How did the arms of the Milky Way help create the solid Earth?

Earth’s journey through the Milky Way may have helped create the planet’s first continents.

A new study has found that comets may have bombarded Earth every time the early solar system traveled through the spiral arms of our galaxy. These recurring bombs, in turn, helped shape our planet’s continental crust, researchers suggested on August 23 in geology.

Previous theories suggested that such effects may have played a role in the formation of land masses on Earth. The team says there is little research explaining how these effects occur so far.

Other scientists say it’s an interesting hypothesis, but it’s not the last word when it comes to explaining how the Earth acquired its land masses.

To look back in time, chronological geologist Chris Kirkland and his colleagues turned to geological structures known as cratons.SN: 12/3/10). The remains of the ancient continental crust of the Earth are among the oldest rocks of the planet. Using materials from Craton in Australia and Greenland that are billions of years old, the team measured the chemistry of more than 2,000 pieces of rock. The analysis allowed the researchers to determine the exact ages of the rocks, and whether they formed anew from magma deep in the Earth or from previous generations of existing crust.

When Kirkland and his colleagues looked for patterns in their measurements, the team found that new crust appeared to form in batches at roughly regular intervals. “Every 200 million years, we see a pattern of increased scale production,” says Kirkland, of Curtin University in Perth, Australia.

This timing rings the bell: it is also the frequency with which the Earth passes through the spiral arms of the Milky Way (SN: 12/30/15). The solar system revolves around the galactic center a little faster than the motion of the spiral arms, periodically passing and bypassing them. The team suggests that cosmic encounters with more stars, gas and dust within the spiral arms have affected the young planet.

The idea makes sense, the researchers say, because the higher density of material in the spiral arms would have resulted in more gravitational pull on the comet reservoir in our solar system’s periphery (SN: 8/18/22). Kirkland and his team suggest that some of these encounters would have sent comets into the inner solar system, and a small portion of these icy residents would have collided with Earth.

Earth was likely covered in oceans billions of years ago, and the energy provided by all those comets broke the planet’s oceanic crust — the relatively dense rocks that have existed since early Earth’s history — and extracted copious amounts of material during the shock release. waves in the planet. This mess, Kirkland says, would have paved the way for parts of the Earth’s mantle to melt. The researchers suggest that the resulting magma would have naturally separated into a denser portion – introducing more oceanic crust – and a lighter, more buoyant liquid that eventually turned into continental crust.

That’s one hypothesis, but it’s far from a hard blow, says Jesse Remink, a Penn State geologist who was not involved in the research. For starters, the impacts of comets and meteorites are known to be difficult to track, especially in that distant time, he says. “There are very few impact diagnoses.” And it’s not well known whether these effects, if they occurred in the first place, would cause magma to be released, he says.

In the future, Kirkland and colleagues hope to analyze moon rocks to look for the same pattern of crust formation (SN: 7/15/19). Our closest celestial neighbor, Kirkland says, would have been frustrated by the same amount of stuff hitting Earth. “You would expect that he would also be prone to these cyclical-effect events.”

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