Dinosaur-Killing Asteroid Turned The Sky Black For More Than A Year

Those were the last blue skies seen for at least a year. Elenarts/Shutterstock

When an asteroid slammed into the Earth 66 million years ago, so much soot was thrown into the atmosphere that it finished off the dinosaurs and wiped out two-thirds of all plant and animal species. Now, a new study has calculated the extent of the darkness, and it's hardly surprising that the dinosaurs didn’t survive. In fact, it’s amazing that anything other than bacteria made it through.

Even before the discovery of the Chicxulub crater, iridium enrichment in sediments laid down around the time of the non-avian dinosaur extinction convinced many scientists an asteroid was responsible. The same layers also contain vast quantities of carbon in the form of soot.

“The soot is collocated with the iridium, and therefore must have been injected during the time required for the iridium to be removed from the atmosphere and reach the ground,” Dr Charles Bardeen, of the National Center for Atmospheric Research, argues in the Proceedings of the National Academy of Sciences.

The paper models the quantity of soot injected by a 10-kilometer-wide (6-mile-wide) asteroid and the fires that sparked immediately, rather than fresh bouts that occurred some years later. Estimates of the amount of soot this event created vary widely, so the paper ran the models with five figures, from 750 Terragrams (830 million tons) to 35,000 Terragrams (39 billion tons). Each of these were considered with and without large amounts of water accompanying the soot into the upper atmosphere.

Using a climate model developed to explore links between the atmosphere, ocean, and ice caps, the authors found that the soot was heated by sunlight, causing much of it to rise to altitudes as high as 90 kilometers (56 miles). The sootiest scenarios would have produced coagulation into larger particles, causing 90 percent to come to ground within a year. However, smaller soot injections would have seen a larger proportion stay airborne for a long time.

Under the 35,000 Terragrams scenario, the Sun would have been as faint as the Moon today, leaving insufficient sunlight for photosynthesis for nearly two years. Even the low-soot model would have blocked 90 to 95 percent of light for two years, allowing only shade-tolerant plants to grow. On all models, temperatures drop by 28ºC (50ºF) on land and 11ºC (20ºF) in the oceans.

The ozone layer would also have been destroyed, taking seven years to recover.

The paper notes that some other substances that were probably injected into the atmosphere at the same time have been ignored, either because they would have rained out too quickly to have a major climatic impact or because their presence has not been confirmed. Nevertheless, none of these were likely to have made the Earth a more habitable place in the immediate aftermath of the impact.

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