When you look up at the moon’s pockmarked face, you’re actually staring at Earth’s early history. The rain of asteroids that pummeled the lunar surface hit our planet, too; it’s just that erosion and plate tectonics blotted out the evidence. In fact, no rocks anywhere in the world survived to tell the story of the first 500 million years of Earth’s 4.5 billion-year existence, a tumultuous period of frequent impacts known darkly as the Hadean.

Now, scientists have capitalized on the moon’s long memory to uncover Earth’s own past. The researchers found that much of our planet’s surface probably melted repeatedly after large collisions during the Hadean eon. Some of these impacts likely vaporized the oceans and sanitized the planet of any early life that may have gained a foothold, according to a new study in the journal Nature.

While scientists have long recognized that large and frequent impacts shook the Hadean Earth, the new study marks the first attempt to quantify what might have happened.

The researchers started out by translating recent estimates of the cratering history for the moon — published over the past few years by the same group of scientists — into similar estimates for the Earth.

“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.

Researchers found that almost every spot on Earth melted at some time during the Hadean. Some places may have melted multiple times, and that may have extended down to a depth of 12 miles. This helps explain why no ancient crust survived, Marchi and his colleagues wrote.

But is their model correct?

“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.

McClatchy News Service