Since the dawn of humanity, we’ve wondered if the Earth has a beginning and an end – and, if so, when it was and when it will be. The question remained in the philosophical field for a long time, from Aristotle to James Hutton, the English geologist who in 1788 would write that the planet has “neither trace of a beginning, nor prospect of an end…”
But scientists are stubborn and continued to investigate. In the 19th century, several attempts were made to establish the age of the Earth in absolute numbers. It has long been known that the planet has been cooling down since its formation. Sir William Thomson, known as Lord Kelvin, one of the most respected scientists of his day, calculated the time it would take for the Earth to cool from being a blazing fireball to its present state, and came up with numbers between 20 and 40 million of years. For those familiar with the natural record, however, such figures were very modest, especially in light of Darwin’s newly proposed theory of evolution (which himself even calculated numbers around 300 million years using estimated rates of erosion of relief). Why would Kelvin’s calculations be wrong?
In 1885, the German physicist Wilhelm Roentgen presented the world’s first radiograph. Several scientists dedicated themselves to understanding the cause and nature of this phenomenon, including the French physicist Antoine-Henri Becquerel, whose hypothesis was to test whether the fluorescence emitted by some minerals exposed to the sun, such as uranium salts, produced the same effect as rays X on a photographic plate. Bad weather in Paris in February 1896 thwarted his plans. He wrapped the uranium salts in a black cloth, along with the photographic plates and the metal cross he intended to X-ray, and left them in the drawer.
Days later, he found that, even covered, the uranium salts produced an X-ray of the metal cross on the plate. Radioactivity had just been discovered “by chance” – in science, as in life, luck does not usually favor those who are no longer prepared for it. In 1903, Henri Becquerel and Marie Curie and Pierre Curie shared the Nobel Prize in physics for the discovery. Marie Curie coined the term radioactivity, in addition to discovering two new elements, radium and polonium, and being the first person to win two Nobel Prizes.
The discovery of radioactivity opened scientists’ eyes to a spontaneous and natural energy source, as the radioactive decay of elements such as uranium involves the transformation of mass into energy. Lord Kelvin’s calculations did not take into account the heat constantly produced by the decay of huge amounts of atoms of elements such as uranium and potassium within the earth’s crust. That is, we cannot calculate the Earth’s cooling time because the radioactive decay of the elements causes it to be constantly reheated.
This was just the first rabbit that the discovery of radioactivity pulled out of a hat. The correspondence exchanged between physicists Ernest Rutherford and Frederick Soddy, one working in Canada and the other in Scotland, is enriching as it gives us a clear vision of the spirit of the times. In one of the letters, describing the natural transformation of a radioactive element into a more stable one, emitting energy in the process, Soddy says, “Rutherford, this is transmutation!” To which the caller responds, “For God’s sake, Soddy, don’t call it a transmutation. They’ll cut our heads off like alchemists!”
In 1904, Rutherford went to present his findings at the Royal Institution, the mecca of science in London: “I walked in and soon saw Lord Kelvin in the audience; I realized that I would be in trouble in the last part of the lecture, when I would speak of the age of the Earth.” He was 35 years old. I knew that Kelvin was the most respected scientist then, and that it would not be easy to challenge the authority of his 81 years. “To my relief he quickly fell asleep,” Rutherford writes. And then, at the end, when the elder awoke, “a sudden inspiration came to me, and I said that Lord Kelvin had limited the age of the Earth ‘since’ no new source of heat was discovered. ‘This prophetic utterance concerns what We are considering today, the radio! Behold!’ And the old man smiled at me…
Rutherford and Soddy discovered not only that one element transforms into another naturally, but also that this transformation occurs at fixed rates in nature. Measuring, for example, the amount of uranium (the radioactive element) and lead (the stable element that it naturally turns into) in a rock or mineral, and knowing the rate at which uranium naturally turns into lead, is possible to calculate how much time has passed since this mineral or rock formed on the Earth’s surface.
From then on, it was just a matter of technological development until equipment was created to measure these elements in very small concentrations. In 1956, Chicago geochemist Claire “Pat” Paterson came up with the first age of meteorites and the Earth, calculated from the amounts of uranium and lead in rock samples, and finally resolved the question of the trace of a beginning, there are 4.55 billion years. Since then, several other methods using radiogenic systems have been developed.
Equipment capable of measuring potassium and argon, father and son respectively in radioactive decay, have reached space: the Curiosity probe, launched by NASA in November 2011, has a microwave-sized unit that can collect rock and soil samples and dating directly on the surface of Mars. In 2014, the first completely off-Earth dating of rocks around 4.2 billion years old from the crater Gale were published, which were deposited on the bottom of a lake, indicating that the red planet contained free water at that time – and it was probably a lot more like ours.
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Fabrício Caxito is a geology professor and philosophy student at UFMG.
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