How The Existence Of Lead-206 Proves The Earth Is Far Older Than 4,000 Years

In 1844, physicist William Thomson (aka Lord Kelvin) came up with an idea of how to measure the Earth's age. He assumed that in the beginning, the Earth was a big molten blob in space. We're not saying the man was temperature-obsessed, but looking at how long it would take the planet to cool to the state it's in now, he reasoned that he could figure out the age of the world itself.

Doing the math years later, he estimated Earth to be around 20 million to 400 million years old. This was at odds with a number of things known at the time, such as geology, Darwin and Russell's ideas of how long it would take animal evolution to take place, and the fact that he only believed the Sun to be less than around 20 million years old.

Religious thinkers, though having the advantage of being more precise with their estimates, were even further from the truth. For example, in the 16th century, James Ussher, the Archbishop of Armagh, attempted to calculate the age of the Earth based on the Bible and medieval historical accounts. His influential estimate placed Earth's creation date to October 22, 4004 BCE, a Sunday, at sunset.

In the 19th century, scientists began to question how old the Earth really is, as we began to realize how much time things like geological processes and evolution take. Charles Darwin himself believed that the Earth was far older, estimating the age of rocks in the Weald in England to 300 million years based on erosion patterns, before he was pressured to revise that figure downwards.

While estimates based on geology rather than written texts got closer to the answer, scientists needed a way of dating the Earth further back in time, and more accurately. In around 1905, Sir Ernest Rutherford realized that we could do this with uranium and lead.

All isotopes of uranium are unstable and radioactive, and over time take a meandering path before becoming a stable form of lead. Knowing about this decay process, it could be used to make better estimates of the ages of rocks. Then, by finding the oldest rock you can and dating it, you can put a lower bound on the age of our planet.

"Uranium, which scientists call a parent isotope, radioactively decays to lead by ejecting protons and neutrons from the atom’s nucleus. Once it gets to be lead, the atom is stable and not radioactive anymore," geologist and science communicator Rudy Molinek explained to the Tiny Matters podcast. 

"But, importantly, lead doesn’t fit in the zircon’s crystal structure when it first forms, so any lead contained in a zircon crystal would have had to come from that initial uranium breaking down. Scientists know how long this radioactive decay takes, so by comparing the amount of uranium to the amount of lead in a zircon crystal, they can figure out exactly how old it is."

As another little radioactive treat, uranium-238, which decays into lead-206, has a half-life of around 4.5 billion years, meaning it would take that amount of time for half of it to decay. So, finding rocks on Earth that contain significant amounts of lead and little radium would be impossible were the Earth just thousands of years old. 

While not perfect, Rutherford's work was the first to provide evidence for anything on Earth being older than a billion years old. Since then, we have dated far older rocks, with the oldest being from the Acasta Gneiss Complex of northwest Canada, dating to around 4.02 billion years ago.

This gives us a nice lower bound, but to figure it out further, scientists also date rocks that came from elsewhere in the Solar System. Earth's oldest rocks could have slid down into the mantle, making aging the Earth difficult, but meteorites smashing into the Earth in recent geological timescales, and rocks collected from the Moon, do not have this issue. Looking at these rocks, scientists now believe the Earth to be around 4.54 billion years old, making 4004 BCE look like seconds ago.