Earth got a "face-lift" early in its history, wiping out most of its original crust, according to a new model of the ancient barrage of asteroids called the Late Heavy Bombardment.
Earth itself is about 4.5 billion years ago, but it's rare to find rocks older than those formed about 3.8 billion years ago. One reason older rocks may be missing is that they were destroyed when asteroids and comets pummeled the Earth, moon and inner planets of the solar system, scientists report today (July 30) in the journal Nature.
"The surface of the Earth was heavily affected by all these collisions," said lead study author Simone Marchi, a planetary scientist with the Southwest Research Institute in Boulder, Colorado. "There's no doubt the crust was excavated, mixed and buried as a result of this bombardment." [Photo Timeline: How the Earth Formed]
According to the model by Marchi and his co-authors, the meteor storm resurfaced Earth's outer crust and destroyed much of the planet's original rocks, similar to how a dermatologist's microdermabrasion wand buffs away skin, giving patients an instant face-lift. They estimate that from one to four giant impacts by bodies 620 miles (1,000 kilometers) across before 4.2 billion years ago likely sterilized the planet, Marchi told Live Science. And there were three to seven smaller impacts by bodies 310 miles (500 km) across, which would have vaporized Earth's ocean into steam.
"If you look at this model, Earth only became habitable after 4.2 billion years ago," Marchi said.
Because there is little evidence on Earth to constrain the timing of such impacts, the researchers turned to the moon and to meteorites. They looked at the distribution of craters on the moon, and the age of collisions recorded in meteorites from asteroids such as Vesta. The team also analyzed the presence of iron-loving elements (the so-called highly siderophile elements), which hint at the timing of collisions after the solar system formed.
"We now have a full model for the bombardment of the inner solar system," Marchi said.
The study not only pinned down the timing of the Late Heavy Bombardment, it peered back into Earth's past, looking at how impacts reshaped the planet in the first 500 million years of its history. The researchers suggest Earth's cosmic crash-up was punctuated in time and space, with asteroids and meteors hitting the Earth in bursts, and the Late Heavy Bombardment that pummeled the planet starting about 4.2 billion to 4.1 billion years ago.
"This study makes a substantial contribution towards understanding the conditions on the early Earth," said Oleg Abramov, a research space scientist with the U.S. Geological Survey's Astrogeology Science Center in Flagstaff, Arizona, who was not involved in the study. "Its findings are generally in good agreement with previous estimates of crustal melting, ocean vaporization and sterilization by impact bombardment. This creates confidence that the scientific community is converging on an understanding of how impacts have fundamentally reshaped the early Earth."
Windows in time
Little of Earth's original crust escaped unscathed, the new model suggests. And that might explain another ancient Earth puzzle. Some of the only survivors from this hellish early period, known as the Hadean, are tiny minerals called zircons. The zircons are like time capsules. The crystals are layered, with each layer offering a window into a different geologic time period, all the way back to when the zircons formed 4.4 billion years ago.
The chemistry preserved in the zircons suggests they formed from rocks that were buried relatively deep in Earth's crust, and had contact with water. Marchi and his colleagues take these chemical signals as evidence that the rocks forming the zircons were buried by impacts. Other studies have suggested early plate tectonics or volcanism buried the zircons' parent rocks.
"These results pose an interesting idea," said Aaron Cavosie, a professor at the University of Puerto Rico in Mayaguez, who was not involved in the study. But there's a crucial piece of evidence for impacts missing from the geologic record, Cavosie said. No one has yet found shocked zircons, which are crystals fractured by the force of meteorite impacts on Earth.
"By the end of the Late Heavy Bombardment, the crust would have contained a global distribution of shocked zircons; the new model does not explain the absence of these grains," Cavosie said. "The absence of shocked Hadean grains remains a mystery."
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