How old is the Earth? It may seem like a simple question to answer. The typical ballpark estimate is that our planet is about 4.5 billion years old. But the closer planetary scientists look, the squishier that story gets. Nuances about how our planet formed could change the age of Earth by half a billion years or so.
“It’s easy to talk about age, but it gets more complicated when you zoom in,” says geology professor Thomas Lapen, who chairs the department of Earth and atmospheric sciences at the University of Houston. As scientists tried to determine more accurate measurements of the Earth’s age, they had to grapple with the details of how our planet came to be.
“When you’re born, it’s an instant of time,” Lapen explains. But planetary formation is a process that takes millions of years. To assign an age to the Earth, astrophysicists, planetary scientists and geologists must determine at what point in the process could be considered the birth of the Earth.
When was the Earth “born”?
About 4.6 billion years ago, gas and dust orbited the newly formed sun. Over the first millions of years of the solar system, particles collided and merged into asteroids and planet seeds. Those space rocks were impacting each other, some growing larger and larger, shaping the solar system as we see it today.
But the planets aren’t just big piles of rock. As they accumulate material, these celestial bodies also differentiate into core, mantle, and crust layers (at least in the case of Earth and the other terrestrial planets). Accreditation and differentiation take time, probably thousands of years. Some people might consider a point in that phase of the Earth’s formation as the birth of our planet. But Lapen says that he thinks of it as the concept of the Earth, and birth came later, when a cataclysmic event also created the moon.
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According to the widely accepted giant impact theory, during the chaos of the early days of our solar system, the proto-Earth collided with another small body about the size of Mars. When the two collided, the debris collided into the moon orbiting the Earth.
This effect is thought to be essentially “resetting” the materials in the planet, says Lapen. At the time, a proton-Earth could have been covered by a thick magma ocean. When the powerful collision occurred, the matter of the two bodies mixed together and coalesced into the planet and moon system we know today. Evidence for such “rearrangement” comes from terrestrial and lunar rocks that contain identical forms of oxygen, Lapen explains.
“Most likely, the composition of Proto-Earth was destroyed or changed,” says Lapen. “In my mind, the Earth was not the Earth as we know it until the moon formation event.”
If this event marked the birth of our planet, that would make the Earth somewhere between 4.4 billion and 4.52 billion years old. But sifting through ancient evidence is needed to determine a more specific age of our planet.
Assign a number to the age of our planet
Like detectives searching for clues to an ancient crime, planetary scientists must look at the evidence that exists today when piecing together the early history of our planet. But with all the turmoil during that chapter—the raging magma ocean and intense geological turnover—the proof is hard to come by.
One way to constrain the age of the Earth is to search for the oldest rocks on the planet, Lapen explains, which were formed just after the magma ocean had hardened into a solid surface. For that date, scientists look to zircons found in the Jack Hills in Western Australia – the oldest known minerals.
To determine the age of these crystals, a team of scientists used a technique called radiometric dating, which measures the uranium they contain. Because this radioactive element decays into lead at a known rate, scientists can calculate a mineral’s age based on the ratio of uranium to lead in the sample. This method showed that the zircons are about 4.4 billion years old.
These rocks suggest that the Earth-Moon system must have formed at some point before 4.4 billion years ago, because the rock record “would have been destroyed by the disappearance of the moon,” says Lapen. So the planet is no younger than 4.4 billion years old. But how much older could it be? To answer that, says Lapen, scientists turn elsewhere – including the moon.
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Rocks on Earth’s satellite bodies are better preserved than those here, because the moon does not undergo processes such as plate tectonics that would melt and reset its surface. There are two main sources for these clues: in lunar meteorites that fall to Earth and in the samples collected directly from the moon during NASA’s Apollo program.
Like proto-Earth, the young moon was also covered in a magma ocean. The oldest rocks taken from the lunar surface show when the lunar crust was formed. Scientists radiometrically dated zircon fragments collected during the Apollo 14 mission, correcting the calculations for cosmic ray exposure, and determined that the lunar crust hardened about 4.51 billion years ago.
There would have been a period of time between the collision and the bodies merging, cooling and differentiating, says Lapen, so this date also has a window of uncertainty, about 50 million years.
“It’s very challenging to go ahead with the exact event,” he says. Lapen estimates that the Earth-Moon system probably formed between 4.51 billion and 4.52 billion years ago, but some scientists say calculations could be as far off by 50 million years.
Another way to constrain that time window is to look at rocks that were there when the proto-Earth was being formed. When the planets solidified from the debris around the young sun, all the material did not combine into the globes and moons we see today. Some remained preserved in asteroids or comets.
Sometimes the time capsules of that solar system come to us as debris that falls to the surface of our planet. The oldest such space rock, according to Lapen, is the Erg Chech 002 meteorite. It is thought to be a fragment of the tonnoid crust of a primitive protoplanet from the early solar system. Thus, the dating of the Erg Chech 002 meteorite provides insight into the time when the proto-Earth was likely at a similar stage in its generation.
“If ‘the birth of the Earth’ is defined as the time of formation of the first nucleus or protoplanet from Earth that eventually grew by accretion to form the present Earth,” says Lapen, “then it may have been as long ago shin. the age [Erg Chech 002].” Scientists have calculated that this chunk of undulating crust crystallized about 4.565 billion years ago.
Can the age of the Earth be refined?
In terms of human timescales, the uncertainty is 50 million years around when the massive and imprecise Earth-Moon system formed. But on planetary time scales, especially billions of years ago, “it’s a good estimate,” Lapen says.
“The further back we look, things are often as precise as the gaps in the record. It’s a relatively short period, where a lot of things were happening – the impact was there, everything had to come together, and cool, and differentiate into a solid rocky body with a core, mantle and crust,” a he says.
However, scientists are not made. There is always the opportunity to get more accurate and precise measurements of the Earth’s age, according to Lapen, especially when researchers get additional samples from the moon, meteorites, and asteroids.
Rocks found by China’s Chang’e 5 mission to the moon, for example, are still being studied. NASA’s Artemis program also plans to collect moon rocks. And NASA’s curatorial team is also providing more material for study from the Apollo program, which gives researchers the opportunity to study old lunar rocks with new technologies, Lapen adds.
If these new efforts reveal lunar samples from parts of the moon’s surface that crystallized earlier than previously studied, he says, that could further constrain estimates of when the Earth and the moon were born.