Earth and giant meteorites go way back, but new research confirms that what had been proposed as the oldest impact crater on the planet – the 100-kilometer (62-mile) wide Maniitsoq structure – isn't actually an impact crater at all.
Through a combination of field mapping, rock dating, and geological chemical analysis techniques, researchers have been able to show that features previously argued to be the faint signature of a long-eroded crater were anything but. The alleged structure is as much the product of the same geological processes as those that created the surrounding region.
Estimated as being around 3 billion years old, the ricks within the Maniitsoq structure date from the Archean era (4-2.5 billion years ago), a period in Earth's history that geologists have little solid evidence to go on when it comes to impact craters.
"Our results conclusively rule out the proposal that much of the Archean rock mass in the Maniitsoq region formed by an Archean meteorite impact, which leaves the 2.23 Ga Yarrabubba structure in Western Australia as the oldest confirmed terrestrial impact structure," write the researchers in their published paper.
"The source craters for Archean-aged impact ejecta remain elusive on Earth."
The idea of Greenland hiding the oldest impact crater on record was first put forward in 2012, but the Maniitsoq site was never widely accepted as being such a crater: even from the start, it didn't meet enough of the necessary criteria to make it a conclusive discovery.
Ticking every last impact crater criteria box shouldn't be required when dealing with geological evidence that's billions of years old, the researchers behind the original proposal argued – rocks can change a lot over such massive periods of time.
A magnetic anomaly across the site, a central section of pulverized rocks possibly formed by impact shock, unusual rock crystal structures, and other changes perhaps caused by hot seawater seeping through the cracks caused by the meteorite were put forward as signs of a massive strike.
Closer examination for this latest study revealed the magnetic anomaly disappears at a larger scale, and together with the pulverized rocks, could be explained by normal geological processes.
What's more, rocks supposedly melted during the meteorite impact turned out to be some 40 million years younger than originally thought.
"I try to keep an open mind about everything in science, especially until you see the rocks themselves," geologist Chris Yakymchuk, from the University of Waterloo in Canada, told Massive Science. "[But] after seeing the rocks, it was kind of 'huh? These don't look that different from rocks I've seen elsewhere in the world.'"
"So either we missed impact structures everywhere on Earth or this wasn't one."
Further investigation showed that there was nothing unusual in the crystal structure of rocks at the Maniitsoq site, and an analysis of 5,587 zircon grains found no evidence of a massive shock impacting on the geology of the region. As for the rush of hot seawater, a look at the type of oxygen isotope on the zircon failed to show signs it had ever occurred.
With the Greenland site out of the running, that leaves the Yarrabubba site in Western Australia – dated to 2.229 billion years ago and 70 kilometers (44 miles) wide – as the oldest impact crater discovered so far.
While the researchers behind the new study admit it's easier to prove something didn't happen than to prove something did happen, it now seems virtually certain that the 3-billion-year-old Maniitsoq structure was not created by a giant meteorite.
"You have to take everything together and say, okay, what is the simplest explanation for all the features we see? And the simplest explanation is that this is not an impact," Yakymchuk told Massive Science.
The research has been published in Earth and Planetary Science Letters.