Earth’s journey through the Milky Way could have helped create the planet’s first continents.
Comets may have bombarded Earth each time the early solar system traveled through our galaxy’s spiral arms, a new study suggests. Those recurring bombardments in turn helped trigger the formation of our planet’s continental crustresearchers propose on August 23 in geology.
Previous theories have suggested that such impacts could have played a role in the formation of Earth’s landmasses. But there has been little research to explain how those impacts happened, until now, the team says.
It’s an intriguing hypothesis, other scientists say, but it’s not the last word when it comes to explaining how the Earth got its land masses.
To look back in time, geochronologist Chris Kirkland and his colleagues turned to geological structures. known as cratons (Serial number: 3/12/10). These relics of the Earth’s ancient continental crust are some of the oldest rocks on the planet. Using material from cratons from Australia and Greenland that are billions of years old, the team measured the chemistry of more than 2,000 rock fragments. The analysis allowed the researchers to determine the exact ages of the rocks and whether they had formed anew from molten material deep within the Earth or from earlier generations of existing crust.
When Kirkland and his colleagues looked for patterns in their measurements, the team found that new crust seemed to form in jets at roughly regular intervals. “Every 200 million years, we see a pattern of increased crust production,” says Kirkland of Curtin University in Perth, Australia.
That moment a bell rang: it is also the frequency at which the Earth passes through the spiral arms of the milky way (Serial number: 12/30/15). The solar system rotates around the center of the galaxy slightly faster than the spiral arms, periodically passing through and overtaking them. Perhaps cosmic encounters with more stars, gas and dust within the spiral arms affected the young planet, the team suggests.
The idea makes sense, the researchers say, since the higher density of material in the spiral arms would have caused more gravitational tugs on the cometary reservoir at the periphery of our solar system (Serial number: 8/18/22). Some of those encounters would have sent comets zooming in on the inner solar system, and a fraction of those icy denizens would have collided with Earth, Kirkland and his team propose.
Earth was probably mostly covered by oceans billions of years ago, and the energy released by all those comets would have fractured the planet’s existing oceanic crust, the relatively dense rock present even earlier in Earth’s history, and excavated large amounts of material while launching a shock. waves on the planet. That chaos would have paved the way for parts of Earth’s mantle to melt, says Kirkland. The resulting magma would have naturally separated into a denser part, the precursor to more oceanic crust, and a lighter, buoyant liquid that eventually became continental crust, the researchers suggest.
That’s a hypothesis, but it’s far from a success, says Jesse Reimink, a Penn State geoscientist who was not involved in the research. For starters, comet and meteor impacts are notoriously hard to track, especially in the distant past, he says. “There are very few diagnoses of impacts.” And it’s not clear whether such impacts, if they occurred in the first place, would have resulted in the release of magma, he says.
In the future, Kirkland and his colleagues hope to analyze moon rocks to look for the same crusting pattern (Serial number: 7/15/19). Our closest celestial neighbor would have been hit by about the same number of things that hit Earth, says Kirkland. “One could predict that he too would be subject to these periodic impact events.”