The dark chapter of the Earth’s history (the first 500 million years) called the Hadean saw the planet being bombarded by asteroids on a frequent basis. The pockmarked surface of the moon would exist on Earth had it not been for around 4 billion years of erosion and the plate tectonics not covered up this evidence.

Hadean Earth Asteroids
Asteroids

This comes from a group of scientist who published a study in the journal Nature yesterday. The group also believes that the earth melted numerous times during the Hadean eon following larger collisions that likely vaporized the oceans killing all early life, according to Julia Rosen of the Los Angeles Times.

Asteroids melted early Earth: Sources of the findings

“The reason is very simple: If you have a crater, you had an impact,” said Simone Marchi, a geologist at the Southwest Research Institute in Boulder, Colo., and lead author of the Nature study.

Marchi and others based their findings on the frequency of craters on the lunar surface, dated these impacts using the moon rocks collected during the Apollo missions and modeled how asteroid frequency has changed over time. This work was limited to craters up to 100 miles across as larger objects did not strike the moon.

For objects above this threshold, the researchers looked to the Main Asteroid Belt between Mars and Jupiter where it could find the larger objects or “massive impactors.”

Using these means the authors of the study believe that nearly the whole of the Earth’s surface melted at some time, with some areas melting multiple times down to a depth of twelve miles. Whether or not their findings are correct is anyone’s guess.

“It’s very difficult to test that because, of course, we don’t have an impact record,” said James Day, a geologist at the Scripps Institution of Oceanography in San Diego, who was not involved in the study.

The group also believes that Hadean zircons, the indestructible crystals that make up the only “record” of early earth that have been found in Canada and Australia and the fact that most have been dated between 4.1 billion and 4.2 billion shows that the Earth was melting and the ocean’s boiling.

Asteroids – Not everyone agrees

But again, this is up for debate. T. Mark Harrison, a geologist and zircon expert at UCLA, says the argument does not fit the geologic evidence.

“They fundamentally misunderstand what a zircon is going to do,” says T. Mark Harrison, a geologist and zircon expert at UCLA who thinks this far to facile of an observation.

“If you can’t get them to fit your model, then your model probably doesn’t work,” adds Day from Scripps who thinks the authors view on zircons is probably pushing it a little far.”

“It’s inevitable that there were asteroid impacts at this time,” he said. “Their size and scale are what they try to quantify, but our knowledge very sketchy.”