for National Geographic News
About 1.8 billion years ago, a meteorite or comet the size of Mount Everest slammed into what is now Canada.
According to James Mungall, a University of Toronto geologist, the impact turned part of the Earth's crust inside out and dusted the surface with a rare metal.
Mungall and other experts studying impact craters, such as this one in Sudbury, Ontario, hope to understand how a period of continual bombardment about four billion years ago shaped the planet.
Until now researchers had found scant evidence that a meteorite could pierce through Earth's upper crust and alter its compositional makeup.
"Over a few hundred million years when this was going on, there must have been a lot of mixing going on in the upper crust," said Mungall, who studies the Sudbury impact site.
David Kring is a planetary scientist at the University of Arizona in Tucson and an authority on impact craters. He said the findings from Sudbury are similar to those he and his colleagues have reached from studying a crater in Chicxulub, Mexico.
"I don't think it is yet widely appreciated, but impact cratering has the capacity to redistribute the chemical elements in the Earth's crust," Kring said.
As well, Kring adds, an emerging theory in the field of impact crater research is that the largest of these impact events early in Earth's history may have created the conditions needed for the evolution of life.
The impacts, he explains, would have heated water in the Earth's crust and created vast hydrothermal vent systems. Many scientists believe these unusual underwater ecosystems helped give rise to early life.
Researchers assumed volcanic activity mostly created hydrothermal vent systems. "But four billion years ago a dominant source was impact-generated hydrothermal systems," Kling said.
The field of impact crater research is just coming into prominence in the scientific community. Mungall says that 15 years ago scientists couldn't even agree that the Sudbury crater resulted from a meteorite impact.
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