TWO massive objects that hit the Earth about 35 million years ago were believed to be comets. But an analysis of one of the craters is showing that the culprit was asteroid. If the second crater was also caused by an asteroid, it would mean that asteroids this big can strike the Earth at intervals of only a few thousand years.
The two impacts that created the 100-kilometre Popigai crater in Siberia and the 85-kilometre Chesapeake Bay crater just off the shore of Virginia happened a mere 10,000 years apart. As such massive events are expected to occur only once in 26 million years, the thinking was that the two had to be related and that only a comet shower could have been responsible. Massive asteroids were thought to be loners. So, when initial measurements on samples from Popigai pointed to an asteroid, 鈥渨e were rather surprised,鈥� says Philippe Claeys of the Free University of Brussels (VUB) in Belgium.
Then Claeys and his colleagues carried out 30 separate measurements of the ratios of platinum-group elements in samples from the crater site. They confirmed that the object striking Popigai was something called an ordinary chondrite with low-iron content, and not a carbonaceous chondrite, the telltale signature of a comet. If the Popigai and Chesapeake Bay objects come from the same place, as their close timing suggests, then the Earth was hit by an unexpected shower of massive asteroids.
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The shower could have come from a collision between asteroids in the main belt between the orbits of Mars and Jupiter, but models show it usually takes millions of years for big pieces to break free of the main belt and end up on course for Earth. Such pieces may escape faster if the collisions occurred near the inner edge of the asteroid belt. In any case, it鈥檚 unlikely that two asteroid fragments from the same collision would hit the Earth within such a short space of time.
David Nesvorny of the Southwest Research Institute in Boulder, Colorado, says it is more likely that the two objects were parts of a larger asteroid torn apart by an earlier close encounter with Earth, much as Jupiter鈥檚 gravity disrupted comet Shoemaker-Levy 9. The encounter would have left the objects in slightly different orbits, so they could have hit Earth at different times.
Last month the US Geological Survey drilled a test core from the centre of the Chesapeake Bay crater, collecting rocks that should be suitable for platinum-group measurements, says Greg Gohn, head of USGS鈥檚 Chesapeake Bay Impact Crater Project. Chemical compositions matching what was found at Popigai will solidify the case for an asteroid shower.