`Earth was bombarded by asteroids 3-4 billion years ago`

Analysing tiny "spherules" embedded in layers of rock, a team from the Purdue University in the US said they found the "first real solid evidence" of asteroid bombardment.

London: Changes in the asteroid belt between Mars and Jupiter some three to four billion years ago hurled a devastating barrage of space rocks at our planet that probably delivered the ingredient crucial to the beginning of life, scientists have claimed.
Analysing tiny "spherules" embedded in layers of rock, a team from the Purdue University in the US said they found the "first real solid evidence" of asteroid bombardment.

According to the team, the spherules were created when asteroids crashed into the Earth, vaporising rock that expanded into space as a giant vapour plume. Small droplets of molten and vaporised rock in the plume then condensed and solidified, falling back to Earth as a thin layer.

These particles were preserved in layers of rock, and the scientists analysed them to record precise information about ancient asteroid impacts on Earth, 3.5 billion to 35 million years ago.

"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. This is the first real solid evidence that this asteroid bombardment actually happened," study author Prof Jay Melosh was quoted as saying by the Daily Mail.

The period of heavy asteroid bombardment between 4.2 billion and 3.5 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 between Mars and Jupiter, Melosh said.

Some of the asteroids were bigger than the nine-mile rock that wiped out the dinosaurs, but the 700-million-year barrage may have delivered chemicals crucial to the beginnings of life on our planet, the team said.

When the team looked at the number of impactors as a function of size, they got a curve that showed a lot more small objects than large ones, a pattern that matches exactly the distribution of sizes in the asteroid belt.

"For the first time we have a direct connection between the crater size distribution on the ancient Earth and the sizes of asteroids out in space," Melosh said. (

Because craters are difficult to study directly, impact history must be inferred either by observations of asteroids that periodically pass near the Earth or by studying craters on the moon.

Now, the new technique using spherules offers a far more accurate alternative to chronicle asteroid impacts on Earth, Melosh said.

"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," Melosh said.

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

For asteroids larger than about 10 km in diameter, the spherules are deposited in a global layer. "Some of these impacts were several times larger than the Chicxulub impact that killed off the dinosaurs 65 million years ago," said Purdue physics graduate student Brandon Johnson.

He said: "The impacts may have played a large role in the evolutional history of life. The large number of impacts may have helped simple life by introducing organics and other important materials at a time when life on Earth was just taking hold."

"A 40-km asteroid would have wiped out everything on the Earth`s surface, whereas the one that struck 65 million years ago killed only land animals weighing more than around 20 kg."


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