经历恐龙之杀星体末日的感受：逐幕记

A great Tyrannosaurus rex strides through the conifer trees of her territory, sniffing the air. She picks up the scent from the carcass of a dead horned dinosaur, Triceratops, that she was feeding on yesterday. She walks over and strips off some more shreds of meat, but the smell is foul even for her.

She goes down to the lake to drink and small crocodiles and turtles scuttle into the water. But she hardly sees them. Of more interest is an armoured dinosaur, Ankylosaurus, lurking nearby. However, she knows this dinosaur won’t be an easy kill and she isn’t desperate enough for food to risk a fight. Little does she know there are bigger dangers ahead. She looks up and sees a bright light racing downwards accompanied by faint crackling and sizzling noises.

Our T. rex has excellent hearing for low frequency sounds and she is disturbed by the vibrations she can feel. But her upset only lasts for a moment. In a flash, she has been burnt to a crisp and her world changed forever.

This all happened 66 million years ago, when a huge asteroid famously hit the Earth in the area of what is now the Caribbean. At the end of the Cretaceous period, sea levels were 100–200 metres higher than today, so the shores of the Caribbean lay far inland over eastern Mexico and the southern United States. The impact happened entirely within these waters.

The event triggered instant changes to our planet and its atmosphere and led to the extinction of the dinosaurs and about half Earth’s other species. But what would it have been like to experience such a gargantuan impact? What would you have seen, heard or smelled? And how would you have died – or survived?

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As experts on meteoritics and palaeontology, respectively, we’ve created a detailed timeline, based on decades of research, to take you right there. So let’s start by travelling back in time to the very last day of the Cretaceous.

T-minus one day

All is calm and the Cretaceous day proceeds as usual. In what will soon be ground zero, it is pleasantly warm, about 26°C, and wet. It often is. For about a week, the asteroid has been visible only at night. Because the giant rock is heading straight towards Earth, it looks like a motionless star. There is no dramatic tail; this is a rocky asteroid rather than a comet.

In the last 24 hours, the light becomes visible during the daytime. But it still looks like a star or planet, getting brighter in the final few hours before impact.

T equals 0: the impact

If you were close by, you would first have experienced a brief light and sound show. Minutes to seconds before the impact, you’d have seen the bright fireball, and its accompanying crackling or fizzing noises. This sizzling sound is a result of the photo-acoustic effect: the intense light of the fireball warms the ground, which then heats the air above it, causing pressure waves, or sound.

Next, a deafening sonic boom, which occurs because the asteroid is travelling faster than the speed of sound. But the asteroid is so huge, perhaps 10km in diameter, that it almost certainly hits the ground before any living creature near the impact zone has time to run for cover.

The asteroid’s enormous energy forms a crater through a series of processes that together take only a few seconds. As the asteroid collides with the surface, its kinetic (movement) energy is instantly transferred to the surface as a combination of kinetic, thermal (heat) and seismic energy (released during earthquakes) . This results in a series of shock waves that heat and compress both the asteroid and its target.

As the shock waves propagate, rocks fracture, break up and are ejected, producing a bowl-shaped depression, or transient cavity, about ten seconds after impact. The heat and compression also melt and vaporise large volumes of material, including the asteroid itself, releasing a fountain of incandescent vapour (its temperature is more than 10,000 K, or 9726.85°C) .

Over the next few seconds, the cavity increases in size to many times the diameter of the original asteroid. Simulations suggest that around 20 seconds after impact, the transient cavity is at least 30km deep – deeper than the deepest depth currently known on Earth, the 11km Challenger Deep valley, part of the Pacific Ocean’s Marianas Trench. The rim of the crater is over 20km high – more than twice the height of 8,900m Mount Everest.

But this enormous feature lasts for less than a minute before it starts to collapse. Within three minutes of the impact, the centre of the crater has rebounded to form a peak several kilometres high. The peak only lasts about two minutes before collapsing back into the crater.

Whether a dinosaur or a dung beetle, if you were near the transient cavity you would have been incinerated instantly by the blast. But even if you were up to 2,000km from the epicentre, you’d likely have been killed quickly by the thermal radiation and supersonic winds now spreading out from the impact site.

T-plus 5 minutes

Five minutes after the impact, the winds have “eased” to those of a category 5 hurricane, flattening everything within about 1,500km of the impact. Destroying everything, that is, which has not already been burnt. Atmospheric temperatures in the region rise to over 500K (226.85°C) . This would feel like being inside an oven – causing burns, heatstroke and death. Wood and plant matter ignite, creating fires everywhere.

Because the asteroid struck the sea, the atmosphere is also filled with super-heated steam, making the hurricane-force winds even deadlier.

Next come the tidal waves, triggered by the vast quantities of displaced rock and water. These 100-metre megatsunamis first strike the shores of what is now the Gulf of Mexico, engulfing the land before depositing huge amounts of debris as they retreat.

By now, the crater has almost reached its final dimensions – 180km across and 20km deep. But making an enormous hole in the ground isn’t the only outcome of the impact. All the rock and vapour displaced during the collision has to go somewhere. Several locations in Northern America show that metre-sized blocks of debris from the impact were thrown distances of hundreds of kilometres.

So if you were 2,000km to 3,000km from the epicentre and survived the first few seconds, you’d most likely die from overheating, earthquakes, hurricanes, fires, tsunami-driven floods or being hit by impact melt.

But what is happening much further away? In the first five minutes after impact, dinosaurs roaming the Cretaceous forests of what are now China or New Zealand are so far undisturbed.

But it won’t be long before that changes.

T-plus one hour

Shockwaves on land and sea are only minor inconveniences compared with the fire that is still radiating down from the sky. Some of the impact energy has been transferred into the atmosphere, heating the air and dust to incandescence.

An hour after impact, a belt of dust has circled the globe. Deposits of solidified molten droplets (impact spherules) and mineral grains have been found in numerous locations from New Zealand in the south to Denmark in the north. In these locations, you would not have been aware of the tsunamis around the Americas or the wildfires, but the skies would certainly have begun to darken.

T-plus one day

By now, huge tsunamis are moving east across the Atlantic and west across the Pacific, entering the Indian Ocean from both sides.

They are still around 50m high – causing death and destruction across many coasts around the world. By comparison, the 2004 Boxing Day tsunami reached heights of up to 30 metres. Tsunamis kill fishes and marine life that are washed high on the shore and then dumped, just as they kill coastal trees and drown land animals. But the tsunamis gradually fade away and probably don’t wipe out any entire species – at least on their own.

The hurricane force winds have also died down, but tropical storm strength winds are whipping up debris and causing further chaos and destruction across the tsunami-affected areas. The burning sky is also triggering wildfires across the globe – which, in turn, carry ever more soot into the atmosphere. The sooty signature of these wildfires has been found deposited as carbon particles in sediments from the K-Pg boundary – a 66-million-year-old thin clay layer.

Further away, in what is modern Europe and Asia, the skies continue to fill up with dust and soot, as they do everywhere. Temperatures start to drop as sunlight is blocked. Trees and plants in general, including phytoplankton, close down as if for winter, unable to photosynthesise. Any animals that rely on warm conditions ultimately hunker down and die.

T-plus one week

It’s getting darker and darker. Simulations of solar radiation reaching the Earth’s surface following the impact indicate that, after about a week, the solar flux (the amount of heat and light per a certain area) is just one thousandth of that prior to the impact. This is caused by particles of dust and soot in the atmosphere.

The continued decrease in light levels is accompanied by a global drop in surface temperatures of at least 5°C. This means that most of the dinosaurs and other large flying and swimming reptiles probably die from freezing within the course of this first week (smaller reptiles with slower metabolisms or more flexible diets could survive longer) . Cooling temperatures and cloud cover also lead to rain. But not just any rain. Storms of acid rain fall across the Earth.

Two separate mechanisms generate acid rain. The first is down to the geology of the impact region. The asteroid happened to hit an area of sediments rich in sulphur, which vaporised and caused sulphur oxides (acidic and pungent gas compounds composed of sulphur and oxygen) to be part of the plume of plasma blasted into the atmosphere. Second, the energy of the collision was sufficient to turn nitrogen and oxygen into nitrogen oxides – highly reactive gases that can form smog.

The dropping temperature ultimately allows water vapour to condense into drops, and the sulphur and nitrogen oxides dissolve to form sulphuric and nitric acids. This is sufficient to generate a rapid drop in pH. Early models suggest that the pH of the rain might be as low as 1 – the same acidity as battery acid.

At this point, Earth is not a great place to be. Rotting vegetation, choking smoke and sulphur aerosols combine to make the planet stink. Plants and animals on land and in shallow seas that have survived the darkness and cold succumb to the corrosive acid rain and ocean acidification. Acid rain also kills trees by leaching nutrients such as calcium, magnesium and potassium from the soil. Shallow marine shellfish, crustaceans and corals also die as acid seawater destroys their skeletons.

T-plus one year

Winds die down, wildfires are extinguished and the oceans are once again calm. It might appear that the asteroid collision is just a scar on the ocean floor. But its effects are still destructive. The atmosphere is still filled with dust and the Sun hasn’t shone for a year. Temperatures have continued to drop, with the average surface temperature now 15°C lower than before the impact. Winter has come.

Any dinosaurs or marine reptiles that survived the first week of freezing conditions would have died very soon after. A year after the impact, only rotted skeletons of these behemoths remain. Here and there, smaller animals like mammals the size of rats and insects would be nestling in crevices, barely surviving on their reserves and decaying plants.

Indeed, it has not been a good year for life on Earth: over 50% of plants have died out because of the cold and lack of sunlight. And similar losses have occurred among terrestrial animals and species in the acidified, shallow sea waters.

While most plant groups and many of the modern groups of insects, fishes, reptiles, birds and mammals recover reasonably rapidly, things don’t look great for other species. Dinosaurs and pterosaurs living on land are extinct, as are many marine reptiles, ammonites, belemnites and rudist bivalves in the oceans. Ammonites and belemnites are high in their food chains, and so suffer not only from the cold and acidification but also from the loss of abundant food resources, such as smaller marine organisms.

T-plus ten years

The Earth is still in the grip of a fierce winter. Although most of the sulphur has rained out of the atmosphere, dust and soot particles remain. The average surface temperature is still about 5°C lower than before the impact. The main oceans have not frozen, but inland lakes and rivers around the world are iced over.

Clearly, there were no humans about at this time – there weren’t even any larger mammals. But given the only species that survived were those that could burrow or live below water, it is unlikely that you could have survived this long.

Surviving plant and animal groups such as turtles, smaller crocodiles, lizards, snakes, some ground-dwelling birds and small mammals repopulate the Earth at this point. But they are forced back to limited areas of relative safety a long way from the impact site. These areas are now receiving sufficient sunlight for plants and phytoplankton to photosynthesise again. As leaves and seeds provide the basis for the food chains on land and in the sea, life begins to rebuild.

Eventually, life returns to the devastated landscapes, but ecosystems are very different and the dinosaurs are no more.

T-plus 66 million years

Today, 66 million years after the impact, the scars of the collision are hidden within geological strata – and scientists have started deciphering them. It was in 1980 that researchers first reported evidence of the impact. In their classic paper, Luis Alvarez, a Nobel-prize-winning physicist, and co-authors, described a sudden enrichment in the element iridium in a specific clay layer in Denmark and in Italy.

Iridium is rare in surface rocks because most of it was sequestered in Earth’s core when the planet first formed. However, iridium is found in meteorites, and Alvarez and colleagues inferred that the rate of accumulation of the metal in the sediments was so high that it could only have been produced by impact of a gigantic meteorite.

Because the scientists had only observed the iridium spike in two locations, the impact hypothesis was rejected by many scientists at the time. However, through the 1980s, iridium spikes were identified in clay layers at more and more locations – in muds laid down on land, in lakes, in the sea.

Support for an impact hypothesis strengthened when a crater of the correct age was found in 1991. The crater is buried beneath younger rocks, but clearly visible in geophysical surveys, lying half on land in the Yucatán Peninsula of Mexico, and half offshore. Since 1990, evidence for the impact has increased, not least when scientists discovered that there was indeed a sharp cooling event at the end of the Cretaceous.

In total, it is estimated that half the species of plants and animals alive at the end of the Cretaceous disappeared. It was once thought that surviving groups such as many plants, insects, molluscs, lizards, birds and mammals somehow escaped unscathed. But detailed study shows that this is not the case – they were all hit hard.

But, by chance or luck, enough individuals and species were able to survive the cold and absence of food, or were in parts of the world where the effects were less extreme. As the world returned to normal, they had the opportunity to expand rapidly into their old niches, but also to occupy the space vacated by extinct groups. In fact, one important consequence of the extinction of the dinosaurs, apex predators in their heyday, was the successful spread and evolution of mammals.

When Alvarez and colleagues first described the drop in temperature following the impact, they called it a “nuclear winter”, reflecting the political climate of the early 1980s. Now we might be more inclined to describe the effects as a global climate change – similar events are currently resulting from increased carbon dioxide in the atmosphere (flooding, temperature fluctuations) .

It is salutary to think that without the asteroid collision, primates might never have reached the level we are at today. But it is equally salutary to consider that modern humans are causing some of the same changes to the atmosphere that ultimately killed our reptilian forbears and may one day also lead to our own demise.

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Monica Grady receives funding from the Leverhulme Trust for an Emeritus Fellowship and from the STFC. She is affiliated with The Open University, Liverpool Hope University and the Natural History Museum, London.

Michael J. Benton does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.