An international team of scientists has identified an ancient 100 kilometer-wide crater near the Maniitsoq region of West Greenland.
According to Dr. Iain McDonald of Cardiff University, the 3-billion-year-old crater was formed as a result of a massive asteroid or comet impact at least a billion years before any other known collision on Earth.
"The spectacular craters on the Moon formed from impacts with asteroids and comets between 3 and 4 billion years ago," explained McDonald.
"The early Earth, with its far greater gravitational mass, must have experienced even more collisions at this time – but the evidence has been eroded away or covered by younger rocks." Indeed, a crater previously thought to be the oldest on Earth formed 2 billion years ago. The chances of finding an even older impact were believed to be, literally, astronomically low.
"This single discovery means that we can study the effects of cratering on the Earth nearly a billion years further back in time than was possible before," McDonald confirmed.
To be sure, locating the evidence was quite difficult as there was no obvious bowl-shaped crater left to find. Over the 3 billion years since the impact, the land has been eroded down to expose deeper crust 25 km below the original surface. All external parts of the impact structure have been removed, but the effects of the intense impact shock wave penetrated deep into the crust - far deeper than at any other known crater - and these remain visible.
However, because the effects of impact at these depths have never been observed before it has taken nearly three years of painstaking work to assemble all the key evidence. "The process was rather like a Sherlock Holmes story," said McDonald. "We eliminated the impossible in terms of any conventional terrestrial processes, and were left with a giant impact as the only explanation for all of the facts."
Only around 180 impact craters have ever been discovered on Earth and around 30% of them contain important natural resources of minerals or oil and gas. The largest and oldest known crater prior to this study, the 300 kilometer wide Vredefort crater in South Africa, is 2 billion years in age and heavily eroded.
"It has taken us nearly three years to convince our peers in the scientific community of this but the mining industry was far more receptive. A Canadian exploration company has been using the impact model to explore for deposits of nickel and platinum metals at Maniitsoq since the autumn of 2011," he added.