A fossil meteorite unlike anything seen before has been uncovered in a Swedish quarry. The mysterious rock may be the first known piece of the "bullet" that sparked an explosion of life on early Earth.
Roughly 100 fossil meteorites have emerged from the limestone quarry west of Stockholm, which is being mined for flooring. All of the meteorites are part of an iron-poor class called the L chondrites. They date back about 470 million years to the Ordovician period, when Earth experienced a mysterious burst of new species.
Now miners working in the Swedish quarry have found a meteorite fragment that is not an L chondrite. Analysing its microscopic crystals, Birger Schmitz at Lund University and his colleagues found that the rock dates to the same time period but is of a kind completely unknown to science.
Cloud of destruction
About 515 million years ago, our planet was going through an evolutionary slump. A burst of diversity that happened during the Cambrian period had tapered off, and few new types of animals were emerging. Mysteriously, about 25 million years later life sprang back into action in the early part of the Ordovician, generating loads of species. So what triggered the second explosion?
Fossil meteorites from the quarry suggest that during this time, impacts were tens to hundreds of times more frequent than they are today. The meteorites may have been born when two asteroids collided and broke apart between Mars and Jupiter. The larger object spawned the cloud of L chondrites that bombarded Earth for about 10 million years. According to one popular idea, this intense meteor shower caused just enough destruction to open up ecological niches and drive life to diversify into a richer assortment. But the fate and identity of the smaller asteroid has long been a mystery.
The fact that the latest fossil comes from the same rock layers as the L chondrites suggests that it is a piece of that second asteroid, says Schmitz. The theory says that most of the smaller asteroid was vaporised during the collision, so it also makes sense that only scant fragments of it would remain.
Crash, bang, wallop
David Harper at Durham University, UK, agrees. "The team may at last have identified the impactor responsible for the break-up of the parent body of the L chondrite meteorites," he says. In which case, he adds, it is a direct remnant of one of the most violent events in our solar system's history.
"This was the largest documented asteroid break-up event of the past 3 billion years," says Schmitz. "The asteroid impact at the end of the Cretaceous period, believed to have killed the non-avian dinosaurs, was tiny in comparison."
Journal reference: Earth and Planetary Science Letters, DOI: 10.1016/j.epsl.2014.05.034
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