It’s no secret the early Earth took a beating from above, but now it seems the planet sustained a longer bombardment than initially thought.
Scientists used to think this pummeling — a spike of violent impacts during the lunar Late Heavy Bombardment – lasted several hundred million years at most. But new simulations that incorporate an extended asteroid belt, combined with evidence pulled from the Earth’s own rocky record, suggest Earth’s pummeling lasted for billions of years, two teams report online April 25 in Nature.
“It seems highly likely that these impacts affected the Earth’s biosphere in profound ways,” says planetary scientist and study coauthor Bill Bottke of the Southwest Research Institute in Boulder, Colo.
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Reconstructions of the early solar system peg the Late Heavy Bombardment as beginning about 4.1 billion years ago, triggered by a cataclysmic rearrangement of the outer planets. At that time, Jupiter and Saturn’s orbits fell in lockstep, unleashing a gravitational tidal wave that sent their ice giant brethren farther out while simultaneously flinging smaller bodies — asteroids, comets and such — toward the inner solar system.
Earlier theories suggest the turbulence ended by about 3.7 billion years ago.
Yet there’s evidence for enormous impacts as recent as 1.8 billion years ago on Earth. And the moon’s impact history isn’t resolved either, with some scientists questioning the ages of lunar basins and the type of asteroids that may have scarred its face. These discrepancies suggested to Bottke that the original story needed some revising. “Where are these impactors coming from? We must be missing a source,” he says.
Bottke considered whether a larger reservoir of asteroids could explain the observations. In his new version of events, the asteroid belt’s inner boundary used to be closer to where Mars currently resides, rather than out at 2.1 times the Earth-Sun distance, where it is now.
Bottke’s simulations show that as the outer solar system spasmed, it perturbed this reservoir, called the E-belt, hurling asteroids inward over billions of years. These asteroids are a hotter, more cooked kind of space rock than most of those hanging around today, and can explain both the odd composition of lunar impactors and evidence for fresher impacts.
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Bottke says he initially hated the result, since it didn’t produce an impact spike as expected, and instead showed a gradual decline over billions of years. “I fought against it for a year,” he says. “Then I realized this signature could explain something.”
That something — the longer bombardment, and younger craters — matches the story told by tiny beads embedded in Earth’s sediments. Called impact spherules, these glassy spheres formed after giant impacts ejected a plume of vaporized rock, which coated the globe and condensed into tiny, part-extraterrestrial BBs. The age of the spherules suggests a gradual decline in Earth’s pummeling. “You can do all the modeling you want, but if there’s no specific evidence to support those models, then you’re left with a model without a home,” says Donald Lowe, a geologist at Stanford University. “This is a good marriage of geology in the field, and models and calculations done in the lab.”
The spherules form layers, or beds – signs of impacts that linger long after craters have been erased by Earth’s tectonic activity. “If you find one of these spherule layers, you can estimate the thickness of it, and you can estimate the size of the asteroid that created that layer,” says graduate student Brandon Johnson of Purdue University in Indiana, a coauthor of the second study. “It’s the first time we’ve been able to get an idea of what size objects were hitting the Earth.”
Johnson’s calculations suggest that most of the earlier impactors were enormous, up to 70 kilometers wide – much bigger than the dinosaur-killing asteroid that arrived 65 million years ago. The spherule layer formed by that collision is only 3 millimeters thick. Other layers, like one called S3, are around 35 centimeters thick. “We would all be gone,” says planetary scientist and coauthor Jay Melosh of Purdue University.
But Melosh and others speculate that a prolonged bombardment wasn’t all bad news for Earthly life at the time. “It may have toughened up our ancestors because they had to muscle through all these giant impacts,” Melosh says.
The collisions might also have kicked life into a higher gear by altering or delivering organic molecules. And although the collisions were huge, it’s unlikely that any sterilized the planet. “If you’re going to wipe out life, you have to at least boil the ocean,” says planetary scientist Norm Sleep of Stanford University. “That takes about a 300-kilometer diameter projectile.”