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It was the dust storm to end all dust storms. When a huge asteroid hit Earth 66 million years ago, triggering a mass extinction that wiped out the dinosaurs, it sent vast clouds of dust flying tens of kilometres up into the air. We now think these spread around the planet at speeds of up to 6 kilometres per second, cloaking it within hours.

If anyone had watched from space, they would have seen our planet become swathed within just a few hours of the asteroid hitting, says Joanna Morgan at Imperial College London. 鈥淵ou鈥檇 see this dust cloud expanding from the impact site and everything getting covered.鈥�

The ensuing extinction wiped out all dinosaurs except for birds 鈥� which evolved from other dinosaurs 鈥� and many other species except for a few land animals.

Morgan and her co-author Natalia Artemieva at the Planetary Science Institute in Tucson, Arizona, first decided to simulate such a dust cloud around 2007. Developing the model and simulating the behaviour of the atmosphere and the rocks and dust in the ejecta was painstaking. 鈥淚t鈥檚 taken over 10 years,鈥� says Morgan.

Extraterrestrial puzzle

They were trying to solve a mystery about the event, which concerns a key piece of evidence for the asteroid impact: a layer of extraterrestrial material, about 3 millimetres thick, laid down in rocks from the time of the collision. Strangely, the layer is the same everywhere it is found in the world. 鈥淚t鈥檚 a constant thickness with a constant composition,鈥� says Morgan.

That is odd. For example, when volcanoes erupt, the heaviest lumps of material land close by and only the lightest bits travel great distances. As a result, sites far away from the eruption have less material. 鈥淭he layer thickness decreases with distance from the site,鈥� says Morgan.

The asteroid hit Earth near what is now Chicxulub on Mexico鈥檚 Yucat谩n peninsula, so there should be a thinner layer of material at sites far away from Mexico.

Morgan and Artemieva came up with an explanation after studying what happened when comet Shoemaker-Levy 9 hit Jupiter in 1994. 鈥淎s it impacted, there was horizontal spreading of dust,鈥� says Morgan. The suggestion is that a fast moving impactor heats the atmosphere, which causes it to expand and help move dust sideways.

They wondered if the same thing happened to the material thrown up by the asteroid impact on Earth. 鈥淭he atmosphere heats up rapidly and expands, so instead of the ejecta coming straight through the atmosphere and landing, it actually gets sent somewhere else.鈥�

The pair鈥檚 simulations show that the rocks and dust thrown up by the impact formed an expanding circle around the impact site. This rose 50 kilometres into the sky, above the dense part of the atmosphere, so it could spread relatively unimpeded.

The overall result was that the dust moved around Earth at up to 6 kilometres per second, cloaking it within hours. This explains how the material from the impact became so evenly spread.

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