think heavy elements come from supernovas?", the explanations given, in my view, lack conviction.
Supernovas are stars which explode dramatically, giving out huge amounts of energy over a short time, maybe a few weeks. We know quite a lot about them. Here are some extracts from the Wikipedia article .
"Supernovas are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months. During this short interval a supernova can radiate as much energy as the Sun is expected to emit over its entire life span.
The explosion expels much or all of a star's material at a velocity of up to 30,000 km/s (10% of the speed of light), driving a shock wave into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.
The word supernova was coined by Fritz Zwicky and Walter Baade in 1931. Supernovas can be triggered in one of two ways: by the sudden re-ignition of nuclear fusion in a degenerate star; or by the collapse of the core of a massive star. Although no supernova has been observed in the Milky Way since 1604, supernova remnants indicate that on average the event occurs about once every 50 years in the Milky Way. They play a significant role in enriching the interstellar medium with higher mass elements."
So when a supernova blows itself apart, the part blown out into space, the 'supernova remnant', is claimed to contain heavier elements. The part left behind is a neutron star, consisting almost entirely of neutrons.
It's a logical deduction, then, that the star which exploded had a sphere of neutrons at its heart. It seems very possible that every star has a neutron core. This is not the accepted position, in fact there is no accepted position. If you google 'What's at the centre of the Sun?', you will not find any definite answer.
An associated Wikipedia article, "Supernova nucleosynthesis", deals with the supposed source of heavy elements.
"Supernovas are a key source of elements heavier than oxygen. These elements are produced by nuclear fusion (for iron-56 and lighter elements), and by nucleosynthesis during the supernova explosion for elements heavier than iron. Supernovas are the most likely, although not undisputed, candidate sites for the r-process, which is a rapid form of nucleosynthesis that occurs under conditions of high temperature and high density of neutrons. The reactions produce highly unstable nuclei that are rich in neutrons. These forms are unstable and rapidly beta-decay into more stable forms.
The r-process reaction, which is likely to occur in type II supernovae, produces about half of all the element abundance beyond iron, including plutonium and uranium. The only other major competing process for producing elements heavier than iron is the s-process in large, old red giant stars, which produces these elements much more slowly, and which cannot produce elements heavier than lead".
What's dubious about the idea of supernovas making heavier elements?
The conventional theory, then, is that elements heavier than iron are made during supernova explosions, through masses of neutrons impacting on the material ejected by the explosion. This ejected material is then spread through surrounding galactic areas. It's usually called interstellar gas and dust, and it is this material which supposedly aggregates together to form planets and solar systems.
Although it is quite reasonable that heavy neutron bombardment will raise the atomic weights of nuclei hit by them, when you look at the supposed mechanisms and timings, it doesn't look so good.
First off, all the transmutations involved would have to occur within the active period of the supernova, only a few weeks. Most of the element-forming processes in stars take place much more slowly, over millions to billions of years.
Then there is the number of steps involved. The heaviest common isotope of iron has 84 nucleons (protons or neutrons) in its nucleus. The commonest isotope of uranium has 238 nucleons. Therefore, to make uranium from iron would involve at least 154 successful captures of neutrons for each nucleus. This, all over a period of a few weeks.
How was the material to make Earth gathered together?
According to conventional theory, the Earth and the rest of the Solar System was formed by aggregation of interstellar gas and dust. Most of the hydrogen went to the growing Sun, while the planets used up much of the non-gaseous dust. The outer 'gas-giant' planets also retained a lot of hydrogen, and its derivatives such as methane, in their atmospheres.
There is scope for a radical revision of this picture , but that need not concern us here. Let's look at some of the implications of the current picture, using evidence from meteorites, solid rocky or metallic objects which fall to Earth from space. All the evidence supports the idea that meteorites were formed at the same time as the Earth, elsewhere within the developing Solar System, and chance collisions and orbital changes have since caused some of them to hit our planet.
Meteorites vary very considerably in their nature and composition. Accurate methods have been developed to determine the ages of meteorites and rocks. The oldest-known meteorite to date is called the Allende Meteorite. This is believed to have broken up in Earth's atmosphere over Mexico, in February 1969.
Figure ITE19. Section through an Allende meteorite. The cube on the right has 1-centimetre sides. From .
The Allende meteorite, a type called a carbonaceous chondrite, is notable for possessing abundant, large calcium-aluminium-rich inclusions [CAIs], which are among the oldest objects formed in the Solar System . Here is more from the Wikipedia article.
"Allende contains chondrules and CAIs that are estimated to be 4.567 billion years old, the oldest known matter. This material is 30 million years older than the Earth and 287 million years older than the oldest rock known on Earth. Thus, the Allende meteorite has revealed information about conditions prevailing during the early formation of our solar system.
The meteorite was formed from nebular dust and gas during the early formation of the solar system. It is a "stone" meteorite, as opposed to an "iron," or "stony iron," the other two general classes of meteorite. Most Allende stones are covered, in part or in whole, by a black, shiny crust created as the stone descended at great speed through the atmosphere as it was falling towards the earth from space.
When an Allende stone is sawed into two pieces and the surface is polished, the structure in the interior can be examined. This reveals a dark matrix embedded throughout with mm-sized, lighter-colored chondrules, tiny stony spherules found only in meteorites and not in earth rock. Also seen are white inclusions, up to several cm in size, ranging in shape from spherical to highly irregular or "amoeboidal." These are known as calcium-aluminum-rich inclusions, so named because they are dominantly composed of calcium- and aluminum-rich silicate and oxide minerals.
Like many chondrites, Allende is a breccia, and contains many dark-colored clasts or "dark inclusions" which have a chondritic structure that is distinct from the rest of the meteorite. Unlike many other chondrites, Allende is almost completely lacking in Fe-Ni metal."
When you look at it, this evidence is totally in conflict with the idea that the Earth, and the other rocky bodies of the Solar System, formed from aggregating interstellar dust and gas originating from supernova explosions. How could such dust, necessarily fairly uniform in composition, come together to form a mixed pudding like the Allende meteor? How could all the material known on Earth or from space have the same age, of around 4.6 billion years? How could it all be derived from a single supernova explosion of this age?
Then there is the matter of the percentage composition of heavy elements such as uranium in meteorites and in the Earth's crust. According to , many analyses have been made of the uranium in the rocks forming the continental and oceanic crusts, and in samples of the Earth's mantle [assumed to be MORB]. The article goes to say:
"We can have some confidence that these measurements are robust for the crust and upper mantle of the Earth, but less confidence that we know the abundance of uranium in the lower mantle and the outer and inner cores. While on average the abundance of uranium in meteorites is about 0.008 parts per million, the abundance of uranium in the Earth's 'primitive mantle' - prior to the extraction of the continental crust - is 0.021 ppm.
Allowing for the extraction of a core-forming iron-nickel alloy with no uranium (because of the characteristic of uranium which makes it combine more readily with minerals in crustal rocks rather than iron-rich ones), this still represents a roughly twofold enrichment in the materials forming the proto-Earth compared with average meteoritic materials.
The present-day abundance of uranium in the 'depleted' mantle exposed on the ocean floor is about 0.004 ppm. The continental crust, on the other hand, is relatively enriched in uranium at some 1.4 ppm. This represents a 70-fold enrichment compared with the primitive mantle. In fact, the uranium lost from the 'depleted' oceanic mantle is mostly sequestered in the continental crust".
How can we reconcile these conflicting views? It's not new to suggest that both meteorites and other planetary bodies may have been subject to catastrophic processes in the past, even the break-up of a planet which once orbited in what is now the Asteroid Belt.
The widely varied nature of meteorites is readily understandable if they are fragments or re-workings of larger planetary bodies with histories similar to that implied by the Heartfire Model. If a proto-planet broke apart catastrophically, material from its Mesolayer might well provide the wide variety of meteorite varieties, including types not found on Earth's surface.
Formation of heavy elements in the Mesolayer
The Heartfire Model of the Earth assumes a central Core rich in compacted neutrons, which are stable when surrounded by similar highly-compacted matter. At the outer surfaces of the Core, density is abruptly much less, and some decay of neutrons into hydrogen atoms can take place.
Figure ITE21. The Heartfire Model of the structure of the Earth.
The inner surface of the Mesolayer, the next layer out from the Core, is made up of normal atoms. These are subject to prolonged impact of neutrons escaping from the Core, which will transmute the inner Mesolayer nuclei to heavier and heavier elements -- neutron bombardment is a standard way of creating heavier elements.
Very gradually, perhaps over millions of years, the Core/Mesolayer boundary will move inwards towards the centre as Core material is converted over into Mesolayer material. This is ordinary matter which has been heavily irradiated with neutrons and achieved a binding-energy balance as in Figure ITE17.
If the Mesolayer is, as suggested, a place where great pressures are applied to a supercritical fluid facing a neutron source, it could very conceivably generate MORB-like solid rock at its boundary. The product would essentially be the result of prolonged bombardment by neutrons of hydrogen atoms and the heavier elements formed in the process. It would be an equilibrium mix, with elements right up to Uranium formed and breaking down, until such time as the Core/Mesolayer boundary had moved inward and was no longer supplying a high neutron flux.
* * * * * * * * * * * * * * * * * * * * *
References and Links
25. Nuclear Binding Energy. http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html .
26. Supernova http://en.wikipedia.org/wiki/Supernova .
27. Evolved star fusion shells. http://en.wikipedia.org/wiki/File:Evolved_star_fusion_shells.svg .
28. The Cosmic Origins of Uranium. http://www.world-nuclear.org/info/inf78.html .
29. Allende meteorite. http://en.wikipedia.org/wiki/Allende_meteorite .
30. David Noel. The Cosmic Smog Model For Solar System Formation. In preparation.
* * * * * * * * * * * * * * * * * * * * * * * *
The above is an extract from Inside The Earth -- The Heartfire Model.
Latest version on Web, 2017 Jan 15