A violent battering of the early Earth by comets and asteroids that went on for 500 millions years was crucial to the evolution of our planet and may have prepared it for life, researchers say.

The collisions occurred during what is known as the Hadean eon, from the Earth's formation around 4.5 billion years ago until 4 billion years ago.

"It was thought that because of these asteroids and comets flying around colliding with Earth, conditions on early Earth may have been hellish," says planetary scientist Simone Marchi at the Southwest Research Institute in Boulder, Colorado.

The assumed hellish conditions gave the period its name, with Hadean taken from Hades, the god of the underworld from Greek mythology.

And yet, although there were impacts large enough to melt the entire surface of the Earth, the researchers say, those impacts were separated by many millions of years during which condition were less extreme.

"Generally speaking, there may have been something on the order of 20 or 30 impactors larger than 200 km (120 miles) across during the 500 million years of the Hadean, so the time between such impactors was relatively long," Marchi says.

"There may have been quiet tranquil times between collisions -- there could have been liquid water on the surface," he says.

Since subsequent plate tectonics and erosion have erased all evidence of the those Hadean impacts, the researchers have looked to the moon -- which still bears the scars of the long bombardment history it shares with the Earth -- to uncover the past of our own planet.

The frequency at which different size craters show up on the lunar surface allowed them to determine the sizes and shapes of the rubble floating around in the early solar system and estimate the impact history on the early Earth, they say.

That history suggests that during the Hadean eon almost every spot on Earth would have been melted as some point, which explains why none of our planet's ancient crust survived, Marchi and his colleagues say.

The oldest rocks ever found on the present Earth are 3.9 billion years old, too young to yield any clues to the earlier impact period, they say.

However, those same rocks -- from shortly after the Hadean asteroid activity -- have yielded the oldest traces of life on earth, Marchi points out.

"It's really a critical time for understanding the evolution of the Earth," Marchi says. "Much of what we see today, including ourselves -- that is, life -- is due to that early evolution. It's very important to understand what the conditions of the Earth were during that time."

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