The asteroid that wiped out the dinosaurs set off an intense heat wave that briefly boiled the Earth’s atmosphere – but it didn’t burn off all the plants.

Humanity has not been unlucky enough to observe at first hand the effects of a large impact, so to investigate whether a massive asteroid would spark off a global wildfire we had to turn to the laboratory. We have modelled, for the first time, the heat generated by the impact and what it meant for the planet’s plants. Our research is published in the Journal of the Geological Society.

This all happened 65m years ago at the end of the Cretaceous period, when dinosaurs still roamed the earth. Suddenly, between 60 and 80% of all living species became extinct. Until the 1980s, this catastrophic loss of life was a mystery, but then scientists found a clue – traces of the element iridium in rocks of this age. Iridium generally falls to Earth with extraterrestrial objects. This suggested a massive asteroid collided with the planet and that this could be responsible for the mass extinction.

Ten years later, scientists found the 65m year-old, 180km wide, Chicxulub crater on the Yucatan Peninsula in Mexico, which finally provided the smoking gun that could explain the apparent chaos at the end of the Cretaceous era. A crater that wide suggests that an asteroid or comet approximately 10km wide hit the Earth.

The impact would have released a huge amount of energy – equivalent to more than a billion Hiroshima bombs. The asteroid itself was vaporised as it smashed into the Earth and in doing so vaporised and blasted out particles of the rock that it hit.

A huge glowing ball of hot rock and vapour rushed up from the impact site at huge speeds, ejecting it way above the atmosphere up into space. As it hit the cold of space it decelerated, cooled and rained back through the atmosphere, re-solidifying and forming tiny droplets of rock known as “spherules”.

As these spherules fell through the atmosphere they were subject to the same frictional-drag which causes space shuttles to become super-heated as they return to earth. This in turn meant as the particles rained down through the atmosphere they delivered a massive blast of heat to the ground. Scientists call this a “thermal pulse”.

This heat pulse has widely been suggested to have ignited global wildfires and has been cited as a cause of the mass extinction.

Recreating the impact

New computer modelling techniques have enabled us to generate better estimates of the heat pulse resulting from this impact. We found it wasn’t evenly distributed across the surface of the Earth. Areas close to the impact site experienced a strong but very short-lived pulse – reaching a peak heat flux of around 50kW/m2 (20 times higher than a human can tolerate) for around one and a half minutes.

Further away, the maximum heat flux was lower – a peak of 20kw/m2 – but lasted much longer – up to seven and a half minutes.

We teamed up earth scientists with fire safety engineers to investigate whether this heat would lead to a massive global wildfire. The heat pulse from the asteroid impact was recreated using state-of-the-art apparatus usually used to test the flammability of furnishings and materials. This provided, for the first time, the ability to test whether the heat pulse from the impact could have set fire to the world’s plants.

Our research reveals that the short sharp blast of heat felt closer to the impact could not have ignited live plants.

However the longer drawn out pulse further away from the impact may have started fires in some locations, implying that localised fires may have occurred. But critically “global firestorms” were unlikely.

This turns our understanding of the heat pulse on its head as its effects would have been greater further away from the impact. Earth scientists will have to reassess their understanding of the fossil record of life. Until now they have assumed the heat pulse was strongest closer to the crater, but now patterns of extinction and survival must be reinterpreted by considering a more severe heat pulse further away.