A new terrestrial bombardment model based on existing lunar and terrestrial data sheds light on the role asteroid bombardments played in the geological evolution of the uppermost layers of the Hadean Earth (approximately 4 to 4.5 billion years ago).

"When we look at the present day, we have a very high fidelity time-line over the last about 500 million years of what's happened on Earth, and we have a pretty good understanding that plate tectonics and volcanism and all these kinds of processes have happened more or less the same way over the last couple of billion years," said Lindy Elkins-Tanton, director of the School of Earth and Space Exploration at Arizona State University.

But, in the very beginning of Earth's formation, the first 500 million years, there's a less well-known period which has typically been called the Hadean (meaning hell-like) because it was assumed that it was wildly hot and volcanic and everything was covered with magma - completely unlike the present day, researchers said.

Terrestrial planet formation models indicate Earth went through a sequence of major growth phases: accretion of planetesimals and planetary embryos over many tens of millions of years; a giant impact that led to the formation of our Moon; and then the late bombardment, when giant asteroids, dwarfing the one that presumably killed the dinosaurs, periodically hit ancient Earth.

While researchers estimate accretion during late bombardment contributed less than one per cent of Earth's present-day mass, giant asteroid impacts still had a profound effect on the geological evolution of early Earth.

Prior to four billion years ago Earth was resurfaced over and over by voluminous impact-generated melt.

Furthermore, large collisions as late as about four billion years ago, may have repeatedly boiled away existing oceans into steamy atmospheres.

Despite heavy bombardment, the findings are compatible with the claim of liquid water on Earth's surface as early as about 4.3 billion years ago based on geochemical data.

A key part of Earth's mysterious infancy period that has not been well quantified in the past is the kind of impacts Earth was experiencing at the end of accretion.

"This study is a major step in that direction, trying to bridge that time from the last giant accretionary impact that largely completed the Earth and produced the Moon to the point where we have something like today's plate tectonics and habitable surface," Elkins-Tanton said.

The new research shows that asteroidal collisions not only severely altered the geology of the Hadean Earth, but likely played a major role in the subsequent evolution of life on Earth as well.

The research was published in the journal Nature.

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