The lunar surface.
Los Angeles: When you look up at the moon's pockmarked face, you're actually staring at Earth's early history. The rain of asteroids that pummelled 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 capitalised 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 following large collisions during the Hadean eon. Some of these impacts likely vaporised the oceans and sanitised the planet of any early life that may have gained a foothold, according to a new study published Wednesday in the journal Nature.
While scientists have long recognised 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 last 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, Colorado, and lead author of the Nature study.
Dr Marchi and his colleagues studied the frequency of different sized craters on the bare lunar surface to determine how often rubble of various shapes and sizes came hurtling through the solar system. They dated the impacts using moon rocks collected during the Apollo missions and by modelling how the number of incoming asteroids might have changed over time.
That worked well for objects up to 160 kilometres across, Dr Marchi said.
"Above that size, we cannot use the moon for making a prediction simply because larger objects did not collide with it," he said.
So they turned to the main asteroid belt, a cloud of debris between Mars and Jupiter that probably would have formed another planet if not for Jupiter's powerful gravitational pull.
The asteroid belt may not have been the source of early projectiles, but it provided a good match for the size distribution of smaller objects that hit the moon. Dr Marchi and his colleagues figured it would also provide a sensible approximation for larger asteroids, too, so they used it to estimate the frequency of massive impactors.
The research team pulled these findings together to come up with a model for what would have happened as these objects collided with the young Earth - including how much melting would have occurred.
They found that almost every spot on Earth melted at some time during the Hadean. Some places may have melted multiple times and that melting may have extended down to a depth of 20 kilometres. This helps explain why no ancient crust survived, Dr 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.
Instead, the study authors compared their results to the few threads of evidence that hold clues about early Earth history, such as how much gold, osmium and similar elements reside in the mantle. If these elements - known as siderophiles, or iron-loving elements - were part of Earth's original composition, they would have migrated to the iron-nickel core. The fact that they exist in the mantle suggests they were probably injected into Earth's outer layers by asteroids later, providing a rough estimate of how much mass was added over time.
Dr Marchi and his collaborators found that their model matched these observations if the Earth encountered dozens of projectiles larger than 160 kilometres wide during its first billion years of existence.
The researchers also compared their results to Hadean zircons, tiny indestructible crystals that comprise the only direct vestiges of early Earth. A handful of samples older than 4 billion years have been found in Canada and Australia, where they have been recycled through numerous violent iterations of the rock cycle. The scientists say their model explains the curious observation that, for some reason, most zircons from the early part of Earth's history date back to a narrow window of time between 4.1 billion and 4.2 billion years ago.
Los Angeles Times