Dinosaur-Killing Impact set Earth to Broil, not Burn

The asteroid impact that ended the age of dinosaurs 65 million years ago didn't incinerate life on our planet's surface – 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 Km 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 "broil" (260 °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.

Yet some species survived, and the global layer of impact debris doesn't contain as much soot as would be expected from burning the world's forests, raising questions about the extent of post-impact wildfires.

To explain the discrepancy, Tamara Goldin of the University of Vienna and Jay Melosh 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. "The actual ejecta themselves were getting in the way of the thermal radiation [in the atmosphere] and shielding the Earth," Goldin told New Scientist.

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's forests from igniting. "With the short pulse [of intense heat], it's really hard to get ignition" 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.

"Now we have models and data that match," says Claire Belcher of University College, Dublin, who was not involved with the study.

Climate change
Wendy Wolbach 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 "makes sense", she told New Scientist.

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 "impact winter" 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's Deccan Traps.

Journal reference: Geology (vol 37, p 1135)

Source : here