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The first published theory was due to Descartes ( although published long after his death, in 1664, apparently due to fears of being accused of heresy like his contemporary Galileo ). It was an early forerunner of the Planet Capture theory, championed by Thomas Jefferson Jackson See at the beginning of the Twentieth Century.
In 1878, George Howard Darwin, son of Charles, of Evolution theory fame, proposed the first major theory; that an early rapidly spinning Earth became deformed by the Sun's gravity and flung off a chunk of itself to become the proto-Moon, the so-called Fission theory.
When a few years later, the geologist Osmond Fisher suggested that the Pacific Basin may have been the site from which the satellite was torn away, the theory caught the popular imagination and was enshrined in text-books well into the Thirties of the Twentieth Century, although the Fisher addition to the Fission theory soon suffered from the development of Plate Tectonics which gave a more plausible explanation for the Pacific Basin.
The third major competing theory to be developed, was the so-called Co-accretion theory, whereby the Earth and Moon formed in the same region but independently from the same primeval material from which the whole Solar System had formed.
The three competing theories finally succumbed, around 1970, to the diverse findings from the results of the Apollo missions. Indeed it was one of the major aims of these missions to settle once and for all, by actually setting foot on the Moon and collecting on-the-spot rock and soil samples and other data, many outstanding questions, and ultimately to elucidate the origin of our natural satellite.
So what then are the findings which must be incorporated in a successful theory?
Capture of a planet from elsewhere failed because lunar rocks have the same oxygen isotope composition as Earth.
Dynamical analysis also indicated that capture was an unlikely event.
The Fission theory of Darwin failed because analysis of angular momentum and energy requirements showed that it was impossible for the present Earth/Moon system to form in this way.
A new theory was necessary, and the Giant Impact Hypothesis was born.
For these reasons I believe that the Giant Impact Hypothesis does not measure up, and perhaps a more radical approach is necessary.
Basing my reasoning on this conclusion I have formulated a scenario which will be thought by many to be too radical no doubt, yet if a certain suspension of belief in some preconceived opinions is accepted, it provides an integrated explanation for all the relevant observations.
If the formation of a moon, the size of ours, was the outcome of an evolutionary process, one would expect to find other planets in the Solar System with similarly large moons. However since this is not observed, and our moon appears to be unique, it is justifiable to look for even a possibly catastrophic solution, as of course was also done in formulating the Giant Impact Hypothesis.
It is generally accepted that a massive bombardment took place during the late phase of planetary formation, and I assume that this was proceeding during the time of the Moon's formation, which as mentioned above, has been accurately dated from the Apollo Moon rocks to about 50 million years following the Earth's formation.
What constituted this early bombardment?
The general opinion is that this question is answered by examination of meteoritic debris which is supposed to represent primordial matter from the solar disc from which accretion of all the planets took place.
Examination of this remnant material, collected over time as fallout on to our planet's surface, has indicated a dearth of the high atomic weight elements, ( higher than iron in the Periodic Table ), in a reduced metallic state, and in particular of metallic, naturally occurring actinides. This is considered to be representative of the primordial condition.
Looking back 4.6 billion years into the past, such assumptions must be considered to be problematical, and at least open to reasonable doubt.
It is here that I ask for the aforementioned suspension of belief.
I have previously suggested in my theory of the Earth's magnetic field that the primordial bombardment contained not only the smaller lighter meteoritic bodies observed today, but also many larger bodies, some of which were metallic or contained metallic cores, in some cases made up of highly concentrated radioactive heavy elements such as the naturally occurring actinide, uranium for example.
For the present I am making this as an assumption, but I intend to justify it in considerable detail in a later publication.
I suggest that the reason that we do not observe these earlier bolides is that, due to their denser constitution, they were preferentially scavenged gravitationally by the growing planets.
This is precisely what I am now proposing happened in the case of the proto-Earth late in its development, but in this case the outcome was unusually catastrophic.
Here I will introduce a slight digression, the purpose of which will soon become obvious.
My major thesis in my theory of terrestrial magnetism is that an enriched kernel of uranium metal exists in the inner core of the Earth, and its radioactive decay provides the engine for the production of the magnetic field.
It has been discovered at Oklo in Gabon in Africa that about 1.8 billion years ago, at least seventeen natural self-controlled nuclear fission reactors operated, on and off for millions of years, and even acted as self-sustaining breeder reactors before eventually shutting down because of loss of water as moderator, due to climate change.
Nuclear reactors using uranium as fuel require a certain minimum ratio of U235 to U238 in order to achieve criticality, and of course a sufficiently large and concentrated body of uranium ore.
1.8 billion years ago all the required conditions existed at Oklo. In particular the ratio U235/U238 was in excess of about 0.03 which has been found by modern reactor technology to be sufficient. However due to the faster decay rate of U235 compared to U238, the ratio dropped below the critical value, consequently for the bulk of the 1.8 billion years following, no natural reactors have been possible.
At the present time the naturally occurring ratio has reduced to 0.0072, so it has been necessary to develop techniques to concentrate U235 to achieve criticality.
Before 1.8 billion years ago the ratio was far higher than necessary, but apparently oxygen was needed to concentrate the uranium in the strata, and oxygen only began to build up sufficiently in the atmosphere to enable this concentrating process to occur, at about 2 billion years ago. If natural reactors existed prior to the presence of oxygen they would have needed another method under possibly reducing conditions. It is conceivable that such processes may yet be discovered and even earlier reactors found. Extrapolating back to 4.6 billion years ago, the ratio was far higher than the minimum required for criticality.
The point of this digression is to raise the question of criticality in the putative uranium metal bolides, and even in the kernel of the Earth itself.
While criticality may have been theoretically possible under some early conditions, the lack of adequate neutron moderators, and/or the presence of neutron absorbers, may have hampered its achievement or continuation.
In the event of criticality occurring in the kernel of the early Earth, a shut-down would soon be expected, due to build up of neutron absorption by fission and radioactive decay products unable to diffuse away within the matrix of the inner core and its kernel, which is maintained as a dense solid by the enormous pressure at the Earth's center.
On the other hand, in the case of small heavy metal bolides travelling through space, the intense cold would remove radioactively generated heat quickly enough to maintain it in a solid state, once again preventing the diffusional removal of perhaps any pre-existing neutron absorbers or radioactive decay products.
Combined with the lack of an adequate neutron moderator, achievement of criticality in these bodies would not be expected even with highly concentrated uranium.
However should one of the radioactive bolides have sufficient energy on impact with the proto-Earth to pierce any developing crust and become incorporated into the outer mantle, everything changes.
Shortly thereafter, should the impacting object be of sufficient size, ( of the order of tens of kilometers in diameter ), large amounts of radioactively produced heat would accumulate in the vicinity, adding to the mantle's own intrinsic heat, and cause liquefaction of the mantle.
The higher density bolide would begin to sink deeper into the planet's mantle, into regions of higher and higher temperature.
The radioactive heat generated is unable to escape, and builds up rapidly in the vicinity, as the object continues to sink towards the core/mantle boundary, under the action of gravity and differential density.
Long before reaching the boundary, the temperature of the object's core rises above the melting point of uranium. In the molten state the sinking process allows a differential separation process to occur based on the different atomic weights of the radioactive isotopes. The denser U238 separates from the lower atomic weight U235, resulting eventually in a highly purified region of fissionable U235.
Earlier in the same process, any neutron absorbing, lighter weight fission products which may have been present in the object during its passage through space, would have been left behind, leaving the bolide's kernel in a potentially highly unstable nuclear condition, far exceeding the minimum critical mass required for a nuclear explosion.
Except for the absence of an adequate moderator to thermalize high energy neutrons produced from self-fissioning processes, the kernel of the bolide material would now be potentially an incredibly powerful atomic bomb.
However this absence would soon be rectified.
For 50 million years the Earth's own kernel had been producing helium from radioactive decay, and the helium had been diffusing outwards. Helium is an excellent neutron moderator and its concentration would be higher at greater depths. Consequently the lower the bolide kernel sinks, the more likely it is to encounter concentrated amounts of helium, sufficient to act as a neutron thermalizing agent.
Is there a neutron producing source able to cause initiation?
U238 itself undergoes spontaneous fission at a slow but nevertheless significant rate, producing high energy neutrons. Of themselves they would be unable to cause nuclear initiation, but in the presence of helium their energies could be lowered sufficiently to produce a chain reaction.
Also other intrinsic neutron sources exist; for example B11 emits neutrons by an alpha-neutron reaction to become N14; Be9 undergoes an alpha-neutron reaction to become C12, and also a gamma-neutron reaction from sufficiently high energy gamma radiation to become Be8. Should small concentrations of these elements be present in the surroundings, allowing the unstable kernel to come into close enough contact, alpha and gamma radiation produced in the kernel could conceivably be capable of causing initiation.
Whatever the specific mechanism, at some point below the core/mantle boundary, conditions were produced, sufficient to raise the Fermi k constant far above unity, and a chain reaction was initiated.
I say at some point below the core/mantle boundary for reasons which will become clear later.
Because of the immense pressure under which the reaction occurred, deep within the Earth, explosive separation of the reacting atoms was hindered, preventing shut-down. Neutron production continued to increase exponentially, feeding the chain reaction, until an unimaginable explosion commenced. Incredible temperature was developed, further liquefying an enormous volume of the Earth's outer core and mantle.
In a mind boggling explosion the planet was rent asumder as a chunk of its outer core with the overlying mantle was ejected into space.
The giant projectile thus produced was soon to reform into a spherical shape to become our natural satellite.
The Moon is thought to contain a small iron core of approximately 3% of its mass. Until now it has been difficult to reconcile this with the iron core of the Earth which is approximately 30% of its mass.
In the present scenario this no longer presents a problem, since all that is necessary to explain the apparent discrepancy is to suppose that the nuclear blast was initiated within the outer layers of the metallic iron/nickel outer core of the Earth, and that a small portion of the Earth's outer core was incorporated into the partially molten projectile which was to become the Moon.
During the subsequent sphericalization the dense iron/nickel material sank within the Moon, under the action of gravity and differential density, before it cooled.
Some feeling for the relative magnitude of the event can be obtained from a rough calculation, as follows:
The Moon's core is 3% of its mass.
Assuming the Moon's core to be mainly iron and nickel, with perhaps some small amount of uranium from the bolide, a value of 10000 kg per cubic meter would be a conservative guess at its density, given a small increase due to pressure at depth.
Mass of the Moon is 7.36E22 kg ( using Hewlett Packard nomenclature )
Therefore the volume of the Moon's core is 2.21E17 cubic meter
The diameter of an assumed spherical core is approximately 750 km
The nuclear chain reaction must then have been initiated at least this approximate distance below the core/mantle boundary of the Earth to have a chance of incorporating the required volume of iron/nickel core in the Moon.
Assuming that the chunk torn out of the Earth was cone shaped with its apex at this distance below the core/mantle boundary, the cone height would be 3.65E6 m.
Taking the average density of the mantle to be 3500 kg per cubic meter, the diameter of the cone's base at the surface of the Earth can be calculated.
The result is about 4800 km.
For comparison, something like 4 or 5 circles of this diameter would fit onto the Pacific Ocean Basin.
This is a very rough estimate of course, for illustrative purposes only.
The Moon can thus be regarded as a massive projectile ejected vertically outwards from the Earth. At the moment of detachment from the mother planet it received the tangential velocity of the Earth, and commenced a spiralling trajectory.
It would seem most likely that centrifugal force would have projected the Moon roughly equatorially from the Earth, which brings me to the matter of the plane of the Moon's orbit about the Earth.
That plane lies at approximately 5 degrees out of the ecliptic plane, in which the vast bulk of the solar planets revolve, which may indicate that at the time of its formation, the Earth's axis of rotation may have been inclined to the vertical to the ecliptic plane by this amount.
Since it seems most likely that all the major planetary bodies were created at the origin of the Solar System with their rotation axes perpendicular to the ecliptic plane, ( and all rotating on their axes in the same sense, and revolving in the same sense about the Sun ), it would appear that the Earth had already suffered a collision during the massive bombardment process which was sufficient to displace its angle of axial rotation by this amount.
Of course the present angle of the Earth's rotation axis is now at approximately 23.5 degrees out of vertical to the ecliptic plane, indicating that a rather major impact occurred at some time after the Moon's formation, without however affecting the original lunar orbital plane.
Due to the strong gravitational interaction which persisted after the Moon's ejection, and the molten state of the new satellite, the Moon's shape would have been distorted from spherical for a long period due to massive tidal forces as it spiralled away, and it is likely that from the very beginning, the same side of the Moon was locked into the Earth's gravitational field.
As indicated from the results of the Apollo missions, even today the Moon is asymmetrical, with its center of gravity displaced towards the Earth, a much thicker crust on the rear side made up of mountainous, less dense anorthositic material, and on the side perpetually facing the Earth, dense volcanic mares, with their mysterious gravitational anomalies known as mascons, underlying them.
Is this not precisely what would be expected to result from a giant nuclear explosion, such as I have described, occurring deep within the mother planet?
Does the near side of the Moon resemble the Earth's deep mantle and outer core, while the far side is more akin to the Earth's outer core and crust, allowing of course for differential effects from the nuclear explosion?
The lack of water and other volatiles is only to be expected from the high temperature generated in the ejection process.
The healing of the gigantic wound in the Earth must have been a spectacular event, as the semi-molten or plastic magma in the sides of the enormous scar flowed together under the action of gravity, to become again an equilibrated sphere.
The primeval crust must have cracked and shattered as it adjusted to a slightly smaller Earth's surface, and even at this early stage the event may have laid the basis for the mighty fault cracking which today runs completely around the globe on the ocean beds, like the seam on a baseball, giving rise to much associated volcanism.
Should the crust have been cool enough at this early time to allow a watery sphere to exist, vast clouds of superheated steam must have dominated the atmosphere of the globe and been driven off into space.
The initial orbital component of the Moon's subsequent spiral path is merely the tangential velocity of the Earth's surface itself, and would not be sufficient to place the new satellite in a stable Keplerian orbit, consequently the new body is simply a projectile.
The fate of projectiles is two-fold. If it has been given sufficient kinetic energy to reach what is referred to as escape velocity it will escape entirely from the Earth's gravitational field never to return, moving away until perhaps it is trapped in some other planet's field, falls into the Sun, or even wanders off into interstellar space.
On the other hand, if escape velocity is not reached, it must slow down until it reaches its apogee and stops, only to inexorably return to the Earth, reuniting with it at the same velocity with which it started.
Thus on this scenario, part of the challenging question I posed at the beginning of this article is disposed of; the Moon is not in a stable Keplerian orbit and is doomed either to crash back into its mother planet in the distant future, with equally devastating effects to those of its birth, or to disappear eventually from our gravitational field forever.
However since radar ranging results indicate that the Moon is still receding at a very slow rate after 4.6 billion years, the Earth has at least that amount of time before any such cataclysmic reunion will occur, and it is likely that the inner Solar System will have been destroyed before this, by the expansion of the Sun as a Red Giant in its own death throes.
A major plus for the nuclear scenario is that the Moon was generated as one huge chunk, thus eliminating the need to deal with tens of thousands of fragments, followed by an unlikely orbital accretion process, which is inescapable on the Giant Impact theory.
On separation from the Earth, angular momentum is conserved, as some is transferred from the Earth's axial rotation to become largely orbital angular momentum of the Moon and a small amount of rotational angular momentum, since it rotates about its own axis in order to maintain the orientation of the same face to the Earth.
Angular momentum is rigidly conserved in the Earth/Moon system in the absence of any dissipation, and its distribution varies as the Moon spirals further away from its parent. However due to tidal effects of each body on the other, a small amount of dissipation does occur. Bulges are produced on each body by the other's gravitational attraction causing frictional dissipation. The bulges are not obvious in solid matter, although they are nevertheless measurable. Of course the most obvious effect is the production of ocean tides on the Earth due to the rotation of the Earth beneath the Moon every 24 hours. Friction occurs between the moving water and the ocean floor causing dissipation.
Due to the its rotation the bulges on the Earth move away from the center-to-center line between the two bodies and exert their own subtle effects.
To disentangle all these effects would require complex and sophisticated mathematical treatment, the results of which, in any case, would undoubtedly remain problematical due to the inherent complexities of the actual system.
For the present purposes I will consider such effects to be negligible.
Setting 1/2mv^2 = GMm ( 1/R - 1/r )
where
m is the mass of the present moon
M is the mass of the present Earth
R is the radius of the present Earth
r is the average orbital radius of the present Moon
v is the initial vertical velocity of the Moon,
one obtains the kinetic energy required to be 4.53E30 J, which gives the initial velocity v, to be 11.09 km per second.
This figure is just less than, but very close to the escape velocity,
( 11.2 km per second ), of a projectile from the Earth's surface.
The rate at which the Moon is receding is given by radar ranging measurements as about 4 cm per year. This is so slow that it can possibly be accounted for by the, above mentioned, subtle interplay of the tidal bulges on the Earth produced by the Moon's gravitational field in conjunction with rotation of the Earth, which causes the bulges to be off the Earth/Moon line-of-centers. This effect can lead to a very slight increase in the orbital distance over time and a slowing of the Earth's axial rotation rate.
It will halt only when the Earth's rotation has slowed to the point where both bodies are gravitationally locked, and present the same faces to each other.
In view of the very low rate of retreat, and the finding that the initial ejection velocity was slightly lower than the escape velocity, it would appear to be most likely that the Moon is virtually at its apogee as a projectile, and will soon, ( geologically speaking ), begin its inexorable fall back to its mother planet - a disaster, fortunately not to occur, for at least another 4.6 billion years. From our point of view the question of whether the Moon is leaving us, or is going to return, is merely of academic interest.
The fission energy of U235 is 200 MeV or 3.204E-11 J
All fission energy produced is available as kinetic energy for the Moon boost
The density of U235 is 1.91E4 kg per cubic meter.
Under these assumptions the mass of U235 required to produce 4.53E30 J is 5.52E16 kg.
While bodies of the size of this meteorite may be considered huge by modern standards, this should not be regarded as a guide to that of objects during the time of the early massive bombardment, nor is the average composition of present day meteorites to be regarded as necessarily representative of that of the meteoritic population extant during the primordial period.
Harking back to my request at the start, for a temporary suspension of belief in some entrenched wisdom, I intend to justify theoretically the various assumptions I have made, in a future publication.
Unfortumately the subject matter is complex and would take me too far afield to attempt to develop it adequately at this point.
Although, as I have indicated throughout, I make no claims for high accuracy in my calculations considering the wealth of approximations and assumptions involved, nevertheless it is rather gratifying to get so satisfactorily into the ball-park, so to speak.
| Mercury | 2.94E6 |
| Venus | 4.38E6 |
| Earth ( alone ) | 1.18E9 |
| Earth plus Moon | 3.94E11 |
| Mars | 3.27E8 |
| Jupiter | 3.62E11 |
| Saturn | 2.46E11 |
| Uranus | 4.26E10 |
| Neptune | 2.67E10 |
It appears from the above figures that the angular momentum per unit mass of the Earth/Moon system is quite different from, and much larger than, the other inner so-called rocky planets, Mercury, Venus and Mars, although rather less so in the case of Mars.
On the other hand, the Earth/Moon combination, Jupiter, Saturn, Uranus, and Neptune are much more comparable.
On these grounds at least, the Earth/Moon system seems more akin to the gas giants, and therefore, rather than make an issue of the larger size of the Earth/Moon system, perhaps the problem should be redirected to the question of why the angular momentum per unit mass of Mercury and Venus are so small relative to the rest of the Solar System planets?
Rather have these planets somehow lost their primordial share, bestowed on the matter of the Solar System at the time of its coming into being?
Due to the fortuitous circumstance that the power of the nuclear event was very close to, but did not exceed that necessary to enable escape, the Earth was bequeathed a very long lived satellite - more than long enough to be appreciated by the first sentient beings to appear on the planet, 4.6 billion years after its tumultuous appearance.
First developed, November 2002
First published on the Web: January 31, 2003
Since publishing the above theory I have devoted further thought to the details of the processes required to produce the nuclear explosion postulated for blasting the Moon out of the proto-Earth into orbit.
Rather than proposing that the nuclear explosion was produced by a slow neutron moderated chain reaction as I previously suggested, I now believe that a fast neutron reaction, incorporating a nuclear fuel breeding process, is capable of providing a more satisfactory sequence of events.
I therefore wish to propose that the bolide containing the uranium kernel was incorporated during the accretion process of the Earth at the position where the core/mantle boundary was eventually to form.
During its passage through cold interplanetary space I assume that neutron absorbing poisons were present from past radioactive decay or fission in quantities sufficient to stall temporarily any chain reaction, even though the ratio of U235 to U238 would have been far more than sufficient for criticality in the absence of neutron absorbers.
Since conditions of microgravity close to zero would prevail at the kernel, diffusion of neutron poisons could only occur, if at all, along a self concentration gradient, ( something which seems in itself to be unlikely to exist ), because without gravity, buoyancy and convection effects would be inoperative, even if internal temperature were raised due to the normal radioactive decay of uranium.
No one can say for sure how long the accretion of the Earth's mantle lasted, but during this period, vast amounts of matter were added to the planet, no doubt on a scale ranging from minute particles to enormous chunks, as judged by the visible craters remaining on planets, natural satellites, and asteroids.
The accretion of the mantle would undoubtedly have been accompanied by a great increase of temperature, due to gravitational and kinetic energy release, resulting in melting and slumping, causing huge mantle-quakes.
Our bolide, buried to great depth, would have undergone these effects. The kernel would have melted due to both the environmental surroundings and to its own radioactively generated heat, largely trapped within.
Once incorporated at the core/mantle boundary, the molten kernel would be subjected to gravity, allowing the process of diffusion due to differential density to come into play.
Neutron absorbers produced by fission, are roughly half the atomic mass of uranium, and would therefore tend to rise through the molten kernel, leaving a relatively purified region behind.
Due to the immense pressure, diffusion at first would be extremely slow, almost comparable to diffusion through solids, proceeding on a timescale of possibly millenia or greater, but nevertheless increasing inexorably in rate as the surrounding temperature rose.
Eventually however, the neutron absorber concentration would have dropped in the cleared region as the poisons diffused upwards, until local criticality was achieved, enabling a fast neutron chain reaction to be initiated in U235 from self fission of U238.
It has been assumed by Herndon (1), and confirmed theoretically by Seifritz (2) that the ratio of U235 to U238 in the early Earth was more than sufficient to cause a fast neutron reaction to occur.
It is well known that fast neutrons produced from U235 can breed Pu239 from U238. Furthermore Pu239, as well as being itself a fissile fuel, decays radioactively with a half-life of 24131 years to regenerate U235. This is of some importance, because should the fast neutron reaction be poisoned, during the ensuing dormant period, U235 therefore will be partially replaced.
These nuclear reactions can produce more fissile material from conversion of the fertile, but non-fissile U238, than the system started with, and are the basis of the fast breeder reactors developed in a number of countries for power generation.
As I mentioned in the above earlier publication, natural breeder reactors have been discovered at Oklo in Gabon, Africa to have operated on and off for millions of years, about 2 billion years ago, until environmental conditions changed, causing their termination.
Although in the cleared region some breeding would have occurred, the reaction at first would have been more or less a "fizzle", as more poisons were soon have been generated, shutting down the reaction again. Nevertheless considerable amounts of energy would have been generated, causing increased heating of the kernel before the shut-down.
The process so far would have had the effect of producing some fissile Pu239, ( a more fissile material than U235 ) to take the place of the U235 used up, and a reduction of U238.
Initially U238 would have been in large excess in the melt, and would have acted as an absorber, since it results in no fission directly. However after breeding had occurred, the ratio of fissile material, ( U235 and Pu239 ), to non-fissile material, ( U238 ), would have increased, and hence in future chains the absorbing effect of U238 would be relatively less, giving a more efficient conversion process.
The other major effect is that due to the temperature increase, the diffusion rate of fission poisons would increase, leading to a faster clearance rate and a shorter shut-down period.
Initially the shut-downs would last for very long periods - perhaps millenia - during which Pu239 would be decaying with a half-life of 24131 years to U235. ( U235, of course, decays negligibly slowly with a half-life of 7.1E8 years ). Poison reduction would continue throughout the bulk of the kernel, not only in the lower initiating region, until eventually another chain would start another breeding reaction, slightly more efficiently due to the more favorable fissile/non-fissile ratio, in a somewhat larger cleared region, with a slightly increased release of energy, before the next shut-down occurred.
However now there would be a slightly lower concentration of poisons overall, and also a slightly more favorable fissile/non-fissile ratio for the next cycle.
So the cycles would continue at relatively low neutron flux, in this stop/start fashion, but with ever increasing efficiency, until conceivably a high fissile/non-fissile ratio would result.
The next major factor which must be considered is poison "burn-up".
As the cycles continued, the neutron flux generated in succeeding chain reactions would increase.
At a sufficiently high neutron flux the absorbing poisons would begin to be "burnt up" and converted into neutron non-absorbent nuclides which present no problem to chain continuation.
Once this phase had been reached, there would no longer have been any need for the slow thermal diffusion process for clearing the poisons; the chain reaction would have reached a point of positive feedback. Increased flux would increase burn-up, which would allow increased flux, which would increase rate of burn-up even more, and so on, until the poisons would have been burnt up as fast as they were produced. Thus they would no longer present any impediment to the growth of the chain reaction.
The rate of the chain reaction would then be increasing exponentially, almost instantaneously engulfing the whole kernel.
At this point, ( if one can say 'point', since the timescale would now be in milliseconds ), the kernel would have been reacting supercritically as a giant plutonium bomb - but a bomb with a difference.
The difference is that it would be imprisoned under the enormous overburden of the Earth's mantle.
It is this weight which would have enabled the nuclear chain reaction to proceed to completion, by holding the reacting nuclei together, unlike man-made nuclear weapons which blow themselves apart, terminating the chain reaction.
Now the effect on the surroundings during this build up of nuclear power must be considered.
The kinetic energy of the fission products would be instantly converted to an immense amount of heat in the surrounding mantle matrix, and unable to be quickly dissipated by the slow process of thermal diffusion, the local temperature would rise to millions of degrees. Volatilization of a large volume of the mantle in the vicinity would occur, producing an enormous pressure bubble.
The overlying mantle would already be largely molten due to gravitational energy release, with only perhaps a light differentiated crust.
In a short space of time, sufficient pressure would be generated, not only to overcome gravitational attraction of the portion of the mantle overlying the bolide's position, but also to provide enough propulsion to blast a moon-sized blob of molten mantle, plus a small portion of the Earth's outer core, away from the mother planet into orbit, to form the proto-Moon.
The mathematical calculations done for the previous scenario are not materially altered, since neither the density nor the atomic weight of Pu239 is significantly different from that of U235 or U238, given the order of accuracy pertaining, except for the fact that considerably less initial kernel uranium would be necessary, due to the conversion of a large portion of the U238 to fissile Pu239 during the initial breeding process.
Assuming for the sake of argument, that a fissile/non-fissile ratio of 4 were attained by conversion, the initial mass of uranium in the kernel, required for the Moon boost, would be 6.90E16 kg, rather than 2.28E17 kg, as calculated earlier.
This equates to an equivalent sphere of 19 km diameter, compared to 28 km for the earlier scenario.
All other speculations remain the same, except that this scenario should also be applied to the possibility of formation of more than one moon, either having shorter lifetimes before falling back, or alternatively, exceeding the escape velocity and being lost to interplanetary space.
I believe that the mysterious mantle plumes, or hot-spots, which are scattered throughout the Earth's mantle, may have been produced by the inclusion of smaller radioactive uranium masses during the accretionary period, and are continuing to generate sufficient heat from normal radioactive decay to power the long lasting plumes of molten mantle, which have been, and are still, responsible for much of the Eath's volcanic activity today.
1 Herndon, J. M. (2003) Nuclear Georeactor Origin of Oceanic Basalt He3/He4, Evidence, and mplications.
PNAS, USA, Vol. 100, No. 6, pp. 3047-3050.
2 Seifritz, W. (2003) Some Comments on Herndon's Nuclear Georeactor. KERNTECHNIK, 68, 4, pp. 193-196.
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