As we now stand on the threshold of the coming ASI Machines rapidly taking over the Earth and then beginning to embark upon the exploration of our Milky Way galaxy, the eternal question still remains. Why has this not already happened at some previous time during the past 10 billion-year history of our galaxy? Why haven't alien ASI Machines already explored and populated our galaxy many billions of years ago as I discussed in Welcome To The First Galactic Singularity and many other posts. The most plausible explanation seems to be that the Rare Earth (2000) hypothesis of Peter Ward and Donald Brownlee keeps making our Rare Earth rarer every day. In this post, I would like to discuss another factor of importance that makes our Earth so rare. The Earth has the very heavy and long-lived radioactive actinide elements thorium-232, uranium-235 and uranium-238 in its mantle. In this post, I will discuss how these relatively rare elements have helped to keep the Earth habitable for the many billions of years required for intelligent carbon-based life to arise and then go on to build ASI Machines to take their place in the Universe. This finding is based on two recent papers. The first paper is at:
A nearby neutron-star merger explains the actinide abundances in the early Solar System
https://www.nature.com/articles/s41586-019-1113-7.epdf?author_access_token=8yMiQNhpUG-6FOa72e3PPtRgN0jAjWel9jnR3ZoTv0MtbjjF39muJud1U0NXLNaLnYx5mKZXfm35mGmMmtjgUuIGiy24-TaLmxN9nEU8ZpboIwYw-coPO7AupWbhmYJOq1forKxwR6hvW0UxlnlPrQ%3D%3D
Here is a temporary free version of the above paper:
https://www.nature.com/articles/s41586-019-1113-7.epdf?author_access_token=8yMiQNhpUG-6FOa72e3PPtRgN0jAjWel9jnR3ZoTv0MtbjjF39muJud1U0NXLNaLnYx5mKZXfm35mGmMmtjgUuIGiy24-TaLmxN9nEU8ZpboIwYw-coPO7AupWbhmYJOq1forKxwR6hvW0UxlnlPrQ%3D%3D
The second paper is at:
Binary neutron star populations in the Milky Way
https://arxiv.org/pdf/2305.04955.pdf
The above papers explain why star systems with planets that contain the radioactive actinide elements thorium-232, uranium-235 and uranium-238 are quite rare in our Milky Way galaxy. I will then expand upon why these long-lived radioactive isotopes are so important for the future exploration of our galaxy by ASI Machines from the Earth.
Radioactive Elements Are Needed to Melt and Differentiate a Rocky Silicate-Based Planet
When a rocky silicate-based planet like the Earth first forms, it can subsequently completely melt from the heat produced by infalling asteroids and the decay of radioactive elements. As it melts, it then differentiates into a molten iron-nickel core with a very hot semi-molten mantle. Eventually, the planet cools enough to produce a thin outer crust over its entire surface. This is very important for the rise of a somewhat intelligent form of carbon-based life to appear on the planet and then go on to build ASI Machines that can go on to explore the galaxy. Intelligent carbon-based life needs to arise on a rocky silicate-based planet that contains some dry land and oceans of water that also has a relatively thick atmosphere containing a good deal of oxygen. The oxygen in its atmosphere is first required to drive the energy-hungry metabolisms of its first complex carbon-based forms of life. Later, intelligent carbon-based life requires oxygen to produce fire and all subsequent technologies. Since you cannot light a fire underwater, intelligent marine life will probably never produce ASI Machines.
Figure 1 - A rocky silicate-based planet can develop a strong magnetic field when it contains a molten conducting core that has convection currents that are deflected into helical motions by the Coriolis effect when the planet rapidly spins. The Coriolis effect also causes hurricanes to spin in opposite directions in the northern and southern hemispheres.
For a rocky silicate-based planet to maintain such an atmosphere, it needs a magnetic field to deflect the solar wind from its home star. Otherwise, the atmosphere will be blown away as happened to the small planets Mercury and Mars that do not have magnetic fields. For a rocky silicate-based planet to have a magnetic field it needs three things. It needs a molten core composed of a conducting metal such as iron and nickel. It also needs these molten metals to rise and fall in its core in large-scale convection currents. Finally, the planet needs to rotate with a sufficient angular velocity to have a magnetic dynamo form to create a strong magnetic field. Mercury and Venus barely rotate and the iron core of Mars solidified many billions of years ago because the planet is too small. None of these rocky silicate-based planets have a strong magnetic field like the Earth.
Figure 2 - Rocky silicate-based planets also need a hot mantle for plate tectonics to develop.
A hot interior is also necessary for plate tectonics to operate. Plate tectonics helps to regulate the atmosphere of a rocky silicate-based planet by subducting carbonate rock back into the mantle. This helps to keep the amount of carbon dioxide in the atmosphere at a low enough level to allow liquid water to exist on its surface. Plate tectonics did not arise on Venus even though it is about the same size as the Earth. Venus does not have a magnetic field either and that is probably why it has no water. Water molecules on Venus would have been blown away by the Sun's solar wind leaving behind the heavier carbon dioxide molecules in its atmosphere that, without plate tectonics, accumulated in its atmosphere and led to a runaway greenhouse effect on the planet. It is thought that water in the Earth's upper mantle lubricates the Earth's plates to allow for plate tectonics to occur. So of the four rocky silicate-based planets in our Solar System - Mercury, Venus, Earth and Mars, only the Earth has a strong magnetic field, a temperate atmosphere and plate tectonics.
Figure 3 - Of the four rocky silicate-based planets in our Solar System, only the Earth has a strong magnetic field and plate tectonics. This is because Mercury and Mars are too small and Venus barely spins.
Now, to melt a recently formed silicate-based planet, a rather short-lived highly radioactive element is required. But that radioactive element cannot be too short-lived either, otherwise, there would be none left by the time the planet began to form. Aluminum-26, with a half-life of 717,000 years, is the perfect isotope to do that. Aluminum-26 is generated by the type II supernovas that result from the bright massive stars in a molecular cloud running out of nuclear fuel and collapsing upon themselves. Type II supernovas are rather common on cosmological timescales. About one type II supernova happens every 50 years in our 10 billion-year-old Milky Way galaxy. They produce the aluminum-26 isotope that can decay and give off enough heat to melt a rocky silicate-based planet such as the Earth. Remember, radioactive isotopes with short half-lives are very radioactive and can quickly produce heat and radiation damage. For example, when a nuclear reactor has a meltdown, it can release iodine-131 with a half-life of only 8 days. That means that iodine-131 is highly radioactive and dangerous. With a half-life of only 8 days, it rapidly releases nuclear energy. But after 10 half-lives of only 80 days, only 1/1024 of the original iodine-131 remains, so it is nearly all gone like a rapidly burning house fire that quickly does a lot of damage but then rapidly fades away into ashes. This is why they pass out iodine tablets during a reactor meltdown so that your body does not take up the rapidly decaying iodine-131.
Figure 4 - Type II supernovas arise when massive stars run out of nuclear fuel to fuse. As these stars run out of nuclear fuel, they form an onion-like structure of heavier and heavier nuclei. But iron Fe nuclei cannot be fused to produce energy and when that happens the entire star collapses under its own weight and is blown apart by huge numbers of neutrinos. Aluminum-26 is produced in this process and is then blown out into the interstellar medium.
Figure 5 - A type II supernova pushes out aluminum-26 into the interstellar medium. Type II supernovas happen about every 50 years in our galaxy.
Figure 6 - Above are some very massive stars in the Orion Nebula that are 10 - 30 solar masses. These stars will quickly burn up all of their nuclear fuel in just a few million years and then produce type II supernovas that will push out aluminum-26 into the Orion Nebula molecular cloud. Such events replenish the amount of aluminum-26 in molecular clouds as the aluminum-26 continuously decays with a half-life of 717,000 years.
Long-Lived Radioactive Elements are Required to Keep a Planet Hot for Billions of Years
But to keep such a planet hot for billions of years, you need the long-lived radioactive elements of thorium-232 (half-life 14 billion years), uranium-235 (half-life 700 million years) and uranium-238 (half-life 4.5 billion years) and those isotopes are not generated by type II supernovas. These long-lived radioactive elements are created when two orbiting neutron stars finally collide together to form a black hole. In the collision process of two neutron stars, many neutrons get smashed together very quickly by the r-process to produce thorium-232, uranium-235 and uranium-238. Normally, a free neutron will decay into a proton and an electron with a half-life of about 15 minutes. Only when a neutron is confined by the strong nuclear force in an atomic nucleus or the confines of a neutron star held together by intense gravity can a neutron persist for billions of years. In the r-process, an atomic nucleus is bombarded with huge numbers of neutrons so quickly that the neutrons do not have time enough to decay. It is possible to inject huge numbers of neutrons into a nucleus because neutrons have no electrical charge to repel them. Only later, do these neutron-rich nuclei have time to have some of the neutrons decay into protons and release electrons. According to the second paper above, neutron stars only collide about once every 30 million years in the Milky Way galaxy. Type II supernovas, the ones that produce aluminum-26, happen much more often, about every 50 years.
The long-lived thorium-232, uranium-235 and uranium-238 atoms in the mantle of a planet act like a nuclear-powered electric blanket to keep the iron-nickel core of the planet from freezing into a solid. For example, the Earth's core is
slowly freezing into a solid iron-nickel inner core. The solid inner core is now about 1,200 kilometers wide, but thanks to the thorium-232, uranium-235 and uranium-238 atoms in the Earth's mantle, the inner core is only growing by about one millimeter each year. According to calculations, as the molten iron and nickel of the outer core crystallizes onto the inner core, the melt just above it becomes enriched in oxygen, and therefore less dense than the rest of the outer core. This process creates convection currents in the outer core, which when combined with the Coriolis effect of a rapidly rotating Earth, are thought to be the prime driver for the currents that create the Earth's magnetic field. These long-lived radioactive atoms also keep the mantle of the planet hot enough to support plate tectonics.
Figure 7 - The long-lived radioactive nuclei thorium-232, uranium-235 and uranium-238 are created when two neutron stars collide to form a black hole. This only happens about once every 30 million years in our galaxy.
Why Does Having Thorium and Uranium Atoms Make the Earth Rare?
The authors of the first paper above explain that by analyzing the amounts of thorium-232, uranium-235 and uranium-238 on the Earth and in meteorites, along with the daughter products of other radioactive elements that have already decayed, they were able to determine that two neutron stars collided about 80 million years before the formation of our Solar System at a distance of about 3,000 light years.
Figure 8 - About 80 million years before the formation of our Solar System, two neutron stars merged into a black hole and ejected large amounts of thorium and uranium atoms into the collapsing pre-solar nebula that formed our Solar System. Since the merger of two neutron stars is relatively rare, most rocky silicate-based planets do not have these long-lived atoms to keep their mantles hot and their cores molten.
But even having thorium-232, uranium-235 and uranium-238 atoms will not guarantee that a rocky silicate-based planet will have a strong magnetic field and plate tectonics. Of the four such planets in our Solar System, Mercury, Venus, Earth and Mars, only the Earth has a strong magnetic field and plate tectonics. The other three such planets have neither because they are either too small or are not spinning fast enough.
Additionally, galactic ASI machines exploring our galaxy will need a compact source of energy. This would be best supplied by fissioning uranium-235, uranium-233 derived from thorium-232, or plutonium-239 derived from uranium-238. When you fission one of these atoms, you get 200 million eV of energy. Compare that to the 2 eV of energy per atom that you get from chemically burning an atom of coal or oil! Nuclear fuel contains 100 million times as much energy per atom and can be fissioned in relatively small reactors. For more on that see Last Call for Carbon-Based Intelligence on Planet Earth and Agile Development of Molten Salt Nuclear Reactors at Copenhagen Atomics.
Comments are welcome at
scj333@sbcglobal.net
To see all posts on softwarephysics in reverse order go to:
https://softwarephysics.blogspot.com/
Regards,
Steve Johnston
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