Sign up for our newsletters Subscribe
The redesigned, saddled-with-bankruptcy Tribune is getting a little...giddy...and Friday's front page went over the top. Sprawled across the top of the page was the headline "The killer comet theory," and in slightly smaller type the five-line subhead, "13,000 years ago, woolly mammoths and saber-toothed tigers roamed North America and early humans hunted food with spears. Then came a mysterious ice age that wiped them all out. Now some scientists say they know what caused it: a comet exploding above the surface of the Earth, leaving tiny 'nanodiamonds' as a telltale sign."
The story itself back on page 14 offered more vivid writing. "First an explosion as powerful as thousands of megatons of TNT rained meteorites down on North America," began reporter Robert Mitchum. "Then forest fires broke out across the continent, sending up a thick layer of soot and dust that blocked out the sun. A sudden ice age ensued, and some of the Earth's largest animals went extinct in a blink of geological time." Mitchum explained that the microscopic nanodiamonds had been found across North America "in a 13,000-year-old layer of rich sedimentary soil called a 'black mat.' Beneath the layer with the nanodiamonds, fossils of the animals are abundant. After that layer, they disappear..."
The occasion for Mitchum's story was an article cosigned by nine scientists that appeared the same day in Science magazine: "Nanodiamonds in the Younger Dryas Boundary Sediment Layer." It was also the occasion for a story in the Friday New York Times, and if you read both stories you were probably left wondering which newspaper to believe.
Mitchum said the scientists believe the meteorite shower was triggered by a comet that "exploded above the planet's surface." The more sober-sided Times story by science writer Kenneth Chang didn't make a single mention of comets. Chang simply wrote that about 12,900 years ago the Earth experienced an "abrupt cooling" that some scientists now say "may have been caused by one or more meteors that slammed into North America."
Mitchum described nanodiamonds as "microscopic particles thought to be found on comets." Maybe so, but not only on comets. Chang told us they're found in meteorites and at impact craters.
So what about the comet angle? Was the idea of a massive "killer comet" exploding above the Earth a piece of cheesy bling hung on the Tribune story to dress it up for the front page?
Actually, no. The paper in Science (it costs $10 to access) is the latest in a series of papers that are trying to build a case for an extraterrestrial catastrophe three millennia ago. A 2007 paper in the Proceedings of the National Academy of Sciences argued for a comet as the cause. "Current data suggest that this impactor was very different from well studied iron, stony, or chondritic impactors," wrote this paper's 20 authors, six of whom also signed the Science paper. "The evidence is more consistent with an impactor that was carbon-rich, nickel–iron-poor, and therefore, most likely a comet." The lack of a crater -- one reason many scientists remain skeptical -- "may be due to prior fragmentation of a large impactor, thereby producing multiple airbursts or craters," the PNAS paper speculated, recalling that in 1908 an object 150 meters in diameter, "either a carbonaceous asteroid or a small, burned-out comet," leveled 2,000 square kilometers of forest in Tunguska, Siberia, yet apparently left no crater. "A debris shower from a heavily fragmented comet would have produced an airburst barrage that was similar to, although exponentially larger than Tunguska, while causing continent-wide biomass burning and ice-sheet disruption, but again possibly, without typical cratering."
On comet! The Tribune got it right. The Times missed an interesting piece of the story.