Researchers have discovered a large impact crater under the North Sea thatillustrates the threat posed by asteroids and comets. Dick Ahlstromreports.
The danger has passed - we are not going to be struck by an enormous asteroid on February 1, 2019. New evidence has emerged, however, about what happens when a passing space rock managed to hit its mark.
Two UK scientists report in Nature this morning about a 20 km-wide impact structure discovered at the bottom of the North Sea. The resultant crater is of particular interest because of its superb preservation, protected by a thick covering layer of sediment that has kept it safe from the elements for at least 60 million years.
Meteor craters on earth are generally poor specimens, beaten up as they are by wind, rain and erosion. They get flattened out and covered over, and movement of the surface crust disturbs their original look.
We look to the moon to understand the structures we see after major impacts and more recently, to data coming from impacts on Jupiter's moon, Callisto. Their distance from us is an impediment, however, hence geologists' delight in finding what has been dubbed the "Silverpit" crater under the bed of the North Sea.
It was defined and mapped by Dr Simon Stewart of BP plc in Aberdeen and Dr Phillip Allen of Production Geoscience Ltd in Banchory, Scotland. It takes its name from a nearby seafloor channel.
Early evidence for the crater came during prospecting for North Sea oil and gas. Silverpit lies well hidden, 130 km from the English coast under 40 metres of water and a further 300 metres to 1,500 metres of seafloor sediments that covered and preserved the crater's structures. These were extensively mapped to a resolution of just tens of metres using three-dimensional seismic reflection data. This allowed the researchers to see through the sediments and get a remarkably clear view on Earth of what previously we could only see through powerful telescopes.
The complete structure measures some 20 km across and includes at least 10 concentric rings located between two and 10 km from the crater's centre. The main bowl-shaped central zone measures about three km across rim to rim and has a typical cone-shaped peak at its centre.
This is surrounded by the rings, which are impact faults, rocks displaced by the impact. The fault types vary depending on how far they lie from the point of impact but look just like the rings seen surrounding much larger impact craters on Callisto.
It had been assumed that only very heavy impactors could produce this ringed effect. "The formation of concentric ringed impact structures at this relatively small scale had not previously been thought possible, especially on the terrestrial planets," the authors write.
We now seem unlikely to suffer the same fate as our planet did over 60 million years ago, at least from asteroid 2002 NT7. Even early on, the US National Aeronautics and Space Administration only gave it a "slight" - but interesting - chance of hitting us. Later calculations done by Mr David Moore, editor of Astronomy Ireland, showed that it had less than a one in 150,000 chance of hitting us. "2002 NT7 will probably miss earth by a million miles or more," he suggests.