* Astronomy

A large fragment has been found of an asteroid that punched a 160km-wide hole in the Earth's surface.

The beachball-sized fossil meteorite was drilled out of South Africa's 145-million-year-old (± 0.8) Morokweng crater.
It is a unique discovery because large objects are widely believed to completely melt or vaporise as they collide with the planet.

Writing in the journal Nature, an international team says the find will further knowledge on asteroid impacts.

“we report the discovery of a large (25-cm), unaltered, fossil meteorite, and several smaller fragments within the impact melt of the giant (> 70 km diameter), 145-Myr-old Morokweng crater, South Africa. The large fragment (clast) resembles an LL6 chondrite breccia, but contains anomalously iron-rich silicates, Fe-Ni sulphides, and no troilite or metal. It has chondritic chromium isotope ratios and identical platinum-group element ratios to the bulk impact melt. These features allow the unambiguous characterization of an impactor at a large crater. Furthermore, the unusual composition of the meteorite suggests that the Morokweng asteroid incorporated part of the LL chondrite parent body not represented by objects at present reaching the Earth.”

The Morokweng crater is one of the largest on Earth, and was formed at the boundary of the Jurassic and Cretaceous periods.
Created by an asteroid measuring about 5-10km in diameter, the impact bowl lies hidden beneath the sand of the Kalahari Desert.
Scientists discovered the meteorite fossil by drilling bore holes into the impact melt - the area where the asteroid fused with the Earth - in the centre of the crater.


S 26° 28' E 23° 32'

"At about 770m down, we came across some dark blocks - one was about the size of a beachball - but we couldn't figure out what it was" - Dr Marco Andreoli, an author on the Nature paper and a geologist at the South African Nuclear Energy Corporation and the University of Witwatersrand.

After chemical and mineral tests - which meant the material was cut up into smaller fragments - the scientists were astonished to find that the rock was a meteorite, a surviving relic from the collision.
When a large impactor strikes the Earth, a colossal amount of heat is produced; and the asteroid material is believed to vaporise or fuse with the surrounding rocks. A 10-km-diameter impactor is thought to generate temperatures of between 1,700-14,000C.
Consequently, scientists can only study these large impacts by looking at the chemical composition of material in the crater.


Morokweng core samples: Surviving asteroid fragments (top) were found hundreds of metres down. Glasses (bottom) produced in the impact's intense heat were drilled from an area close to the crater. Image: Jennie Hills/Science Museum London


"What is amazing is that here we have these fragments - that may not have been attached to the asteroid, or maybe trailing behind it - that smashed into the Earth and survived the fiery furnace in the crater that formed; and then they got trapped. This is remarkable because this is something that people didn't think could happen" - Dr Marco Andreoli.

It meant that computer models of large impacts might now need to be revised, to take into account conditions where some of the asteroid material endures.

"Anything that helps scientists to model what happens when two bodies collide is good news" - Dr Marco Andreoli.

Further investigation into the discovery has also revealed that the chemical composition of the space rock is slightly different to that of other meteorites that have been studied. It is a little more radioactive; there is more uranium, sodium, but less iron and nickel.

"All of our science of meteorites is based on meteorites that fell in the last few thousand years. But all of a sudden we can study a meteorite that fell 145 million years ago, and this opens the possibility that the nature of these impacting bodies has changed over the years" - Dr Marco Andreoli.

People in the UK can see fragments of the meteorite if they visit the Antenna Wing of London's Science Museum from Thursday.

Source BBC