The oldest and 'biggest ever' asteroid crater discovered in Greenland

The oldest and 'biggest ever' asteroid crater discovered in Greenland

• 100km (62mile) wide crater discovered in Greenland - and it is possible crater is more than 500km in size

By Eddie Wrenn

PUBLISHED: 10:42 EST, 29 June 2012 | UPDATED: 11:30 EST, 29 June 2012

A 100 kilometre-wide crater has been found in Greenland, the result of a massive asteroid impact a billion years before any other known collision on Earth.

The previously oldest known crater on Earth formed two billion years ago and the chances of finding an even older impact were thought to be astronomically low.

Now, a team of scientists from the Geological Survey of Denmark and Greenland (GEUS) in Copenhagen, Cardiff University in Wales, Lund University in Sweden and the Institute of Planetary Science in Moscow has upset these odds.

Explosive: This simulated image shows how the impact would have looked - it would have wiped out all forms of higher life if it happened later in Earth's history

Hidden in the runes: Evidence came in the form of broken-up, contorted, melted and hydrothermally altered rocks affected by the impact and influx of sea-water during the impact

The spectacular craters on the Moon formed from impacts with asteroids and comets betweenthree3 and four billion years ago.

The early Earth, with its far greater gravitational mass, must have experienced even more collisions at this time - but the evidence has been eroded away or covered by younger rocks.

Following a detailed programme of fieldwork, funded by GEUS and the Danish ‘Carlsbergfondet’ (Carlsberg Foundation), the team have discovered the remains of a giant three billion-year-old impact near the Maniitsoq region of West Greenland.

'This single discovery means that we can study the effects of cratering on the Earth nearly a billion years further back in time than was possible before,” according to Dr Iain McDonald of Cardiff University’s School of Earth and Ocean Sciences, who was part of the team.

It is possible or even likely that the meteorite hit the sea, for the preserved rocks have been intensely altered by circulating hot aqueus fluids. These fluids were likely derived from sea water that would have been able to penetrate deep into the Earth's crust through the numerous fissures and crush zones generated by the impact.

Boris A. Ivanov at the Institute of Planetary Science, Russian Academy of Science, Moscow, has carried out a series of provisional model calculations, which suggest that the impacting meteorite at Maniitsoq may have had a diameter of more than 30 km, i.e., about twice the size of the Vredefort meteorite and with a mass about ten times larger.

If this meteorite had hit the Moon, the final crater structure would have had a diameter well above 1000km and easily visible from Earth. However, due to the much stronger gravity of our planet, the Maniitsoq structure may have had a diameter of 'only' some 500-600km

Clues: Finnefjeld mountain, which is around 1050m high, is believed to be the crushed core of the structure

The dull grey rocks were crushed to fine powder by the impact, and then cut by white melt sheets

Making an impact: The asteroid is believed to have hit close to where the town of Maniitsoq in Greenland

If an impact of this size hit the Earth today, it would not only be able to pulverise a medium-sized national state but its global effects would also kill all higher life.

Then, three billion years ago, there was not much life to extinguish, but as yet no depositional rocks of matching age have yet been identified that could enlighten the effects of the Maniitsoq impact such as extreme tsunamis, deposition of re-condensated atmospheric glass particles from the evaporated meteorite or other signs of global atmospheric and marine effects.

Finding the evidence was made all the harder because there is no obvious bowl-shaped crater left to find. Over the three billion years since the impact, the land has been eroded down to expose deeper crust 25km below the original surface.

All external parts of the impact structure have been removed, but the effects of the intense impact shock wave penetrated deep into the crust - far deeper than at any other known crater - and these remain visible.

However, because the effects of impact at these depths have never been observed before it has taken nearly three years of painstaking work to assemble all the key evidence.

THE MYSTERY OF EARTH'S MISSING CRATER IMPACTS

If you look at the Moon on a clear night through a pair of ordinary, hand-held binoculars, you'll see a multitude of meteorite craters.

Some are larger than 1000 km in diameter and readily visible with the naked eye.

Through the first 500 million years of Solar System history, both the Moon and the Earth were constantly bombarded with a multitude of small and large meteorites and comets.

Some scientists even think that life was brought to the Earth by comets.

The Moon has preserved the remains of thousands of impacts, but on Earth only about 180 such impact structures are known, and most of them are very small, young and repidly decaying.

Contrary to the Moon, the Earth is a dynamic planet with plate tectonics, mountain belts and erosion, which means that most impact structures are eroded away, destroyed by mountain building processes or buried by younger deposits over geological time.

Until recently, the 2.02 billion years old and 300 km wide Vredefort crater in South Africa was considered to be both the oldest and largest impact structure on Earth.

It is estimated that the impacting meteorite had a diameter of about 15 km. During the development of the final crater structure, a kilometre-thick layer of sedimentary rocks containing the World's largest gold deposits collapsed into the cavity excavated by the meteorite and in this way became protected from erosion and preserved until today.

Also the second largest impact structure on Earth, the 1.85 billion years old Sudbury crater in Canada, hosts world-class mineral deposits - in this case nickel-rich minerals that were melted and concentrated by the extreme heating caused by the impact.

from GEUS

'The process was rather like a Sherlock Holmes story,' said Dr McDonald. 'We eliminated the impossible in terms of any conventional terrestrial processes, and were left with a giant impact as the only explanation for all of the facts.'

Only around 180 impact craters have ever been discovered on Earth and around 30 per cent of them contain important natural resources of minerals or oil and gas. The largest and oldest known crater prior to this study, the 300 kilometre wide Vredefort crater in South Africa, is 2 billion years in age and heavily eroded.

Dr McDonald added that 'It has taken us nearly three years to convince our peers in the scientific community of this but the mining industry was far more receptive. A Canadian exploration company has been using the impact model to explore for deposits of nickel and platinum metals at Maniitsoq since the autumn of 2011.'

The international team was led by Adam A. Garde, senior research scientist at GEUS. The first scientific paper documenting the discovery has just been published in the journal ‘Earth and Planetary Science Letters’.

• The research was supported by the Carlsberg Foundation.
  

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