How old is the Earth? | popular science

How old is the Earth? This may seem like a simple question to answer. The rough estimate is that our planet is about 4.5 billion years old. But the closer planetary scientists look, the more squishy this story becomes. Nuances in the formation of our planet could change the age of the Earth by about half a billion years.

“It’s easy to talk about an age, but it gets more and more complex as you zoom in,” says a geology professor Thomas Lapen, who chairs the Department of Earth and Atmospheric Sciences at the University of Houston. As scientists sought to determine more precise measurements of Earth’s age, they had to grapple with the specifics of how our planet was born.

“When you’re born, it’s a moment in time,” says Lapen. 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 what point in the process could be considered the birth of the Earth.

When was the Earth “born”?

Around 4.6 billion years ago, gas and dust swirled around the newly formed sun. During the first million years of the solar system, particles collided and coalesced into asteroids and seed planets. These space rocks kept bumping into each other, with some growing larger and larger, shaping the solar system as we see it today.

But planets aren’t just big piles of rock. As they amass material, these celestial bodies also differentiate into layers of a core, mantle, and crust (at least in the case of Earth and other terrestrial planets) . Accretion and differentiation take time, probably on the order of Tens of millions of years. Some might consider a point in this stage of Earth’s formation to be the birth of our planet. But Lapen says he sees this as Earth’s conception, and the birth came later, when a cataclysmic event also formed 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 Mars-sized body. When the two collided, the debris merged into the moon orbiting Earth.

This impact is also thought to have essentially “reset” the materials that made up the planet, Lapen says. At the time, a thick ocean of magma may have covered proto-Earth. In the powerful collision, material from the two bodies mixed and merged into the planetary and lunar system we know today. Evidence for such a “reset” comes from Earth and moon rocks that contain identical forms of oxygen, Lapen says.

“Proto-Earth was, in all likelihood, either destroyed or altered in composition,” Lapen says. “In my mind, Earth was not Earth as we know it until the moon formation event.”

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

Assign a number to the age of our planet

Like detectives searching for clues to an ancient crime, planetary scientists must examine the evidence that remains today as they piece together our planet’s early history. But with all the turmoil in this chapter – the churning ocean of magma and the intense geological turnover – proof can be hard to come by.

One way to limit Earth’s age is to search for the planet’s oldest rocks, Lapen says, which formed just after the ocean of magma hardened into a solid surface. For this date, scientists look to zircons discovered 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. Since this radioactive element decays into lead at a known rate, scientists can calculate the age of a mineral based on the ratio of uranium to lead in the sample. This method revealed 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 be erased by the moon formation event,” Lapen says. The planet is therefore no less than 4.4 billion years old. But how long can he be old? To answer that, Lapen says, scientists are looking elsewhere, including the moon.

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Rocks in Earth’s satellite body are better preserved than those here, because the moon does not undergo processes like plate tectonics that would melt and reshape its surface. There are two main sources for these clues: in lunar meteorites that fall to Earth and in samples taken directly from the moon during NASA’s Apollo program.

Like proto-Earth, the young moon was also covered in an ocean of magma. The oldest rocks taken from the lunar surface can indicate when the lunar crust formed. Scientists performed radiometric dating on fragments of zircon collected during the Apollo 14 mission, correcting cosmic ray exposure calculations and determining that the lunar crust had hardened. about 4.51 billion years ago.

There would have been a period of time between the collision and the melting, cooling and differentiation of the bodies, Lapen says, so this date also has a window of uncertainty of around 50 million years.

“Dating the exact event is very difficult,” 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 the calculations could be up to 50 million years old.

Another way to constrain this window of time is to look at the rocks that existed when 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 have remained preserved in asteroids or comets.

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

“If the ‘birth of the Earth’ is defined as the time of the formation of the first proto-Earth core or protoplanet that eventually grew by accretion to form present-day Earth,” Lapen says, “then perhaps that was it. as long ago as the age of [Erg Chech 002].” Scientists calculated that this piece of igneous crust crystallized around 4.565 billion years ago.

Can the age of the Earth be refined?

On a human scale, a 50 million year uncertainty around the formation of the Earth-Moon system seems vast and imprecise. But on planetary timescales, particularly billions of years ago, “that’s a good estimate,” Lapen says.

“The further we look back, often things are less accurate because of gaps in the record. It’s a relatively short period of time, where a lot of things happened – there was the impact, everything must have coalesce, cool and differentiate into robust rocky bodies that have a core, a mantle and a crust,” he says.

However, scientists are not done. There’s always the possibility of getting more accurate and precise measurements of Earth’s age, Lapen says, especially when researchers get additional samples from the moon, meteorites and asteroids.

Rocks acquired by China’s Chang’e 5 mission to the moon are still being studied, for example. NASA’s Artemis program also plans to collect moon rocks. And NASA’s conservation team is also making more materials available for study through the Apollo program, which offers researchers the ability to study ancient moon rocks with new technologies, Lapen adds.

If these new attempts reveal lunar samples from parts of the moon’s surface that crystallized earlier than previously studied, he says, that could still constrain time estimates for the birth of Earth and the moon.

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