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.
On 3 September 2009, the remains of an even larger and much older impact structure near Maniitsoq (Sukkertoppen) in Greenland were 'discovered' at an office in Copenhagen, more specifically at the premises of the Geological Survey of Denmark and Greenland (GEUS). On behalf of his employer, senior research scientist Adam A. Garde was preparing himself for a workshop on nickel and platinum occurrences in the Maniitsoq region of West Greenland. The meeting was organised by the Greenlandic exploration company NunaMinerals A/S and was taking place the following week in Nuuk, Greenland. During his preparations this Thursday morning, Adam suddenly saw a both simple and extreme explanation for several strange geological features in this region. He had worked on these phenomena several times during his career but never really understood them, even though they had constituted the backbone of his dr. scient. thesis in 1997.
Since the idea of the meteorite impact appeared in Septembr 2009, a small group of scientists at GEUS, Lund University in Sweden, Cardiff University in Wales and the Institute of Planetary Science in Moscow has been investigating, documenting and modelling the impact structure, and the first scientific paper has just been published in the journal 'Earth and Planetary Science Letters'. Thanks to support from the Danish 'Carlsbergfondet' and GEUS it was possible to visit the impact structure in both 2010 and 2011, first with helicopter support and then by ship, and carry out a closer study, on site, of the rocks affected by the impact.
There is no obvious surface expression of the crater structure today. The bedrock in this part of Greenland is over 3 billion years old, about two thirds of the age of the Earth, and the impact itself took place exactly 3001 ± 2 million years ago in the middle of a region, where mountain building was actively taking place in a setting that was probably much like the Japanese archipelago today. 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.
In the course of the 3 billion years that have elapsed since the impact, the land has been eroded down to a depth 25 km below the original surface, and has lately also been carved and excavated by the Greenland ice sheet. All external parts of the impact structure have thus long gone, but the effects of the intense impact shock wave penetrated deep into the interior of the Earth and these remain visible today. The unusual size of the impacting extraterrestrial body and the strong gravity of the Earth (compared to e.g. the Moon) meant that much of the crushed and melted material remained at depth instead of being expelled vertically and laterally from the centre during the first seconds of the impact, as is known from all other impact structures on the Earth.
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 1000 km 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 500-600 km. 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, 3 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.
Why are ancient meteorite impact structures of interest to mankind at all? There are several obvious reasons. First, because of the rich endowment with minerals, oil and gas or water resources that such structures may provide. The discovery of a giant impact at Maniitsoq has promoted the exploration company North American Nickel to stake an exploration claim at Maniitsoq and the company will continue their exploration for nickel in the summer of 2012. Secondly, insight into the very complex, ultra high-speed cratering processes is important for the understanding of the initial accretion of the Solar System, and small meteorite craters were used for modelling in connection with nuclear tests during the cold war. However, the physical damage of even the largest nuclear bomb is minimal compared with the impact of a modest meteorite in the 100-m class. Finally, meteorite and comet impacing represent contact with the outer space - a subject that will continue to fascinate philosophers, filmmakers and boys of any age alike.
Why did almost three years elapse from the discovery of the Maniitsoq structure to the publication in a scientific journal? There are several explanations. Firstly, the idea was so radical that the research group wanted to carry out more fieldwork to be sure of their findings. Second, the sheer size of the Maniitsoq structure and the great initial crustal depth of its remnants mean that the technical criteria most commonly used to identify hypervelocity impacting cannot be employed directly. The effects of a giant impact on deep-crustal, ductile rocks at an ambient temperature of around 800°C are in some ways qualitatively different from those in cold, upper-crustal rocks - the targets of all other currently known terrestrial impact structures - and it has been difficult to convince reviewers to accept the evidence. It has been a slow process to document the extraordinary features that characterise the Maniitsoq structure and build up the evidence until it became overwhelming, but it has also been very rewarding to follow up on input from curious, knowledgeable and perceptive reviewers.
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