Uncovering an asteroid

But the velocity of the asteroid wasn't the only surprise. Its composition was very different from that of other asteroid remains found on Earth: it did not contain iron-nickel metal. This seems to indicate that the fragments are from a different part of the asteroid than other known meteorites.

Its effect on living species also goes against what one would expect from an impact of its scale. Large asteroids typically cause mass extinctions, like the one that hit Chixculub, Mexico 80 million years ago and is believed to have wiped out the dinosaurs. "Undoubtedly, life in the immediate vicinity, hundreds of kilometres from the centre of Morokweng, had a pretty dreadful time, but there is currently no evidence that the Morokweng impact created any kind of mass extinction," says McDonald.

No evidence - but patterns of extinction and climate change from the time period when the Morokweng asteroid touched down are pretty hazy. It impacted on Earth right at the boundary of the Jurassic and Cretaceous eras when Africa, South America and Antarctica were still joined together. Extinctions that occurred during this period are not well understood and there is debate as to whether certain extinctions happened at the same time due to a single event, or at different times due to local factors.

Whether or not an asteroid will affect living species has a lot to do with the composition of the ground it crashes into. The Morokweng asteroid impacted into rocks made of granite, amphibolite and banded ironstone which do not release sulphur dioxide and other environmentally-damaging gases. "Unlike the evaporite rocks at the Chixculub crater, these target rocks are pretty benign," says McDonald.

But the speed of the impact could also have a lot to do with its effect on living things. The research team hopes that their work at Morokweng will prompt further investigations into the conditions of the event. "Since numerical simulations are very complex and involve lots of supercomputer time, simulations at velocities less than 20 km/s have not been performed, or at least published," says McDonald. If researchers are able to replicate the features they observed at the crater, it could give us greater insight into how this catastrophic event, and other similar ones, affected our planet.

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