July 01, 2001

Article at Newton Magazine

The Day the Sky Fell In

The K/T asteroid strikes blue Earth, creating huge splash into space

By Wilson da Silva

SIXTY-FIVE MILLION years ago, in one catastrophic flash, the Earth changed forever. Without warning, a mountain-sized rock from space crashed through the atmosphere like a giant flaming fireball. 

When it struck the ground, the energy generated was greater than anything we can imagine – more than 10,000 times the explosive power of all of the world’s nuclear weapons going off in one place at one time. 

A blastwave of fire raced through air for thousands of kilometres, burning any living thing in its path. The shockwave shook the Earth like a bell, triggering tsunamis, landslides and volcanic eruptions around the world. 

The sky fell dark, temperatures dropped. In the dark and desperate years that followed, 70% of all the species on Earth became extinct. Among them, the dinosaurs, magnificent creatures that had ruled the planet for 160 million years.

Was this a freak accident, or could it happen again? 

Like a 100 million atom bombs exploding 

The enormous asteroid that struck the Earth on that fateful day was a monster rock, at least 10 km in diameter. As it punched through the sky over what is today the Yucatan Peninsula in Mexico, it was travelling at tens of kilometres per second. 

It was so large and moving so fast that it didn’t just slip through the atmosphere like a knife through butter: in a split second, it actually blew away all of the air above the ancient volcanic island chain of the Caribbean Sea. The sky filled with a sudden incandescent flash of fury and then, suddenly, the asteroid struck.

At the point of impact, the shallow sea barely 100 metres deep, was vaporised in a hundredth of a second. The ensuing shockwave blasted enormous masses of surrounding water high into the sky, triggering tsunamis in all directions. 

The impact gouged a hole 30 km deep into the seafloor, piercing into the planet’s molten mantle and instantly creating a crater more than 100 km wide. Pulverised rock and dust, from the Earth and the asteroid, was blown up to 40 km high, past the ozone layer and beyond the stratosphere. 

In an instant, the killer asteroid had come to a stop, striking with the energy of a hundred million atom bombs. But the repercussions were still being felt around the world.

Faster than the speed of sound

A cataclysmic blastwave generated by the impact radiated from ground zero at 70,000 km/h, or about 20 km a second. As the ring of air grew in diameter and encountered resistance from surrounding atmospheric air mass, it began to slow. 

Within 10 minutes, the blastwave had travelled 500 km and wind speeds had fallen to 1000 km/h – about the speed of sound, but still many times faster than the wildest hurricanes and cyclones on record. An hour later, the widening ‘blast ring’ had reached a radius of 1,000 km, but was still packing winds of speeds never seen before.

Scientists have calculated that the winds were so strong, they would have flattened forests and blown away dinosaurs up to 1000 km away, in the southern coast of the United States and along most of Central America. 

But the outward blast had created a vacuum over ground zero, and it wasn’t long before the winds reversed direction, crashing back into the impact site at cyclonic speeds.

Meanwhile, the sea at the point of impact would have leaped at least 100 metres into the air. Tsunamis generated by the impact headed outward from ground zero, toward the coasts of Africa, North and South America. 

Because the sea at the point of impact was so shallow, the ensuing tsunamis were not as fierce as if they had been in deep ocean water. Still, by the time they reached continental shores, waves would have been tens of metres high. This would have been enough to reach several kilometres inland, washing away forests and drowning creatures in its path.

Fossilised evidence of these tsunamis have been found in rocks in Texas (where waves were estimated to have reached at least 50 metres high), and boulders the size of cars were washed as much as 500 km away, to what is today Belize.

Shockwaves travel the globe

The whole of the planet would have rung like a bell after the collision. Most of the impact force was transferred straight into the ground – an awesome 10 to 13 on the Richter scale, at least 1000 times stronger than any earthquake on record.

On the exact opposite side of the globe from the Yucatan Peninsula, in what is today the Indian Ocean off the north-west coast of Australia, the seafloor suddenly heaved up in massive swells. Shockwaves from the impact on the other side of the world, having travelled across the Earth’s rocky surface, converge on this side of the planet. Forces estimated at 10 times that experienced in the catastrophic San Francisco earthquake of 1906 are exerted, cracking open fissures spilling lava onto the surface and under the seas.

Some scientists believe this may have triggered a long wave of volcanic eruptions on the floor of the Indian Ocean, forming what we recognise today as India’s Deccan Plateau, a geographical formation created by volcanic activity 65 million years ago. But others dispute this, arguing that volcanoes may have been forming at least a million years before the catastrophic impact.

Even those on the other side of the planet did not escape the gigantic forces unleashed on that day. Within an hour of the impact, the ground was shaking violently around the world. 

Cracks developed in the crust and furious lava flows began, releasing copious quantities of poisonous sulphur gases into the air. Millions would have been killed by these sudden lava flows and volcanic eruptions, or the poisonous gases unleashed from deep inside the traumatised Earth.

Even on the opposite ends of the Earth

It was not a good day to be a dinosaur. In fact, it wasn’t a good day to be on the surface of the Earth at all. Any living creature within sight of the impact area would have died instantly. And those would have been the lucky ones.

Plants and animals within 1,000 km of ground zero would have been swept away, either instantly obliterated or dashed against mountains and cliffs – and then sucked back in again, or been drowned by the tsunamis that quickly followed. 

But as if the hurricanes, earthquakes, volcanoes and tsunamis were not enough, there was also a rapidly growing firestorm to contend with, as well as a hail of boulders, rocks and stones thrown up by the impact. Now, they were headed on a slow arc back toward Earth.

Then things really turned nasty 

A spike of incandescent molten rock at the centre of the impact site would have blasted back up into the atmosphere almost instantaneously, unrestricted by air resistance thanks to the vacuum created by the incoming asteroid only a second before. 

The ‘central ejecta’ – the hot spike of molten rock at the centre of the impact – leaped into the sky at massive speeds. The amount of molten and vaporised material reached temperatures in the thousands of degrees Celsius. It was rich in silica and metallic vapour,  particularly iridium, a metal that is rare on Earth but common in asteroids and other space debris. This central jet reached highest into the upper atmosphere, spreading itself far and wide and enveloping the globe in a fine hot mist.

Below this upwardly shooting spike – and moving rapidly heavenwards just behind it – came an expanding bubble of molten debris. Scientists estimate that the impact on that day catapulted some 300 times the asteroid’s mass into the sky. Nearly 200,000 cubic kilometres of molten rock leapt into the upper atmosphere less than a minute after the initial impact. Billions of tonnes of solid, molten and vaporised rock were propelled up high, at velocities ranging from several kilometres per second over the impact point (or ground zero), to hundreds of metres per second at the edges of the crater.

The plume rapidly ballooned out as it roared higher, its superheated temperature allowing it to spread quickly in all directions across Earth’s atmosphere. This rapidly expelled molten rock soon began to cool as it encountered the extreme cold of the upper atmosphere and near-space. Hot rock began to condense out of this superheated cloud, almost snap-frozen into solids again. 

Soon, the planet’s gravity took hold, and the rocks began falling back down. In contrast to their upward journey, this time the ejecta met air resistance on the way down, as the atmosphere below slammed back into the space left by the asteroid’s path.  As it struck the atmosphere, there were more shockwaves, and the rocks – all 200,000 cubic kilometres – began heating up again as they re-entered the atmosphere. 

All over the world, the skies began to light up as the enormous mass of rock thrown up by the impact began to rain down onto Earth. A sudden global inferno enveloped the planet. Scientists calculate that every square metre of Earth’s atmosphere would have received an average of 10 kg of hot rock plummeting back towards the ground, striking its hapless inhabitants at an average velocity of 300 km/h. 

This fiery re-entry would have triggered an inferno, unleashing more than 50 kilowatts of energy per square metre in the upper atmosphere as they began to re-enter. This is some 30 times the amount of energy in the time that our atmosphere normally receives from the Sun at any one time. 

Luckily, most of this would have dissipated in the higher altitudes, 60 to 70 km up, as the incoming rocks began to burn. An only a fraction of this inferno would have actually made it to the ground, since about half of the heat would have radiated into space. The other half that radiated back down was catastrophically hot, but still had to travel through tens of kilometres of the gases of the lower atmosphere.

The day the air burned

Nevertheless, scientists estimate that 10 kilowatts of heat energy per square metre still made it to the surface within an hour of impact, enough to heat the soil to 400°C. This would have ignited forest fires on every continent, fires that would have driven the ambient temperature even higher. Any creature in the open would have been grilled to a crisp as the surface of the planet turned into an oven set at ‘high’.

Many wonder how any living creature could have survived this hellish scenario. Scientists believe that cloud cover could have played a part: the ‘heat pulse’ from the incoming meteors would have vaporised clouds into steam, and the water droplets would have absorbed a lot of the heat. In parts of the world where cloud cover at the time of the impact was heavy, life may well have been relatively spared. But those living under blue skies or starry nights would have burned alive.

In addition, because the Earth continued rotating as these events took place, the curtain of superheated debris that fell back was not perfectly symmetrical. An analysis of the likely trajectory suggests that the hardest hit areas (after North and Central America), were those to the west – the Pacific Ocean and Asia, with areas of Europe and the Atlantic Ocean largely spared. 

This appears to be borne out by the fact that ‘shocked quartz’ – grains of quartz with criss-cross patterns that can only be produced by massive shockwaves, such as from asteroid impacts – have been found at many sites dated at 65 million years ago, but few have been found in 

It may have lasted for decades

Millions of living creatures perished in the awesome cataclysm that befell our planet on that day, 65 million years ago. But millions more, including whole species, were to disappear in the days and months ahead as a long winter shrouded the Earth in darkness. 

The dust thrown up by the impact now circulated in the upper atmosphere, blanketing the Earth in darkness for several months, and in a twilight state for most of a year. This drastic depletion of sunlight starved plants and was enough to collapse a number of food chains and lead to wholesale slaughter of marine plankton floating on the surface of the oceans.

But that’s not all: the heat pulse of the initial blastwave and the fiery meteor shower that followed ionised atmospheric gases, recombining them into harmful nitric oxides. These mixed with water vapour to form nitric acid, which rained down on the land and sea.

Poisonous metals from the rocks were leached into the rivers and oceans by the waves of acid rain. Scientists have found evidence of large quantities of arsenic, selenium, aluminium and mercury in the soil of the time. 

In addition, deep layers of anhydrite, a mineral rich in sulphur, had lain at the base of the Yucatan Peninsula. It’s estimated that 100 billion tonnes of this sulphur was catapulted into the air on impact. Some of this fell as acid rain over the years, but much of it also crystallised in the upper atmosphere and blocked out sunlight.

Even after the dust from impact and soot from global forest fires cleared, there was enough crystallised sulphur in the upper atmosphere to block out 80% of the sunlight. This plunged the Earth into a deep winter for a decade; on land, where a balmy 20°C might have been the norm, temperatures dropped to as low as -20°C. 

Many plants, without enough light to photosynthesize, began to die out. Animals that relied on the plants for food began to starve. Predators that fed on the dying herbivores at first feasted on the abundant carcasses, but then they too began to starve. 

It was all too much for many species: 75 per cent of them disappeared in the years that followed, including 90 per cent of all plankton species. Some, like the dinosaurs, had been an unparalleled success for millions of years – until their luck ran out, and they too were shown the door to extinction.

How 70% of all life disappeared

As the ‘impact winter’ dissipates, the extremely high concentrations of carbon dioxide in the atmosphere begin to create a greenhouse effect. The Earth's surface heats up, ice caps melt and sea levels rise, triggering violent squalls and downpours. How long these erratic climate lasts depends 

Days after impact 
Shortly after impact, the atmosphere filled with vaporised sulphur dioxide, carbon dioxide, and tiny fragments of rock blown into the air. These were blown across the planet by winds, while on land forest wildfires burned out of control, sending huge amounts of soot into the atmosphere.

Years later 
The vaporised sulphur dioxide, having condensed with the surrounding dust and soot into droplets of concentrated sulphuric acid, turns into a mist that floats in the stratosphere. This blocks out sunlight, darkens the Earth's surface, and restricts plant photosynthesis. This ‘impact winter’ is believed to have lasted from several years to a decade.

Up to a decade later 
Chlorine vaporised in huge amounts from the Earth's surface by the impact finally reaches the stratosphere, possibly destroying the ozone layer. Forest fires have abated, but few plants can survive in the harsh ‘impact winter’. The final blow is acid rain, which may well have wiped out entire ecosystems on the Earth's surface.

A decade to centuries later 
As the ‘impact winter’ dissipates, the extremely high concentrations of carbon dioxide in the atmosphere begin to create a greenhouse effect. The Earth's surface heats up, ice caps melt and sea levels rise, triggering violent squalls and downpours. How long these erratic climate lasts depends on how well the carbon dioxide in the atmosphere has been re-absorbed by the Earth.

Death of a dynasty

Extinctions happen all the time. New species evolve to inhabit a particular ecological niche and succeed for a while. But the tenure of any species on Earth is not guaranteed, and they can go extinct for any number of reasons: climate change destroying their ecological niche, new competitors moving in, or catastrophic changes like ice ages and meteor impacts.

In the 4.5 billion-year history of the Earth, there have been five ‘mass extinctions’, periods of usually thousands to hundreds of thousands of years when large numbers of species disappear. The last mass extinction, 65 million years ago, was unusual for the speed in which species were wiped out. 

It’s also famous for claiming the dinosaurs, the undisputed giants of all time. Dinosaurs arose at the beginning of Triassic period, some 250 million years ago and, by the Jurassic – 45 million years later – they were in their heyday and covered the Earth. But by the start of the Cretaceous, 144 million years ago, dinosaurs began a long but slow decline, for reasons still not well understood. Nevertheless, these magnificent and frightening creatures ruled the planet for another 79 million years … before the unexpected arrival of a giant rock from space bundled them over the edge of extinction.

When palaeontologists dig down to the level corresponding to 65 million years ago, they find massive amounts of fluffy, soot-like carbon deposits – several hundred times anything recorded previously. There seems little doubt that this represents the burnt remnants of sweeping forests and million of animals. 

Based on evidence from a number of sites around the world, chemists estimate that the raging fires that followed the impact claimed 100 billion tonnes of organic material alone. It’s calculated that forests and plants accounted for only half of this ‘biomass’: the rest would have been animals.

Mammals rose from obscurity to dominate the planet  

Of course, there’s always the flipside to any mass extinction: because of the great calamity, creatures that had lived in the shadow of the dinosaurs came to dominate the planet.

No species weighing more than 25 kg made it through the great cataclysm: any large animal to survive would have needed large quantities of food. The larger the animal, the fewer in number exist in nature; there would have been many more possum-like creatures than Tyranosaureses. Hence, if a species is few in number, a disaster that kills 95% of that species can push it to extinction. Whereas an abundant species like possums can easily repopulate after a cataclysm; and because they are small, they can survive on less food.

Only a quarter of all land-based species survived the impact, and half of these were a relatively obscure grouping known as mammals. Small mammals living in burrows could have survived the initial blastwave, and many are likely to have survived the global wildfires. 

Being warm-blooded would have also helped pull through the decade-long winter, and since most were nocturnal, finding food in the year-long darkness was no great challenge. In addition, most had opportunistic diets: relying on no specific food group, they ate everything and anything – seeds, roots, insects and meat. Being able to grow their young in the womb, and then caring for them for long periods would have also helped ensure survival of each new generation.

In a very real sense, the asteroid that wiped out the dinosaurs and caused so much death and destruction 65 million years ago, opened the way for us humans. Large mammals like elephants, bears, gorillas and humans, may not had the chance to evolve if fierce, pack-hunting, swift-moving assassins like dromaeosaurs had survived. 

All of the mammals alive today – from horses to hedgehogs, from walruses to wombats – are descendants of those diminutive mammals that survived the great impact. 

Most were little more than scavengers and fruit eaters. But today, their descendants are found in all manner of climates across the globe. With the exception of insects, mammals have a wider distribution and are more adaptable than any other single class of animal. They are the Earth’s dominant species … but it took the death of the dinosaurs to bring them out of ecological obscurity and populate the planet.

There have been five mass extinctions … and a sixth one may be underway

Extinctions may be bad news for the victims, but they’re also a natural part of evolution, allowing new species to arise and adapt to changing conditions. There have been five mass extinction events since life first arose four billion year ago, and it’s estimated that 99% of all species that ever existed are now extinct. 

But extinctions in nature occur at a steady pace: one species will die off and eventually their vacated ecological niche is replaced by a new species. In this way, a natural balance is maintained.

It’s when rapid changes take place that mass extinctions occur: all life on Earth in endangered and biodiversity crashes. In such conditions, no-one is safe. Survival depends not just on advantageous attributes, but also on luck. A lot of luck.

The first mass extinction took place in the Late Cambrian period, 500 million years ago, shortly after the first plants had evolved on land and at a time when the seas were already teeming with a cacophony of animal life. It’s thought that a dramatic change in sea levels so drastically altered the environment that many living things couldn’t adapt in time, and were wiped out.

The second extinction spasm occurred in the Late Ordovician, 440 million years ago, in which some animal groups lost more than half their species. It seems to have been triggered by a sudden onset of an ice age: the seas retreated as more and more water turned into ice sheets, while conditions underwater changed drastically. This was followed by a long period of relative stability, during which amphibians and reptiles arose.

Then the Earth experienced a double-whammy in a relatively short time, at least in geological terms. There were two mass extinctions: one at the end of the Permian 250 million years ago, which killed more than any other extinction event: 90% of all marine species, and most of the land species. The surviving species had only just begun to recover from this unexplained calamity when, 45 million years later, another mass extinction took place. Although less severe, it a drastic impact on the oceans and pushed many reptiles and early dinosaurs over the edge.

The fifth mass extinction, now known to have been caused by a giant asteroid colliding with the Earth, occurred very suddenly 65 million years ago, at the end of the Cretaceous. It ended the reptilian dominance of the planet and opened the way for mammals.

Most scientists now believe that, as we enter the 21st century, the Earth is undergoing a sixth mass extinction, this time caused by a single species: humans. Every year, an average of 50,000 animal, plant and insect species vanish. It’s estimated that 50% of the species alive today will have vanished by the end of this century.