Shock discovery of ancient space invader

11 May 2006

Deep in a huge crater, scientists have unearthed an astonishing find - the only preserved fragments of an asteroid ever discovered. We've always thought asteroids vaporised when they hit Earth head-on. How did this one survive?Antenna investigates the rocky relic...

This story was published in Nature on 11 May 2006.

Image: David A Hardy/

In South Africa an asteroid crater wider than London has surrendered some amazing space secrets - the first fragments of a large asteroid ever found. The impact crater, called Morokweng, formed when a huge asteroid slammed into our planet 145 million years ago.
Until now, scientists thought a large asteroid's energy turned into heat when it hit, vaporising the asteroid and melting the ground to form a so-called melt sheet. But drilling deep into Morokweng's melt sheet has revealed a beach-ball-sized lump of asteroid, along with lots of smaller fragments.

The team drill deep into the crater to take samples of the melt sheet.

Image: Marco Andreoli

'We got excited as the drilling got to around 800 metres deep when we saw something that was clearly not of earthly origin. This asteroid is preserved just as if it fell yesterday.'
Marco Andreoli, geologist, University of the Witwatersrand, South Africa

Marco Andreoli, geologist, South African Nuclear Energy Corporation and University of the Witwatersrand, South Africa.

Image: Marco Andreoli

'How the asteroid fragments came to be embedded in the melt sheet is a big question; they just should not be there. The melt sheet is actually boiling following an asteroid hit. This would melt away nuggets and boulders of asteroid faster than a box of ice cream in your kitchen oven.'
Marco Andreoli

This rock sample contains fragments of the first asteroid ever discovered intact. The brown 'nuggets' in the sample once orbited the Sun, and are probably older than the planets in our Solar System.

Image: Jennie Hills/Science Museum

Somehow chunks of this asteroid survived the impact. Now experts are looking again at how asteroids, and their smaller cousins meteorites, impact on Earth. One big question is, if this meteorite made it, might others have survived their impact with Earth in the past?
'There is a lot of talk nowadays about the possibility of a cosmic swapping of bacteria-carrying meteorites between the early Earth and Mars about 4 billion years ago. Any improved impact models may affect the possibility of such things occurring.'
Marco Andreoli

Could life from Mars have made its way to Earth?

Meteorites that hit in Antarctica are preserved by the extreme cold, and the white background makes them easier to spot.

Image: NOAA

Most meteorites that make it through our atmosphere quickly weather away. They last longer in dry or cold conditions, so nearly all meteorites we find have fallen in the Antarctic or in deserts. But even these are much younger than the newly discovered rocky relics.
'The Morokweng meteorite was protected from weathering beneath the melt sheet, and is the oldest unaltered meteorite known. And the composition is very different to the meteorites we have found so far. This might be because it hit Earth so much earlier.'
Wolf Maier, meteorite expert, University of Quebec, Canada

Wolf Maier, meteorite expert, University of Quebec, Canada

Image: Marco Andreoli

Samples drilled from the crater are carefully logged and tested to see if they contain any space rock.

Image: Marco Andreoli

The international team of scientists used a battery of tests to prove that this humble boulder was part of an ancient asteroid which once orbited our Sun.
How do we know these fragments of rock were once part of an ancient asteroid?
'We could be quite sure once we found the chondrules. These "drops of fiery rain" are little beads that were molten before the planets formed, swirling around the baby Sun 4600 million years ago.'
Marco Andreoli

The rocky relics found at Morokweng once swirled around the Sun.

Image: NASA

Most scientists believe that a huge asteroid impact caused mass extinction of the dinosaurs.

Image: NASA

Scientists believe the Morokweng asteroid hit our planet at the end of the Jurassic period. It didn't cause huge damage to wildlife, unlike the massive asteroid impact which most scientists believe wiped out the dinosaurs 80 million years later.
'We know we are star dust - our atoms are the remnants of stars that exploded billions of years ago; perhaps the first life on Earth came from space too.
'The Morokweng asteroid suggests we need to think again about the chances of anything contained within large asteroids surviving an impact with our young planet.'
Doug Millard, space curator, Science Museum

Doug Millard, space curator, Science Museum

Image: Doug Millard