The Age Of The Earth

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He tried to calculate the different scientists. There were multiple experiments. To accurately determine the age of our planet, it took more than three centuries.
Now we ssvm that the Earth is already 4.54 billion years (1% accuracy) — a value that changed little from when it first received 57 years ago in 1956; decreased only error. But can we be sure that we have the final number?
Why so much time was required to find him? In search of an answer to these questions we must go back three centuries.

With the first studies 200 years ago, dedicated to the cooling of iron balls, and to accurate estimates of the isotopes contained in meteorites, scientists seek knowledge of the true age of the Earth. The work of different specialists, solving complex problems and numerous experiments have allowed to pass this difficult path.
The Archbishop of the Anglican Church James Asser (James Ussher) from Ireland — one of the many scientists of the seventeenth century, tried to establish the exact date of the creation of the Earth God. In those days knowledge was used to extract by analyzing different kinds of historical texts, including the Bible, and obtained values varying from 3616 to 6984 BC User put in chronological order all the significant characters in the old Testament, beginning with Adam. So he determined that heaven and earth are created in the night from Saturday to Sunday, 23 October 4004 BC The date would have remained obscure if not for an enterprising merchant named Thomas guy (Thomas Guy). Feeling the demand for cheap mass publication of the Bible, guy in 1675 began printing versions of the book, which included the chronology Asser made on the field. SPHERE OF TIME
As the accumulated knowledge of the Geology, scientists began to understand that the history of the Earth is clearly not appropriate in several millennia. French naturalist Georges-Louis Leclerc de Buffon (Georges-Louis Leclerc de Buffon) put forward the hypothesis about the formation of the earth from the jet of very hot material ejected from the Sun under the influence of the comet. He tried to figure out when this could happen by studying empirically the processes of cooling.
For 11 years, Buffon conducted long-term experiments with balls of different radius, made of iron and stone. Pinpointing the time of their cooling, and then extrapolated empirical data on an object the size of the Earth. The results he published in 1775, considering the age of the Earth at least 74 832 years since its formation to the current cooling state. The Buffon believed that the Earth is still much older than her, maybe even 10 million years.
In the ensuing century appeared abundant evidence of long geologic processes that occurred many millions of years. The characteristic sediments have been described from different geological times. Finally, by the mid-nineteenth century method of "hourglass" was seen as very authoritative. The first attempts to evaluate the capacity of rocks on different continents and the speed with which these accumulated sediments (which allowed to obtain a time necessary for their accumulation), given the huge variation — from H m to 2.4 billion years (due to the difference in the speed of formation of precipitation in different places).
Another alternative of steel attempts to measure the rate at which seawater accumulated salt. The rivers bring to the sea salt from their blurry rocks. If we assume that initially the oceans consisted of fresh water, then, in principle, possible to estimate the time necessary for their "pollution" to its current state. This method was fraught with difficulties and has led to a large spread of values (not to mention purely produce hypotheses assumptions).
In 1862, British physicist Lord Kelvin began one of the speeches at the meeting of the Edinburgh Royal society from the attacks on the geologists and their methods of determining the age of the Earth. Like Buffon, Kelvin argued that the Earth was originally in a molten state, and considered "obvious" that, if we know the temperature at which melting of the rock, and the speed at which they are cooled, you can calculate the time during which the crust is formed. The original Kelvin value lay within very wide limits, from 20 to 400 million years, but a few years later, after precise measurements of the melting temperature of rocks (she was much below the expected), Calvin has revised its estimate, reducing it by up to 20-40 million years. Among geologists, this work has caused considerable confusion.
The decade at the turn of the twentieth century brought a number of important discoveries. In 1895, were discovered x-rays, and in 1896 it became known that the uranium also emits a similar "mysterious rays". This phenomenon of radioactivity was discovered by the French chemist Antoine Becquerel (Antoine Henri Becquerel) and then study the couple-physics Maria Sklodowska-Curie (Maria Skiodowska-Curie) and Pierre Curie (Pierre Curie). The name of this phenomenon was given by Marie Curie. As a result of their discoveries in laboratories all over the world swept a boom of research in this direction.
In 1897 Joseph John Thomson (Joseph John Thomson) discovered the electron, and in 1902 Ernest Rutherford (Ernest Rutherford) and Frederick Soddy (Frederick Soddy) proposed a theory of radioactive disintegration, which formed the basis of the doctrine of the atom and atomic energy. They amazed the world with a statement that in the process of radioactive decay, one element changes to another: the uranium turns into radium, which decays, releasing radon gas.
Shortly thereafter, Soddy showed that the result stands out not only radon, and helium. Radon is also unstable and decays into other elements.
A few months later, before Pierre and Marie Curie were awarded the Nobel prize in 1903, Pierre found that the radioactive decay of an atom leaving the electrons releasing energy in the form of heat. Even if Calvin was right, believing that the Earth cooled from a molten state, in any case he did not know that at the same time, radioactive elements inside the Earth produces enough heat to delay the process of cooling for any period of time that may be needed geologists. ROCK OF AGES
The discovery that helium is a byproduct of uranium decay, prompted Rutherford to the next step. He realized that, based on the rate of formation of helium and measuring the amount of uranium and helium in the rocks, through relatively simple calculations, we can estimate the duration of the accumulation of helium and so set the age of the rock. A year later Rutherford became the first man to appreciate the age of rocks using radioactive decay — he got the value of 40 million years. Unfortunately, his method was a mistake and to find her helped Robert Strutt (Robert Strutt), a physics teacher from London's Royal College of science (UK), who drew attention to the fact that helium gas can seep through rocks. This meant that only part of the radiogenic helium had been measured and the obtained age was only a minimal estimate. Search for a more perfect method Strutt asked one of his students, 20-year-old Arthur Holmes.
In 1907, the American chemist Bertram Boltwood (Bertram Borden Boltwood) studied rocks containing uranium. He noticed that along with helium they contained large amounts of lead, and suggested that lead might be the end product in the decay chain of uranium. But Holmes, in turn, realized that if the rights of Boltwood, the age of the rock can be obtained by measuring the content of lead, not helium. Thus defined, what proportion of the uranium had to disintegrate for the lifetime of the object, since crystallization of minerals in it. The scientist decided to try it. In the winter of 1910, he analyzed the content of uranium and lead in 17 of the minerals (see box, "Key experiment").
The results obtained allowed Holmes to come to the conclusion that lead is indeed the final product of uranium decay and now finally found a reliable method of estimating the age of rocks (in different variations it is still being used). The oldest breed in the studied samples was 1.64 billion years, and Earth had to be even older. These results, however, received a hostile reception by the majority of geologists who trust the Kelvin and received numbers.

William Thomson, Lord Kelvin (1824-1907)
British physicist and mathematician from the University of Glasgow. Believed to work on the definition of the age of the Earth, his most important contribution to science.

Soddy Frederick (1877-1956)
British chemist, who explained the essence of the radioactive decay (along with Ernest Rutherford at the canadian McGill University) and isotopes (University of Glasgow), which produced a real revolution in the science of radioactivity.

Alfred Nier (1911-1994)
American physicist from Harvard University, a pioneer in the field of mass spectrometry. He opened the isotope of lead 204РЬ, what made Arthur Holmes to develop a better method of Dating the earth's rocks.

Arthur Holmes (1890-1965)
British physicist and geologist who developed the uranium-lead Dating method. Holmes worked at the University of Durham on a "common scale of geological age".

Claire Patterson (1922-1995)
American geochemist of the California Institute of technology, who was finally able to estimate the age of the Earth, highlighting micrograms of lead in meteorites.
Progress has been slow, and the discovery of isotopes, Frederick Soddy in 1913, only complicated. At that time, the only way to distinguish one isotope from another was to obtain their atomic masses, and only a few laboratories in the world could cope with this task. In addition, Holmes realized that a certain amount of lead, probably present on the Earth initially. He could not determine which of the isotopes of lead formed by decay of uranium, and which were present on Earth from the beginning, therefore, its date could not be accurate. TRIAL AND ERROR
In 1924, Holmes became a Professor at the University of Durham (UK), where he continued work on the creation of a "common scale geological age" and establish the age of the Earth. Among other things, he tried to develop new Dating methods. Although each method initially looked promising, after some time all of them were declared unfit. Finally in 1938 the young physicist Alfred Nier (Alfred Nier), who worked with the new mass spectrometer at Harvard University (USA), tried to identify all known isotopes of lead (chemical symbol PB). He quickly spotted three well-known isotopes of radiogenic origin (from decaying uranium and thorium) — 206РЬ, 207РЬ and 208РЬ. And at the end of the spectrum noticed another tiny splash. Then was finally identified by a primary isotope 204РЬ — a missing element in the uranium-lead puzzle.
1775
Georges-Louis Buffon calculates the age of the Earth, heating up the iron sphere, noting the time during which they cool down and extrapolating these results to the size of the planet. It turns out 74 832.
1862
Lord Kelvin believed that the Earth originated as a molten ball of 20-400 million years ago. He then clarifies this value and gets 20-40 million years.
1902
Ernest Rutherford and Frederick Soddy explain radioactive decay. Two years later, Rutherford for the first time date the age of rocks using radioactive decay. It turns out 40 million years.
1911
Arthur Holmes develops the uranium-lead method of Dating and finds that the Earth is older than 1.64 billion years. Two years later, Soddy opens a previously unknown isotopes of lead, which can significantly improve the accuracy of the method.
1946
After opening Alfreda Research the breed age of 2.48 billion years Holmes uses his data to develop the model, calculate the age of the Earth, which gets the value of 3.015 billion years.
1956
Claire Patterson evaluates the content of lead in the five meteorites that fell to Earth that allows you to find the current value for the age of the Earth, moon and meteorites. It is of 4.55 ± 0.07 billion years.
NIR has engaged in the development of geochronological time scale. Before the Second world war he held a series of very accurate experiments, which allowed to determine the age of 2S of different species from different geological formations. The age of one of the studied minerals of the pegmatite of Manitoba was estimated to be 2.48 billion years old. Intrigued by the results of Nira, Holmes wrote him in may 1945 (after he had finished his work in the Manhattan project to create the atomic bomb, which was then attended by many American physicists).
According to Holmes, the work of Nira is "of great interest not only because it showed that the oldest rocks yet to be found". She, as he considered interesting here: "...the resulting value indicates that modern ideas about the expanding Universe need revision".
By the time the American astronomer Edwin Hubble (Edwin Hubble) determined that the age of the Universe amounts to only 1.8 billion years. But Nira data showed that this may not be true — the whole universe the age of the Earth. Holmes predicted that the data from Nira will help to clarify the age of the Earth. He bought one of the first computers to complete complex calculations and once again turned to Nero 16 February 1946, writing that the age of the Earth should be in the region of 3 billion years, and that the best data set leads him to the value of 3.015 billion years.

The Hyper saline water of the Dead sea in Israel show how much salt can be contained in sea water. The first attempts to determine the age of the Earth included in the calculation of the time required for accumulation of salt in the initially fresh water seas.
By the beginning of 1950-ies the method of uranium-lead isotopes for Dating rocks finally became generally accepted. But Holmes was already unhealthy and have moved away from research by giving the next generation to continue his quest. As technology improvements, another American researcher, Claire Patterson (Claire Patterson), was able to detect vanishingly small amounts of lead in iron meteorites.
The advantage of choosing iron meteorites that the uranium content in them slightly, so any primary lead is not contaminated with lead is of radiogenic origin. Patterson realized that if the Earth (according to the assumptions of astronomers) was formed at the same time as the Solar system, it is possible to use data on the number of primary lead, isolated from meteorites, to determine the current age of the Earth.
Travel time band
If all the rock samples are on the same line (the travel time band) in the diagram, isotope ratios, then they all formed at the same time. The slope of the line gives the age of the rock.
Isotope
Chemically identical atoms of any element containing a different number of neutrons in the nucleus. The sum of neutrons and protons gives the atomic mass of the isotope. Unstable isotope 238U decays to a stable isotope 206РЬ with a half-life of 4.47 billion years (the range of radium).
The mass spectrometer
Tool that separation of ionized particles of matter (molecules and atoms) according to their masses. The principle of operation is based on the influence of magnetic and electric fields on ion beams, flying in a vacuum.
Pegmatite
Coarse-grained igneous rock. Formed at the edge of the magma chamber during the final stage of crystallization. It often contains minerals that are suitable for Dating.
Radioactive decay
Spontaneous change of structure of unstable atomic nuclei by emission of particles or nuclear fragments. The resulting child - in some cases, also turns out to be unstable and after some time could fall apart.
The next three years, Patterson has spent in trying to confirm it. In 1956 he managed to finally prove that the Earth, planets and meteorites had a common origin. He analyzed the lead content in the five meteorites and found that the ratio of their isotopes are formed on a straight line (the travel time band), which gave an age of 4.55 ± 0.07 billion years. In addition, the samples from Earth (and later the moon) lay on the line, that is, the Earth and meteorites formed at about the same time from one solar material about 4.5 billion years ago. So, exactly 300 years after Asser (died in 1656) the true age of the Earth was finally installed.