Astronomers find a shocking culprit who shaped Earth’s continents

The earth is the only one Planet known to have continents. In a new study published in the journal Aug. 23 geologyresearchers propose unexpected seeds for the formation of our world’s first continents — comets that were shot into our solar system as it passed the spiral arms of the Milky Way.

HERE IS THE BACKGROUND – Unlike any other known planet, the surface of the earth has both continents and oceans. Continental crust is less dense and much thicker than oceanic crust, causing it to float essentially higher than oceanic crust on the dense underlying mantle.

The presence of continents on Earth has greatly affected the atmosphere, oceans, climate, and the spread of life. For example, runoff from continents is the main source of a number of important nutrients for the oceans, such as phosphorus, which is needed for the formation of DNA and other biological building blocks.

Previous research uncovered cycles in the production of continental crust. Scientists generally thought these cycles were related to the periodic birth and destruction of supercontinents on Earth’s surface due to plate tectonics — the drifting, crashing, and diving of the vast slabs of rock that currently make up the Earth’s surface.

However, these cycles can also be seen in some of Earth’s oldest rocks, when plate tectonics may not have existed.

WHAT DID THE SCIENTISTS DO? — In the new study, the researchers analyzed data from two locations where the earliest history of continents on Earth is preserved — the North American craton in Greenland and the Pilbara craton in Western Australia. (Cratons are huge stable blocks of the Earth’s crust that form the hearts of continents.)

The decay of uranium into zircon crystals helped scientists determine details about continental formation at both locations, spanning a period of about 2.8 billion to 3.8 billion years ago. Hafnium isotopes in these crystals also helped identify times when magma inflows related to crust formation occurred.

By looking at huge amounts of data from large amounts of rock, the researchers discovered a rhythmic pattern in the formation of the continental crust some 170 million to 200 million years ago. They discovered a similar pattern when looking at oxygen isotopes, which supports their findings. “Only through large datasets can this pattern be identified,” says Chris Kirkland, lead author of the study, a geochronologist at Curtin University in Perth, Australia Vice versa.

WHAT DID YOU FIND? — This pattern corresponds to the time it takes for the solar system to pass through one of the four major spiral arms of the Milky Way, where the density of stars and interstellar clouds is high, as it completes an orbit around the heart of the galaxy.

The gravitational influence of the spiral arms could hurl comets inward toward Earth and the other planets from the Oort Cloud at the very edge of the solar system. The scientists argue that such cometary bombardment may have excavated vast amounts of rock from Earth’s surface, causing the underlying mantle rock to decompress and melt, a bit like popping a cork on a bottle of champagne. The formation of this lively molten rock, in turn, may have triggered the formation of continents.

“While impacts are viewed as catastrophic events, they have also shaped the evolution of our planet, and it appears our continents would not have evolved the way they did without them,” says Kirkland. “Our planet is connected to the structure of the galaxy.”

The researchers found further evidence for this idea in spherical beds, which are rock formations formed by cosmic impacts. These contain deposits of tiny spheres formed either from molten rock ejected during a collision or from vaporized rock that condensed and rained out after impact. They found that the age of the ball beds in Australia and South Africa coincided with the solar system’s movement into the Norma spiral arm, around 3.25 billion to 3.45 billion years ago. Determining the age of more bead beds could provide further evidence, they noted.

“It is becoming increasingly clear that the solar system and the Earth did not form by themselves, but through outside influence,” says study co-author Phil Sutton, a planetary scientist at the University of Lincoln in England Vice versa. “We need to look outward instead of just focusing on internal mechanisms.”

Although Earth experiences impacts from rock bodies in the asteroid belt much more frequently than from comets in the Oort Cloud, collisions from the Oort Cloud would strike with much more energy. That’s because near-Earth asteroids “travel in the same direction as Earth,” says Sutton. “So if there’s an impact from that group, the difference in speed is much smaller.”

Imagine cars driving in the same direction on a multi-lane road. When one car hits another next to it, “the relative speed of impact is pretty small,” he explains.

However, if there were an intersection across that road and one car crashed into another at a 90-degree angle at that intersection, “the damage would be orders of magnitude greater,” says Sutton. “That’s essentially what happens with comets versus asteroids.”

In addition, the gravity of the Sun and Jupiter makes it difficult to knock near-Earth asteroids out of orbit. On the other hand, comets in the Oort cloud have a weak influence from the Sun, which makes them more susceptible to disturbances, Sutton explains.

The researchers emphasize that the time periods during which these massive impacts occur are “extreme and in no way pose a threat to us,” says Sutton. “For example, dinosaurs have been on the planet for about as long as it takes to get from one spiral arm to the next, which is hundreds to thousands longer than humans exist.”

Interestingly, “life is thought to arise around the times that we report periodic crust formation,” says Sutton. “Is that a coincidence or was it the spark that ignited the explosion of life on the planet?”

WHAT’S NEXT? — The role of impacts in the formation of the continental crust likely decreased over time as the average size and number of potential impacts decreased exponentially as the solar system evolved. The formation of plate tectonics would also have begun to play a more important role in continent production on Earth, the scientists note.

“It would be great to see a similar pattern in the timing of lunar impacts,” says Kirkland. “They would predict the same frequency of impacts for the moon.”