AN IMPACT crater has been found off the shores of Britain. But it鈥檚 like nothing else on Earth.
Phil Allen, a consultant geophysicist based near Aberdeen, discovered the crater by chance. Petroleum giant BP had asked him to look at 3D seismic data from a gas field four kilometres below the North Sea. During his analysis, Allen discovered some unusual features in layers of chalk lying above the gas field, one kilometre beneath the seabed (Nature, vol 418, p 520).
What Allen saw looked like a crash site. 鈥淚 was flabbergasted,鈥� he says, 鈥淚鈥檇 never seen anything like it.鈥� It wasn鈥檛 until a meeting with Simon Stewart, a BP structural geologist who also thought it looked like a crater, that Allen took the idea seriously. Stewart had published a paper a year earlier in which he argued that there was a good chance an impact crater would be found somewhere in the North Sea. Based on the frequency of asteroid impacts, the size of the North Sea and the age of the corresponding rocks, Stewart estimated that the chance of the area containing a small impact crater was 1 in 2 (Petroleum Geoscience, vol 5, p 273).
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The newly discovered crater, named Silverpit after the local fishing grounds, is 140 kilometres off the east coast of Britain (see Graphic). An asteroid between 200 and 500 metres across must have caused the crater, which is around 3 kilometres across and 300 metres deep, when it crash-landed between 60 and 65 million years ago. The collision would not have been powerful enough to scatter debris across the planet, but it would have created a huge tsunami that inundated the few rocks poking up above the ocean surface in what鈥檚 now Scotland.
Expensive 3D seismic imaging isn鈥檛 an option for most research groups. Instead, the underlying structures of a few craters, for example Chicxulub in Mexico, have been crudely imaged by 2D seismic data and drill cores. But BP鈥檚 commercial survey of the gas field beneath Silverpit reveals the crater in unprecedented detail. Although the crater鈥檚 size is not particularly noteworthy, its structure is unique.
It鈥檚 surrounded by at least 10 concentric fractures, and the outermost ring is about 19 kilometres across. No other crater on Earth has anything like them. 鈥淚t鈥檚 a fantastic discovery,鈥� says Jay Melosh of the University of Arizona at Tucson. 鈥淕iven the spectacular detail revealed by the 3D seismic reconstruction, Silverpit is likely to teach us a great deal about how such ring systems arise.鈥�
That may in turn provide information about the structure of Jupiter鈥檚 icy moons, because Silverpit looks a lot like craters found on Europa and Callisto, which have similar systems of rings. The discovery of such a site on Earth will give scientists a unique chance to study how these ringed craters form. The current theory is that there must be a soft, watery layer beneath a brittle surface. When an asteroid hits, it creates a bowl-shaped cavity that the soft layer moves inwards to fill. As it does so, it drags the brittle surface and cracks it. At Silverpit, it may be that the underlying layer of muddy shale was soft enough to flow in a similar way, and finding evidence of that would help confirm the theory.