Ancient meteorite leaves telltale traces
The first convincing evidence of a massive meteorite impact that occurred 2.63 billion years ago has been found in northwestern Australia's Pilbara region.
Some scientists have long suspected that certain features of the Jeerinan Formation in the area known as the Pilbara Craton were signs of a large meteor that hit the Earth in the Precambrian period.
But Rasmuassen and Koeberl said the only evidence for this had been small splatters of melted rock called spherules, between 3.4 and 2.6 billion years old.
Until now, the area had lacked the indisputable hallmarks of a meteorite impact: high iridium levels and shocked quartz.
Earth's own iridium is locked away in the planet's core but meteorites are full of the metal and thus a spike in iridium levels is used to detect debris from meteorite impacts.
Quartz crystals in rocks display a particular texture known as shocked quartz, which can only be formed by the impact of a bomb or a meteorite.
Until now, such evidence had not been reported in the Pilbara rocks. But Rasmussen and Koeberl have for the first time published evidence of shocked quartz in layers of the rock also containing melt spherules and high iridium levels.
They said their findings provided "compelling evidence" for an extraterrestrial impact, which was likely to be on land.
But where's the crater?
While they said this was among the largest yet documented impacts in the Precambrian rock record, they do know exactly where the crater is.
By studying comparative rocks in South Africa, they argue the meteorite would have caused a blanket of material to be thrown out across 32,000 square kilometres covering South Africa and Australia, which were joined at the time.
Earth sciences Professor Richard Arculus of the Australian National University in Canberra, described the research as "another notch on the record of confirmed impacts that have happened to Earth".
"It's a nice piece of detective work," he told ABC Science Online.
He said the only life forms present at the time of the impact were single-celled photosynthetic algae. But there was not enough palaeontological evidence to know what effect the meteorite impact would have had on these.
Arculus said the impact was not "as big a deal" as the meteorite that hit at the end of the Cretaceous period that has been implicated in the extinction of the dinosaurs.