New research suggests that the mass extinction event that killed the dinosaurs 66 million years ago was driven by a near-impermeable layer of dust that acted like a lid, trapping heat in the atmosphere and igniting worldwide wildfires. The study, published in the Journal of Geophysical Research: Biogeosciences, revives a theory that extreme heat and fire, rather than cooling and darkness alone, were the primary killers on the day of the asteroid impact.
Brandon and Alexandria Johnson, a husband-and-wife team at Purdue University in the US, combined expertise in impact cratering and atmospheric particulates to model the aftermath of the asteroid strike that formed the Chicxulub crater in Mexico's Yucatán Peninsula. Their findings indicate that the impact ejected a massive plume containing over 1,000 cubic kilometers of vaporized material high into the atmosphere. As this material condensed and rained down as rock droplets known as spherules, it reversed any potential cooling effect from the dust cloud.
According to the researchers, the dust layer trapped almost all radiation within the atmosphere, allowing a staggeringly tiny upward transmittance of just 10⁻²⁸⁶. This trapped energy was reradiated back down to the surface, producing heat fluxes sufficient to cause temperature-induced deaths within an hour or two of the impact for any creatures that could not shelter or burrow. The radiation was also intense enough to ignite lichens, grasses, and pine needles, destroying vast amounts of plant life through wildfires.
Alexandria Johnson said she was surprised by the opacity of the dust layer, asking her husband to redo the calculation to confirm the numbers. She noted that the fact that almost no radiation escaped enhances the idea of a rapid heat pulse killing off life in a very quick manner. The researchers estimated that the dust particles, about as small as those in wildfire smoke, may have blocked out the Sun for decades before settling into what is now seen as the K-Pg boundary, a rock layer a few millimeters thick containing high levels of iridium.
The study breathes new life into a theory that had fallen out of favor, as previous work had suggested that condensed spherules would have blocked the Sun's radiation, causing a global cooling effect similar to large volcanic eruptions. However, the Johnsons argue that the energy added back to the atmosphere as the spherules traveled to the surface created the opposite problem at ground level. The dust layer acted as a lid, keeping almost all radiation that should have escaped back to space and reradiating it downward.
Brandon Johnson said his analysis of rock vaporization thresholds indicates that the plume's mass was large enough to facilitate the transport of material across the planet's atmosphere. Without this transport, the wreckage would have been more localized to the Chicxulub impact site. He hopes to carry out 3D impact simulations that can more accurately incorporate the vaporization of rock to see what happens to the vapour and whether it expands and is deposited globally as expected.
The findings add another piece to the puzzle of how a single asteroid could cause worldwide destruction, a debate that has persisted since the father-and-son team of Luis and Walter Alvarez first presented evidence in 1980 that an asteroid strike triggered the extinction. The research also highlights the role of extreme heat and wildfires as mechanisms behind the mass extinction, which may offer lessons for understanding current climate disasters.
