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Silverpit crater
Its age is thought to be of the order of 65 million years, making its formation roughly coincident with the impact that created the Chicxulub Crater. If Silverpit is indeed an impact crater, this may imply that the Earth was struck at that time by several objects, possibly in a similar event to the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994. Several other impact craters around the world are known to date from roughly the same epoch, lending credence to this theory.
Discovery
The crater was discovered during analysis of seismic data collected by petroleum geoscientists Simon Stewart of BP and Philip Allen of Production Geoscience Ltd, for a region 130 km off the Humber estuary, during a routine search for fossil fuel deposits. Allen noticed a set of concentric rings, but did not know what they were, and hung an image of them on the wall of his office, hoping someone else would be able to shed light on the mystery. Stewart, visiting Production Geoscience on an unrelated matter, saw the map and suggested it might be an impact crater. The discovery of the crater and the impact hypothesis were reported in the journal Nature in 2002 [1]. Silverpit was named after local fishing grounds. Only three years before the announcement of the discovery of the Silverpit crater, it had been suggested that seismic data from the North Sea would have a good chance of containing evidence of an impact crater: given the rate of crater formation on the Earth and the size of the North Sea, the expected number of impact craters would be one. The crater currently lies below a layer of sediment up to 1500 m in depth, which forms the bed of the North Sea at a depth of about 40 m. Studies suggest that at the time of the crater's formation, the area was under 50 to 300 m of water. OriginScientists are generally in agreement that a bolide impact is the best explanation for the origin of Silverpit. However, other mechanisms can lead to the formation of craters, and Silverpit's categorization as an impact crater has been questioned. Evidence in favour of impact originOther mechanisms for producing a crater were considered and rejected by Allen and Stewart when they discovered the crater. Volcanism was excluded because there were no magnetic anomalies in the crater, which would be expected if eruptions had occurred there. Withdrawal of salt deposits below the crater, known to be a mechanism for the formation of some craters, was ruled out because the Triassic and Permian layers of rock beneath the crater appeared to be undisturbed. Another strong indication that an impact had created the crater was the presence of a central peak - something difficult to form except through a meteorite impact. Evidence for alternative interpretationsAnalysis of newer seismic data by Professor John Underhill, a geologist at the University of Edinburgh, led to a suggestion that withdrawal of material at depth was in fact a better explanation [2]. Underhill found that all layers of rock down to the Permian (with an age of about 250 million years) are synclinically folded, and that sediments of this era at the crater are thinned, suggesting that the crater was forming while Permian sedimants were being laid down. The existence of the central peak which seemed to strongly support the impact hypothesis was considered by Underhill to be equivocal. He suggested it may have been an artefact of image processing. Despite Underhill's work, the scientific consensus still seems to broadly favour the impact hypothesis as of 2005. Structure
One possibility is that after the impact excavated a bowl-shaped depression, soft material surrounding it slumped towards the centre, leaving the concentric rings. It is thought that for this to happen, the soft material would have to comprise quite a thin layer, with more brittle material on top. A thin layer of mobile material beneath a solid crust is easy to understand in the context of icy moons, but is not a common occurrence on the rocky bodies of the solar system. One suggestion is that overpressured chalk below the surface may have acted as the soft, mobile layer [4].
The impactFrom the size of the crater and certain assumptions about the speed of an impacting object, the size of the impactor can be estimated. Impacting objects are generally moving at speeds of the order of 20-50 km/s, and at these speeds an object about 120 m across and with a mass of 2.0×109 kg would be required to form a Silverpit-sized crater, if the object was rocky. If it was a comet then it would have been somewhat larger. For comparison, the object which struck the Earth at Chicxulub is estimated to have measured approximately 9.6 km across, while the object responsible for the Tunguska explosion in 1908 is thought to have been a comet or asteroid about 60 m across, with a mass of about 4×108 kg [5]. An object 120 m across smashing into the sea at many kilometres per second would generate enormous tsunamis. Scientists are currently searching for any evidence of large tsunamis in the surrounding areas dating from around that time, but as yet no such evidence has been uncovered. AgeThe position of the crater within the layers of rock and sediment on the sea floor can be used to constrain its age: sediments laid down before the crater's formation will be disturbed, while those laid down afterwards will not be. Allen and Stewart found that Silverpit was formed in Cretaceous chalk and Jurassic shale, but is covered by an undisturbed layer of tertiary sediment. The Cretaceous Period ended about 65 million years ago, and so Silverpit seems to have been formed between 60 and 65 million years ago. The Chicxulub impact, which probably played a major role in the extinction of the dinosaurs, occurred 65 million years ago. This method of estimating the age of a formation is somewhat crude, and the result is questioned by Underhill's non-impact hypothesis. Other possible ways of dating the impact include looking for evidence of ejecta material such as tektites, and deposits from the hypothesised tsunami, which might be found anywhere around the North Sea basin, but may have been subjected to repeated glaciations. As well as allowing a more accurate age determination, these lines of investigation would also confirm the impact hypothesis. Two nearby oil exploration wells penetrate the ring system, and cutting samples from these are currently being analysed. Analysis of samples taken directly from the central crater would also assist age determination, but such samples have not yet been taken. Part of a multiple impact?
The estimated age of Silverpit leads to inevitable speculation about whether it is related to the much larger Chicxulub crater and the extinction of the dinosaurs. The age is as yet not accurately known, and so at the moment scientists can only speculate. However, several other large impact craters of around the same age have been discovered, all between latitudes 20°N and 70°N, leading to the hypothesis that the Chicxulub impact may have been only one of several impacts that happened all at the same time. The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 proved that gravitational interactions can fragment a comet, giving rise to many impacts over a period of a few days if the comet should collide with a planet. Comets frequently undergo gravitational interactions with the gas giants, and similar disruptions and collisions are very likely to have occurred in the past. The scenario may have occurred on Earth 65 million years ago. Evidence for this hypothesis is not yet strong, however, as the ages of the Silverpit crater and other possibly related craters are only known to an accuracy of a few million years. See alsoReferences
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