By Lisa Grossman, Science News
If the dinosaurs didn't die in a firestorm 65 million years ago, they died in an oven.
New research appearing in the December issue of Geology suggests that the heat from the asteroid impact — blamed for extinguishing most life on the planet at the end of the Cretaceous period — wasn't enough to ignite worldwide wildfires, as previously thought.
"We can't say that the whole world spontaneously ignited," says lead author Tamara Goldin of the University of Vienna. "That doesn't fit with these current numbers."
Earlier models, developed by coauthor Jay Melosh of Purdue University and colleagues in the early 1990s, found that the rain of red-hot debris from the impact would radiate so much heat that the surface of the Earth would feel like an oven set to broil (260 degrees Celsius) for at least 20 minutes — long enough and hot enough to directly ignite wood. The world's forests would go up in flames, the model projected, and any unsheltered creatures would be burnt to a crisp.
But controversies remain about the global layer of impact debris at the boundary between the Cretaceous and Tertiary periods. For one, some have observed that it doesn't include as much soot as might be expected from widespread fires, casting doubt on the theory that all the Earth's forests burned. For another, the soot present may not necessarily come from burnt trees.
To resolve the question, Goldin and Melosh considered how the impact debris itself would affect the amount of heat reaching the Earth's surface. Previous models had shown that one-third of the heat from all the debris reached the ground. Some would be lost to space and some to absorption by greenhouse gases. But the new model suggests that a third reaching the ground is too much.
"We thought we'd just press 'go' on the computer and broil a bunch of dinosaurs," Goldin says. "But it didn't work. There wasn't enough radiation getting down."
It all came down to the particles' paths through the atmosphere. The impact tossed some debris into high trajectories, and those particles took a long time to float downward. Other particles flew into low trajectories and returned to Earth more quickly. The new model shows that the first onslaught would actually shield Earth's surface from the heat of debris that came later. After the first layer of particles radiated away heat, the particles formed a cool cloud of dirt around 70 kilometers in the air that blocked heat from subsequent embers.
"Not only are the spherules producing the radiation, they're also protecting the surface from it," Goldin says. "Over time, a bigger and bigger proportion is blocked from getting all the way to the ground."
Ultimately, the Earth's surface still felt like an oven on broil, but for only about eight minutes, the model suggests. The surface felt less heat for less time. This could explain why the species that survived the impact included small, burrowing mammals and aquatic reptiles like crocodiles that could take shelter from the searing heat, says Doug Robertson of the University of Colorado at Boulder.
"I'm really pleased about the model," says Belcher, who says her research on the charcoal content of the boundary layer argues against widespread wildfires. "I think it's been a long time coming."
Robertson still believes worldwide fires occurred, however. While the radiation may not have been enough to directly ignite wood, it could still have kindled pine needles and dry leaves, he points out.
"You don't need to ignite every tree to have a forest fire," he says. "Toss a smoldering cigarette—it won't ignite a log either, but it will ignite a forest fire. It'll hit dry leaves, and poof."
"It's not a closed book on the wildfires," Goldin says. "There's still a lot of work going on, on both sides on this story, to converge on a solution."