The asteroid impact that ended the age of dinosaurs 65 million years ago didn鈥檛 incinerate life on our planet鈥檚 surface 鈥� it just broiled it, a new study suggests. The work resolves nagging questions about a theory that the impact triggered deadly wildfires around the world, but it also raises new questions about just what led to the mass extinction at the end of the Cretaceous period.
The impact of a 10-kilometre asteroid is blamed for the extinction of the dinosaurs and most other species on the planet. Early computer models showed that more than half of the debris blasted into space by the impact would fall into the atmosphere within eight hours.
The models predicted the rain of shock-heated debris would radiate heat as intensely as an oven set to 鈥渂roil鈥� (260 掳C) for at least 20 minutes, and perhaps a couple of hours. Intense heating for that long would heat wood to its ignition temperature, causing global wildfires.
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Yet some species survived, and the global layer of impact debris doesn鈥檛 contain as much soot as would be expected from burning the world鈥檚 forests, raising questions about the extent of post-impact wildfires.
To explain the discrepancy, of the University of Vienna and of Purdue University in Indiana studied how ejecta falling through the atmosphere might affect heat transfer from the top of the atmosphere to the ground. Earlier models considered only how atmospheric greenhouse gases would absorb heat.
The study reveals that the first debris to re-enter the atmosphere just a few minutes after the impact helped protect the surface from the debris that followed. 鈥淭he actual ejecta themselves were getting in the way of the thermal radiation [in the atmosphere] and shielding the Earth,鈥� Goldin told New 杏吧原创.
Burning sky
As a result, the surface felt the full heat from the sky for only a few minutes. As more particles drifted down, they blocked more and more of the heat from above, preventing the world鈥檚 forests from igniting. 鈥淲ith the short pulse [of intense heat], it鈥檚 really hard to get ignition鈥� far from the impact site, Goldin says.
Surface life would have been broiled, but not burnt to a crisp. Animals that were able to take refuge underground or in the water were likely able to survive the short period of intense heat, explaining why not all life was killed.
鈥淣ow we have models and data that match,鈥� says of University College, Dublin, who was not involved with the study.
Climate change
of DePaul University in Chicago, who in 1985 proposed that soot found at the end of the Cretaceous came from global wildfires, agrees. The heat shielding effect 鈥渕akes sense鈥�, she told New 杏吧原创.
Without global wildfires, other mechanisms are needed to explain the mass extinction, Belcher says. These include the idea that dust in the atmosphere cut off sunlight in an 鈥�impact winter鈥� that lasted for years before emissions released after the impact caused long-term global warming.
Acid rain following the impact may also have played a role in the extinction, as could the additional stress on global climate from the massive volcanic eruptions that occurred 65 million years ago in India鈥檚 Deccan Traps.
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