Two Asteroid Events, Not One, Made Twin Craters

For 50 years, East Clearwater Lake and West Clearwater Lake in Québec have been considered twin craters formed together the moment a pair of star-crossed asteroids slammed into the Earth.

But new radiometric dating of rocks and a detailed look at the impact sites suggests the two craters are a "false doublet," created by two separate asteroid impacts about 180 million years apart.

"Even though we did think they formed simultaneously, it looks like they actually didn't,” said geologist Gordon "Oz" Osinski of Western University in Ontario. On Monday Osinski presented the results of the research conducted by he and his colleagues in Canada, Germany and Australia, at the Joint Assembly meeting of U.S. and Canadian geoscientists in Montreal.

The twin craters were recognized as having been created by asteroids about 50 years ago, and at that time it was assumed that a binary asteroid – a pair of asteroids traveling through space together – had made the two lakes. Binary asteroids have been observed for centuries, and the cratering of the Moon and other heavily cratered worlds in our solar system makes it clear multiple crater impacts are not uncommon.

The research into the Clearwater Lakes involved measuring the amounts of radioisotopes in minerals of rocks that were melted by the impact. Radioisotopes are elements that change at known rates into what are called daughter isotopes. Working out the ratios of the parent and daughter isotopes enables scientists to figure out how much time has passed since a mineral – and usually the rock it belongs to – has been molten.

Osinski and his colleagues measured Argon-40 and Argon-39 from impact melt rocks at the 36-kilometer (22-mile) wide West Clearwater Lake and got a date of 286.2 million years, give or take 2.2 million years. Rocks from the 26-km (16-mile) wide East Clearwater Lake were dated to somewhere between 460 to 470 million-years-old.

"We get no ages whatsoever in the 200-million-year range" at East Clearwater Lake, Osinski said. "They are all upwards of 400 million years."

The older age of East Clearwater Lake was glimpsed before, in a radioisotope study done 25 years ago. But that single measurement of about 460 million years was thrown out as unreliable.

"Since day 1 we've assumed they formed from the impact of a binary pair," said Osinski. And so the older date, just didn't fit – until now.

Read more at Discovery News