Clocks Within Atoms

Lord Kelvin had based his calculation of Earth’s age on a fundamental (and, it would turn out, false) assumption: the planet had no source of heat of its own. But there was a hidden heat inside the planet that Kelvin hadn’t counted on. In 1896, 14 years after Darwin died, a French physicist named Henri Becquerel wrapped a piece of uranium salt in a photographic plate. When he developed the plates, he found sharp, bright dots on them. Uranium, he realized, released rays of energy. Seven years later Pierre and Marie Curie showed that a lump of radium released a constant supply of heat.

Becquerel and the Curies had found a source of energy in the basic structure of atoms. Atoms are made out of three building blocks: protons, neutrons, and electrons. Electrons, which carry a negative charge, flit around the edge of atoms, while positively charged protons sit at the center. Each element has a unique number of protons. Hydrogen has one proton, helium has two, and carbon has six. Alongside protons, atoms have neutrally charged particles called neutrons. Two atoms of the same element may have a different number of neutrons. The most common form of carbon on Earth has six protons and six neutrons (known as carbon 12), but there are trace amounts of carbon 13 and carbon 14 as well. These different versions of atoms, known as isotopes, make it possible to tell geological time.

The protons and neutrons in an atom are a bit like piles of oranges at a grocery store: in some arrangements they’re perfectly stable, but in others they will sooner or later fall apart. Orange piles are held together by gravity, but protons and neutrons are held together by other forces. When an unstable isotope breaks down, it releases a burst of energy, along with one or more particles (otherwise known as radiation). In the process it may become a different element. Uranium 238, for example, breaks down by releasing a pair of neutrons and a pair of protons, and turns into thorium 234. But thorium 234 is unstable as well and decays into protactinium 234, which also decays. Through a chain of 13 intermediates, uranium 238 finally settles into a stable form, lead 206.

You can’t predict exactly when a particular atom will decay, but a large collection of them will obey certain statistical laws. In any given period of time, an atom has a certain probability of decaying. Let’s say a pebble has 1 million radioactive isotopes inside it, and this particular kind of isotope has a 50 percent chance of decaying in a year. After the first year, 500,000 of the isotopes will be left. Of those 500,000 isotopes, 50 percent will decay in the second year, leaving 250,000. Year after year, half of the remaining isotopes disappear, until about 20 years later the last isotope disappears. Physicists capture this trailing off in a measurement known as the half‑life: how long it takes for half of any given amount of a radioactive element to decay. Uranium 238, for example, has a half‑life of 4.47 billion years; other elements have half‑lives lasting tens of billions of years, while others have half‑lives of only minutes or seconds.

The laws that govern atoms don’t submit to any simple intuitive sense, but they work. If they didn’t, computers wouldn’t crunch numbers and nuclear bombs wouldn’t explode. And long before computers and nuclear bombs were invented–in fact, within a few years of the work of Becquerel and the Curies–physicists realized that these laws exposed a fatal flaw in Kelvin’s claims of a young Earth. Uranium and other radioactive elements such as thorium and potassium can be found in the Earth, where they decay and give off heat. Kelvin thought that Earth was young because it hadn’t cooled down very much from its origin. Radioactivity allows the planet to stay warm far longer.

A physicist named Ernest Rutherford was the bearer of these bad tidings. Rutherford worked out many of the fundamentals of radioactivity, showing that it was a natural alchemy that could transform one element into another. In 1904 he traveled from Montreal, where he taught at McGill University, to England to give a talk about the new discoveries.

I came into the room, which was half dark, and presently spotted Lord Kelvin in the audience and realized that I was in for trouble at the last part of the speech dealing with the age of the earth, where my views conflicted with his. To my relief, Kelvin fell fast asleep, but as I came to the important point, I saw the old bird sit up, open an eye and cock a baleful glance at me! Then a sudden inspiration came, and I said Lord Kelvin had limited the age of the earth, provided no new source of heat was discovered. That prophetic utterance refers to what we are now considering tonight, radium! Behold! the old boy beamed upon me.

That may be how Rutherford remembered that day, but Kelvin never publicly retracted his old estimate. Two years after hearing Rutherford’s talk, he was writing letters to the London Times maintaining there wasn’t enough radioactivity in the earth to keep it hot on the inside.

Rutherford realized that radioactivity not only showed that Earth was old, but could show how old it was. Any uranium that got trapped inside a cooling rock would decay gradually into lead. And because physicists knew uranium’s half‑life with great precision, it was possible to use the remaining proportions of lead and uranium to calculate how old a rock was.

With this method, geologists were soon estimating the age of various rocks not in millions of years but in billions. They later learned how to make Rutherford’s clock even more accurate. Instead of taking a single measurement of the lead and uranium in a rock, they began to measure their levels in many different parts of it. That allowed them to compare the parts that originally contained very little uranium to others that initially had high levels. If the uranium throughout the rock decayed at a uniform rate, the different samples should all point to the same age. And in many cases they do.

Geologists also learned how to measure time with two clocks at once. In addition to uranium 238, some rocks also contain uranium 235, which decays into a different isotope of lead, lead 207. It also has a different half‑life, of only 704 million years. With two independent tests for the age of a rock, geologists can often narrow down the margin of error even more.

They can also eliminate the uncertainty over whether any uranium or lead has crept into a rock after it was formed. As certain types of rocks form, atoms of zirconium and oxygen combine into crystals known as zircons. Zircons act like microscopic prisons: any uranium or lead atoms trapped inside a zircon have a very difficult time escaping, and few new atoms can enter it. Within its zircon cage, the uranium slowly breaks down into lead without any interference from the outside world. The geophysicists who put a date of 4.04 billion years on the rocks of Acasta did so by dating their zircons. They fired a beam of charged particles at the crystals, blasting out tiny clouds of isotopes that they then measured. Thanks to all the different crosschecks they performed, they were able to estimate its age within a margin of error of only 12 million years. Twelve million years may be a vast gulf of time for us, but for the Acasta rocks, it represents a margin of error of less than 0.3 percent.

The rocks at Acasta are the oldest known rocks on the planet, but they formed when Earth was already 500 million years old. Geologists needed a gift from space to find out the true age of the planet. In the 1940s they began studying the isotopes of lead in meteorites. Most meteorites are jumbles of space junk left behind from the formation of the solar system. In 1953 Claire Patterson, a geologist at the California Institute of Technology, measured the lead and uranium in the meteorite that had carved out the 1.2‑kilometer‑wide Meteor Crater in Arizona. It had practically no uranium left in it, because most of the atoms had turned into lead. This meteorite had formed at the dawn of the solar system and had circled the sun essentially unchanged ever since.

Meteorites and our planet all formed from the same primordial stuff, but each one ended up with different proportions of elements, including uranium and lead. By comparing the amount of uranium and lead isotopes in rocks from Earth and meteorites such as the one from Meteor Crater, Patterson determined Earth’s age. It is 4.55 billion years old.

Why is there a 500‑million‑year gap between the oldest rocks on Earth and its birth? Thanks in part to their ability to date rocks, geologists have discovered that Earth destroys its crust and creates new rock to take its place. The planet’s crust is actually a collection of drifting plates. Magma emerges from the depths of the earth and adds a fresh margin of rock to one side of a plate, and the other side becomes buried underneath its neighboring plate. As the sinking edge of the plate plunges into the planet, it warms up until it partially melts. Any fossils it might carry are destroyed with it.

Continents are floating islands of low‑density rock that sit on top of the moving plates. When a plate slides under its neighbor, a continent does not get sucked down with it. If a rock is lucky enough to be nestled within a continent, it may be spared Earth’s fiery cycle–along with fossils and other clues to life’s history it may hold. The rocks of Acasta are geological freaks.