How old is the earth? | popular science

How old is the earth? It may seem like a simple question to answer. The typical estimate is that our planet is about 4.5 billion years old. But the more closely planetary scientists look, the more sensitive that story becomes. Nuances about how our planet formed could change Earth’s age by half a billion years or so.

“It’s easy to talk about era, but it gets more complicated the bigger you zoom in,” says geology professor Thomas Lappin, who heads the Department of Earth and Atmospheric Sciences at the University of Houston. As scientists sought to determine more precise 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 a moment in time,” Lapin explains. But planet formation is a process that takes millions of years. To assign an age to the Earth, astrophysicists, planetary scientists, and geologists have to pinpoint the point in the process that can be considered the birth of the Earth.

When was the earth “born”?

About 4.6 billion years ago, gas and dust were orbiting around the newly formed sun. Over the first millions of years of the solar system, particles collided and coalesced into asteroids and planetary seeds. Those space rocks kept crashing into each other, some growing bigger and bigger, forming the solar system as we see it today.

But planets aren’t just big rocky piles. Since they collect material, these celestial bodies are also differentiated into core, mantle, and crust layers (at least in the case of Earth and other terrestrial planets). Accumulation and differentiation takes time, most likely on the order of tens of millions of years. Some may consider a point at that point in the Earth’s formation as the birth of our planet. But Lapin says he believes it depicts the Earth, and the birth came later, when a cataclysmic event also caused the formation of 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, proto-Earth collided with another small body the size of Mars. When the two collided together, debris coalesced into the moon in orbit around Earth.

Lapine believes that this effect essentially “reset” the materials that make up the planet. At that time, a thick magma ocean might have covered the primeval earth. Upon the powerful collision, the materials of both bodies mixed together and merged into the planet-moon system we know today. Evidence for such a “reset” comes from terrestrial and lunar rocks that contain identical forms of oxygen, Labine explains.

“Proto-Earth was, most likely, destroyed or its composition changed,” says Lapin. “In my opinion, the Earth was not the Earth as we know it until the formation of the Moon occurred.”

If this event represented the birth of our planet, that would make the Earth between 4.4 billion and 4.52 billion years old. But determining a more definitive age for our planet requires sifting through ancient evidence.

Assign a number to the age of our planet

Like investigators searching for clues to an ancient crime, planetary scientists must consider the evidence left today when piecing together our planet’s early history. But with all the upheaval during that season—a crumbling magma ocean and intense geological circulation—the evidence can be hard to find.

Lapine explains that one way to constrain the age of the Earth is to look for the oldest rocks on the planet, which formed right after the magma ocean solidified into a solid surface. For this date, scientists looked to zircons discovered in Jack Hills in Western Australia – the oldest known mineral.

To determine the age of these crystals, a team of scientists used a technique called radiometric dating, which measures the amount of uranium they contain. Because this radioactive element decays into lead at a known rate, scientists can calculate the age of the mineral based on the ratio of uranium to lead in the sample. This method revealed that the zircon is about 4.4 billion years old.

These rocks indicate that the Earth-Moon system must have formed at some point in time Before 4.4 billion years ago, because the rock record “would be erased by the moon formation event,” says Labine. So the planet is at least 4.4 billion years old. But how much older could it be? To answer that, Labine says, scientists are turning elsewhere, including to the moon.

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The rocks on the body of the satellite are better preserved than those here, because the moon is not subject to processes such as plate tectonics that would melt and alter its surface. There are two main sources for these clues: in lunar meteorites that fall to Earth and in samples collected directly from the moon during NASA’s Apollo program.

Like the Primordial Earth, the Little Moon was also covered by a magma ocean. The oldest rocks from the lunar surface can indicate when the crust formed. Scientists performed radiometric dating of zircon fragments collected during the Apollo 14 mission, corrected cosmic ray exposure calculations, and determined that the lunar crust hardened about 4.51 billion years ago.

There could have been a period of time between the impact and the objects gathering, cooling and differentiating, Labine says, so this date also has a window of uncertainty of about 50 million years.

“Determining exactly when the event will take place is quite a challenge,” he says. Lapine estimates that the Earth-Moon system likely formed between 4.51 billion and 4.52 billion years ago, but some scientists say the calculations could be as far back as 50 million years.

Another way to constrain that time period is to look at the rocks that were present when the proto-Earth was forming. When the planets solidified from the debris around the young sun, not all of the material coalesced into the worlds and their moons that we see today. Some remain preserved in asteroids or comets.

Sometimes these time capsules of the solar system come to us in the form of meteorites that fall to the surface of our planet. The oldest known space rock is the Erg Chech 002 meteorite, Lappin says. It is believed to be part of the igneous crust of a primordial planet from the early solar system. As such, dating the Erg Chech 002 meteorite provides a snapshot of when the proto-Earth was likely at a similar stage in its conception.

“If ‘the birth of the Earth’ is defined as the time of formation of the first proto-Earth core or a protoplanet that eventually grew through accretion to form the present Earth,” says Lapine, “then it may have been as long ago as the age of the Earth.” [Erg Chech 002]. Scientists calculated this piece of igneous crust that crystallized about 4.565 billion years ago.

Can the age of the Earth be refined?

On human time scales, the 50-million-year uncertainty about when the Earth-Moon system formed seems vast and imprecise. But on planetary time scales, particularly billions of years ago, “it’s a pretty good estimate,” says Labine.

“The further back we look, the less subtle things are often because of the gaps in the record. It’s a relatively short period of time, where a lot of things were going on—there was an impact, and everything had to coalesce, cool, and solidify into solid rocky bodies for her core, covering, and shell,” he says.

However, the scientists are not done. There’s always an opportunity to get more precise and precise measurements of Earth’s age, Lapine says, especially as researchers obtain additional samples from the Moon, meteorites, and asteroids.

For example, the rocks obtained by the Chinese Chang’e 5 mission to the moon are still being studied. NASA’s Artemis program also plans to collect lunar rocks. Lapine adds that the NASA management team is also providing more material for study from the Apollo program, which provides researchers with an opportunity to study ancient moon rocks with new techniques.

If these new endeavors reveal lunar samples from parts of the lunar surface that crystallized earlier than previously studied, he says, it could constrain time estimates for when the Earth and Moon were born.

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