Being able to precisely date an object can help us discover when the Earth was formed, help us figure out past climates and tell how early humans lived.
How do scientists do that?
Radiocarbon dating is the most common method to date, according to experts. This method involves measuring the amounts of carbon-14, a radioisotope of carbon - or a copy of an atom with a different number of neutrons. Carbon 14 is found everywhere in the environment.
Thomas Higham, an archaeologist and radiocarbon dating specialist at the University of Oxford in England, said that after it forms high in the atmosphere, plants and animals breathe in, and "everything that is alive takes it."
While the most common form of carbon contains 6 neutrons, Carbon 14 contains two more. This makes isotopes much heavier and less stable than the most common form of carbon. So after thousands of years, carbon-14 eventually decays. One of its neutrons is divided into a proton and an electron. During the escape of the electron, the proton remains part of the atom. With one neutron less and another proton, the isotope decays into nitrogen.
When living things die, they stop absorbing carbon-14, and the rest in their bodies begins the slow process of radioactive decay. Scientists know how long it takes for half a certain amount of carbon-14 to degrade - a time period called the half-life. This allows them to measure the lifetime of an organic piece - whether it's animal skin, a skeleton, ash, or a tree ring - by measuring the ratio of carbon 14 to carbon 12 remaining in it and comparing that amount to the carbon-14 half-life.
Carbon 14 has a half-life of about 5,730 years, making it ideal for scientists who want to study the last 50,000 years of history. "This really interesting part covers human history, the origins of agriculture, the development of civilizations: all of these things happened in the radiocarbon period," Higham said.
Even so, objects older than that lost more than 99% of carbon-14, leaving very little to be detected, said Brendan Culleton, assistant research professor in the Radiocarbon Laboratory at Pennsylvania State University. For ancient bodies, scientists do not use carbon 14 as a measure of age. Instead, they often look for radioisotopes for other elements in the environment.
For the oldest objects in the world, dating with uranium, thorium, and lead is the most useful method. "We use it to determine the age of the earth," Higham said.
While radiocarbon dating is only useful for previously surviving materials, scientists can use uranium, thorium and lead dating to measure the age of objects such as rocks. In this method, scientists quantify a variety of different radioisotopes, which all degrade into stable forms of lead. These separate decay chains begin with the breakdown of uranium-238, uranium-235 and thorium-232.
"Uranium and thorium are very large isotopes that explode in seams. They are always unstable," said Tammy Rittenour, a geologist at Utah State University. All of these "parent isotopes" decay into a different chain of radioisotopes before ending up as lead.
And each of these isotopes have different half-lives, ranging from days to billions of years, according to the Environmental Protection Agency. And just like radiocarbon dating, scientists calculate the ratios between these isotopes and compare them to their respective half-lives. Using this method, scientists were able to date the oldest rock ever discovered, a 4.4 billion-year-old zircon crystal found in Australia.
Another method of dating does not tell scientists the age of an object, but when it was last exposed to heat or sunlight. This method, called luminescence dating, is preferred by geologists who study changes in landscapes over the past million years - and they can use it to discover when a glacier is forming or retreating, and rocks are depositing over the valley; Or when a flood dumped sediments on a river basin.
When the minerals in these rocks and sediments are buried, they are exposed to the radiation emitted from the sediments around them. This radiation expels electrons from their atoms. Some electrons fall back into the atoms, while others get stuck in holes or other defects in the dense network of atoms around them. It takes a second exposure to heat or sunlight to return these electrons to their original positions.
And this is exactly what scientists do. They expose a sample to light, and when the electrons return to the atoms, they emit heat and light, or an illuminating signal.
"The longer this body is buried, the more radiation it is exposed to," said Rittenour. In essence, long-buried objects exposed to a lot of radiation will have an enormous amount of electrons that left their place, which together will emit a bright light when they return to their atoms, and the amount of the luminous signal tells scientists of the time the object was buried.
Forensic scientists use this method to solve crime mysteries, from murder to art forgery.
Source: Live Science
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