Objects called spherules are helping researchers record more precise information about asteroids that crashed to Earth from 35 million to 3.5 billion years ago.

Spherules are created from vapourizing rock that expanded into space as a giant vapour plume. Small droplets of molten and vapourized rock in the plume condensed and solidified, falling back to Earth as a thin layer.

The round or oblong particles, most about a millimetre in diameter, were preserved in layers of rock, and now researchers are analyzing them to glean more accurate information than they would get from studying craters only.

While craters seem to be the most obvious indication that asteroid has struck, their features can be obscured by surface weathering and tectonic processes.

"We can look at these spherules, see how thick the layer is, how big the spherules are, and we can infer the size and velocity of the asteroid," said Jay Melosh, an expert in impact cratering and a professor of Earth and atmospheric sciences, physics and aerospace engineering at Purdue University in Indiana.

"We can go back to the earliest era in the history of the Earth and infer the population of asteroids impacting the planet," he said.

On collision course

Some of the asteroids studied using the new kind of method of extracting information were more than twice the size of the one said to have killed off the dinosaurs 65 million years ago, said Purdue physics graduate student Brandon Johnson.

Johnson and Melosh included the findings in their research paper on spherules, appearing online in the journal Nature on Wednesday.

The period of heavy asteroid bombardment — from 3.5 billion to 4.2 billion years ago — is thought to have been influenced by changes in the early solar system that altered the trajectory of objects in an asteroid belt located between Mars and Jupiter, sending them on a collision course with Earth.

The researchers say it’s possible that asteroids measuring 40 kilometres in diameter struck the Earth during this period, and one that size would have wiped out everything on the Earth's surface.

On the other hand, they say, the asteroid that struck 65 million years ago — which was about 12 to 15 kilometres in diameter — killed only land animals weighing more than around 20 kilograms.

Johnson says the earlier impacts may have introduced important organic material at a time when life in Earth was just taking hold.