For hundreds of thousands of years, the woolly mammoth thrived on earth, striding majestically across the frozen expanses.
Then something happened. The earth changed. And in a remarkably short time, the mammoths (Mammuthus primigenius) had disappeared; the last of them died out 4,000 years ago on the remote Wrangel Island in the cold Arctic north.
Although humans are believed to have played a major role in their ultimate demise, it is not clear what factors may have triggered the climate change that put them in danger. One theory is that Earth was struck by a cosmic event nearly 13,000 years ago that warmed the world beyond what mammoths could tolerate, paving the way for other species to thrive.
This is called the Younger Dryas Impact Hypothesis (YDIH), and to say it is highly controversial would perhaps be an understatement. Nevertheless, some scientists believe the idea is valid and are looking for evidence to support it.
One of them is archaeologist Christopher Moore of the University of South Carolina. “Some of our critics have asked, ‘Where is the crater?'” says Moore. “So far we have neither a crater nor a crater.”
Still, Moore and his colleagues believe that if you do more than just a cursory examination of the Earth, you can find evidence. And they believe they’ve found some of it – in the form of minerals with properties they think are best explained by a comet impact.
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In their recent article, they describe several of these lines of evidence that throughoutthey say, tell a captivating story.
This diverse evidence comes from layers of sediment excavated at archaeological sites around the world. They have all been radiocarbon dated to about 12,800 years ago – the time when the impact is believed to have occurred.
From around 50 locations around the world, including North and South America, Europe, Asia and the Greenland ice sheetEvidence has emerged that could indicate an encounter between Earth and a comet.
Ice cores dug from permanently frozen regions of Greenland have revealed microparticles associated with large-scale fires – so-called combustion aerosols, which spread throughout the atmosphere when matter burns.
Unusually high amounts of platinum have been found in samples from other parts of the world, such as Syria and three widely separated sites in North America. Platinum, Moore explains, is rare in the Earth’s crust but relatively common in comets.
In the same sediment layer, there is an increased concentration of tiny, microscopic iron spheres, called microspheres. These are formed when molten material splashes through the air, as happens, for example, when a meteorite hits the surface or melts and explodes in the atmosphere.
Finally, researchers report for the first time the presence of quartz grains found in shock fractures in the Younger Dryas boundary layer at several well-separated sites in North America. These are quartz that exhibit microscopic fractures as a result of a significant shock wave.
“It’s like putting 75 elephants on a quarter,” Moore says. “What we’re seeing is the result of tremendous pressure.”
The bigger picture that might emerge from these puzzle pieces is a comet that hit Earth about 12,800 years ago, in an impact that may not have left a crater. If the comet exploded in the atmosphere, the resulting shock wave could have washed over the surface and created all of the observed elements, much like the Tunguska event created a huge tumult without leaving a deep scar on the planet’s surface.
However, it is far from conclusive evidence. In a paper published in December last year, a team led by anthropologist Vance Holliday of the University of Arizona found: “The evidence and arguments purporting to support the YDIH include faulty methods, inappropriate assumptions, questionable conclusions, false factual claims, misleading information, unproven claims, irreproducible observations, logical fallacies, and the selective omission of contrary information.”
So we’ll probably need a lot more data before the science is even remotely convincing. But other scientists point out that in the past, many scientific theories that were once rejected or dismissed later gained broad consensus. So while it’s important to remain skeptical, it can pay to keep an open mind.
What cannot be denied is that asteroid and comet impacts are absolutely worth studying in the context of large-scale environmental change, if not to understand history, then to guide our decisions for tomorrow. These events have changed the course of all life on Earth before, and although the solar system is much quieter than it used to be, the likelihood of such impacts occurring again in the future is not zero.
The new paper was published in Air detonations and crater impacts.