Before the fish, mollusks, and trilobites of the Ordovician era got over their shock from a meteor strike gouging a large crater in shallow marine waters, a second strike may have occurred, if evidence presented at the 45th Lunar and Planetary Science Conference is correct.
It is expected that several of the Earth's asteroid craters should be part of a pair created by a binary asteroid. However, while several possibilities have been proposed, no example has been accepted. Now the Lockne/Malingen pair in Sweden has been hailed as the most likely candidate yet to have been formed in this way.
One in seven asteroids larger than 200m across has a companion. Sometimes these are much smaller, almost like a moon, while others are of similar size to the parent. Binary asteroids are thought to form when loosely held together asteroids spin so fast that loose material is thrown off to eventually accrete into a satellite. In 1993 the Galileo spacecraft flew by 243 Ida making it, and its moon Dactyl, the most famous asteroid pair.
With so many asteroids traveling in pairs it is to be expected that the Earth would often have been hit by both. Depending on their orbits and the size of their craters sometimes this would result in a single, possibly misshapen, crater. However, current estimates suggest that 3% should be doublets. Of 184 known impact craters 130 are large enough for pairing to be possible, and indeed several have been proposed. Nevertheless, no universally accepted double crater has been found.
This is not all that surprising. Many craters are old and so badly eroded that even recognizing that they are the result of asteroid impact is often not easy. Even when two craters lie next to each other and are roughly the same age, there is always a possibility of coincidence.
However, Dr Jens Ormo of the Center for Astrobiology in Madrid has produced what he considers the most compelling evidence yet for paired craters from Lockne in northern Sweden and its smaller neighbor, Malingen.
Both are known to date from the Ordovician Period, 458 million years ago, and they lie 16km apart, a little more than double the diameter of Lockne. Malingen is 700m wide, although the asteroids that made them are thought to have been 600m and 250m wide respectively. Their distance apart is near the maximum possible for objects of this size to still have been gravitationally bound.
The Lockne crater has been studied in detail, including dating to 458 million years ago around the time the first jawed fishes were appearing. Ormo presented evidence that the Malingen crater is of a similar age, based on the fossilized chitinozoans in the first sedimentary rocks laid down after the crater was formed. Moreover, neither of the craters shows signs that the sediments that filled it in was disrupted by material ejected from the other.
Dr Gareth Collins, of Imperial College London, was not involved with the research, but told BBC News: "Short of witnessing the impacts, it is impossible to prove that two closely separated craters were formed simultaneously. But the evidence in this case is very compelling. Their proximity in space and consistent age estimates makes a binary-impact cause likely."
Although the two impacts did not cause a mass extinction, Ormo says "The Lockne impactor was big enough to generate what's known as an atmospheric blow-out, where you blow away the atmosphere above the impact site.” Debris would have fallen worldwide, but in insufficient quantities to be traced so long afterwards.
Quebec's Clearwater East and West Craters are probably the best known candidates for double impact craters, and others have been proposed in Russia and Germany.