The tiny rounded nodules are the result of asteroids crashing into the planet, vaporizing rock that expanded into space as a giant vapor plume that then settled back to Earth -- with small droplets of molten and vaporized rock in the plume condensing and solidifying into the spherules.
Preserved in layers of rock, they are providing researchers with precise information about asteroids impacting Earth from 3.5 billion to 35 million years ago, Purdue University reported Wednesday.
"What we have done is provide the foundation for understanding how to interpret the layers in terms of the size and velocity of the asteroid that made them," Purdue researcher Jay Melosh, an expert in impact cratering, said.
The findings support a theory that Earth endured an especially heavy period of asteroid bombardment early in its history, from 4.2 billion to 3.5 billion years ago, when changes in the early solar system altered the trajectory of objects in an asteroid belt located between Mars and Jupiter, sending them on a collision course with Earth.
"Impact craters are the most obvious indication of asteroid impacts, but craters on Earth are quickly obscured or destroyed by surface weathering and tectonic processes," researcher Brandon Johnson said. "However, the spherule layers, if preserved in the geologic record, provide information about an impact even when the source crater cannot be found."