Are we stardust? | Science

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Bubble of gas and dust around a young star in the N44F nebula.
Bubble of gas and dust around a young star in the N44F nebula.

The building blocks that make up all living things on Earth are just a handful, with a huge preponderance of oxygen, carbon, hydrogen, nitrogen, phosphorus, and sulfur, which make up 97.5% of our bodies. In much smaller quantities, nature needs other bio-essential microelements, such as sodium, potassium or calcium, to build us. And even some trace elements, such as copper or zinc, essential for enzymatic processes. And we are convinced that these elements are manufactured inside the stars.

According to the model with which we explain the origin of the cosmos from the Big Bang, the only elements present in the early universe were hydrogen, helium and small amounts of lithium. Elements with atomic numbers between 4 (beryllium) and 25 (manganese) would have formed billions of years later, through the progressive fusion of increasingly heavy nuclei within massive stars, thanks to the combination of the enormous pressures and temperatures of the stellar interiors.

The creation of elements heavier than iron (atomic number 26) would have required the appearance of supernovae: the generation of iron, unlike the manufacture of the lighter elements, does not emit excess energy and is therefore unable to prevent stars from collapsing under their own weight. When collapsing in the form of supernovae, these stars produce high-speed neutrons, which are captured by the nuclei to create precisely the elements heavier than iron.

When stars die, they release all these elements, which will be incorporated eons later into new star systems in formation, and also to their planets and everything they may contain, including processes of organic chemistry or living beings. This is where the adage “we are stardust” comes from the famous phrase of the American astronomer Harlow Shapley in 1929: “We, the organic beings that we call ourselves human beings, are made of the same matter as the stars”.

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This is the reason why the complementary hypothesis of the Japanese geophysicist Mikio Fukuhara to explain the generation of elements heavier than lithium in the universe has generated so much interest. The word “complementary” is important here: Fukuhara’s hypothesis does not discuss the Big Bang model, it only offers an additional possibility to manufacture heavy elements.

According to the Fukuhara hypothesis, the creation of elements lighter than iron might not depend exclusively on the extreme conditions found within very massive stars. His hypothesis raises the possibility that oxygen, carbon and all other elements with atomic numbers up to 25 have also been produced inside the Earth. According to his hypothesis, fusion reactions of increasingly heavy nuclei must also occur in the lower mantle of the Earth, where they would be catalyzed by neutrinos and excited electrons.

In a preliminary model published a year ago, Fukuhara already proposed that nitrogen, oxygen and even water, whose concentrations on Earth have skyrocketed over time, could have been forged in endothermic reactions within the Earth’s mantle. These reactions would involve carbon and oxygen nuclei confined within the crystal lattice of calcium carbonate rocks (aragonite) in the lower mantle, subjected to great pressures and temperatures during the lithosphere subduction process as two tectonic plates converge.

Neutral sprockets

Fukuhara then already pointed out the possible main objection to his hypothesis: the temperatures and pressures that are registered at depths of several thousand kilometers below the earth’s surface are enormous, certainly, but not so much as to force these nuclei to join together overcoming their mutual repulsion. , what does happen inside the stars. However, Fukuhara argues that the presence of certain subatomic particles, known as neutral pions, would be capable of increasing nuclear attraction to the point of forcing fusion. The pions would be generated by electrons excited by the rapid fracturing and sliding of carbonate crystals, as a hitherto unidentified consequence of plate tectonics.

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The new published Fukuhara hypothesis now goes one step further, showing how these fusion reactions in the mantle could explain the production of not only nitrogen, oxygen, and water, but also the 25 lightest elements. To demonstrate the feasibility of this mechanism, he calculated the minimum energy required to start the reaction in each case.

He began the analyzes with three sets of two cores each: magnesium and iron, aluminum and magnesium, and aluminum and silicon. In all three cases, he observed that the combination of temperature, pressure, and catalysis reduced the interaction distance between the nuclei so that they could fuse, producing sulfur and titanium, sodium and silicon, and oxygen and potassium, respectively. Fukuhara plans to perform additional calculations to find out if the mechanism he has identified can also generate elements heavier than iron.

Of course, this fusion mechanism inside the Earth is still just a hypothesis that needs to be tested with additional experiments, involving very high temperatures and pressures. But, if the production of heavy elements in the interior of rocky planets were confirmed with plate tectonics, we would not be exclusively stardust.

Alberto González Fairén is a researcher in the Astrobiology Center (CSIC-INTA) in Madrid, and in the Department of Astronomy of the Cornell University in New York.

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George Holan

George Holan is chief editor at Plainsmen Post and has articles published in many notable publications in the last decade.

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