Asteroid strike made 'instant Himalayas'

The model animation shows the action in one half of the crater

Media captionThe model animation shows the action in one half of the crater

Scientists say they can now describe in detail how the asteroid that wiped out the dinosaurs produced its huge crater.

The reconstruction of the event 66 million years ago was made possible by drilling into the remnant bowl and analysing its rocks.

These show how the space impactor made the hard surface of the planet slosh back and forth like a fluid.

At one stage, a mountain higher than Everest was thrown up before collapsing back into a smaller range of peaks.

"And this all happens on the scale of minutes, which is quite amazing," Prof Joanna Morgan from Imperial College London, UK, told BBC News.

The researchers report their account in this week's edition of Science Magazine.

Their study confirms a very dynamic, very energetic model for crater formation, and will go a long way to explaining the resulting cataclysmic environmental changes.

The debris thrown into the atmosphere likely saw the skies darken and the global climate cool for months, perhaps even years, driving many creatures into extinction, not just the dinosaurs.

The team spent May to June this year drilling a core through the so-called Chicxulub Crater, now buried under ocean sediments off Mexico's Yucatan Peninsula.


Chicxulub Crater - The impact that changed life on Earth

Drill siteImage copyright NASA
Image caption The outer rim (white arc) of the crater lies under the Yucatan Peninsula itself, but the inner peak ring is best accessed offshore
  • A 15km-wide object dug a hole in the crust 100km across and 30km deep
  • This bowl then collapsed, leaving a crater 200km across and a few km deep
  • Its central zone rebounded and relaxed, producing an inner "peak ring"
  • Today, much of the crater is offshore, buried under 600m of sediments
  • On land, it is covered by limestone deposits, but its outline is visible
  • It is evident in an arc of famous sinkholes referred to as cenotes
CenoteImage copyright Max Alexander/B612/Asteroid Day
Image caption Mexico's famous sinkholes (cenotes) have formed in weakened limestone overlying the crater

The researchers targeted a particular zone in the 200km-wide bowl known as the "peak ring", which - if earlier ideas were correct - should have contained the rocks that moved the greatest distance in the impact. These would have been dense granites lifted from almost 10km down.

And that is precisely what the team found.

"Once we got through the impact melt on top, we recovered pink granite. It was so obvious to the eye - like what you would expect to see in a kitchen countertop," recalled Prof Sean Gulick from the University of Texas at Austin, US.

But these were not normal granites, of course. They were deformed and fractured at every scale - visibly in the hand and even down at the level of the rock's individual mineral crystals. Evidence of enormous stress, of having experienced colossal pressures.

Drill coreImage copyright DSmith@ECORD
Image caption The team retrieved many hundreds of metres of rock from the crater

The analysis of the core materials now fits an astonishing narrative.

This describes the roughly 15km-wide stony asteroid instantly punching a cavity in the Earth's surface some 30km deep and 80-100km across.

Unstable, and under the pull of gravity, the sides of this depression promptly started to collapse inwards.

At the same time, the centre of the bowl rebounded, briefly lifting rock higher than the Himalayas, before also falling down to cover the inward-rushing sides of the initial hole.

"If this deep-rebound model is correct (it's called the dynamic collapse model), then our peak ring rocks should be the rocks that have travelled farthest in the impact - first, outwards by kilometres, then up in the air by over 10km, and back down and outwards by another, say, 10km. So their total travel path is something like 30km, and they do that in under 10 minutes," Prof Gulick told the BBC's Science in Action programme.

Imagine a sugar cube dropped into a cup of tea. The drink's liquid first gets out of the way of the cube, moves back in and up, before finally slopping down.

When the asteroid struck the Earth, the rocks it hit also behaved like a fluid.

"These rocks must have lost their strength and cohesion, and very dramatically had their friction reduced," said Prof Morgan. "So, yes, temporarily, they behave like a fluid. It's the only way you can make a crater like this."

Prof Joanna Morgan: "The model we've proven is much more catastrophic"

Media captionProf Joanna Morgan: "The model we've proven is much more catastrophic"

One of the important outcomes of the research is that it provides a useful template also to understand the surfaces of other planets.

All the terrestrial worlds and even Earth's Moon are scarred with craters just like Chicxulub.

And knowing how rocks can move vertically and horizontally in an impact will assist scientists as they attempt to interpret similar crustal features seen elsewhere in the Solar System.

The project to drill into Chicxulub Crater was conducted by the European Consortium for Ocean Research Drilling (ECORD) as part of the International Ocean Discovery Program (IODP). The expedition was also supported by the International Continental Scientific Drilling Program (ICDP).

Schrodinger CraterImage copyright NASA Scientific Visualization Studio
Image caption Schrodinger Crater on the Moon looks exactly the same as Chicxulub and would have been made - according to this analysis - in a very similar way
Artist's impressionImage copyright SPL
Image caption Artwork: The asteroid that made the crater was probably moving at about 20km/s when it hit the Earth

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos


Photo
An artist’s impression of what the Chicxulub crater might have looked like soon after an asteroid struck the Yucatán Peninsula in Mexico. Researchers studied the peak rings, or circular hills, inside the crater. CreditDetlev van Ravenswaay/Science Source

Some 66 million years ago an asteroid crashed into the Yucatán Peninsula in Mexico, triggering the extinction event that obliterated the dinosaurs and nearly extinguished all life on Earth. It struck with the same energy as 100 million atomic bombs, and left behind a 100-mile-wide scar known today as the Chicxulub crater.

Now, a team of geophysicists has drilled into the gigantic cavity under the Gulf of Mexico, targeting a circular series of hills called a peak ring located at its center. What they discovered illustrates that powerful impacts can catapult materials buried deep in a planet’s crust much closer to its surface.

“Chicxulub is the only crater on Earth with an intact peak ring that we can go sample, the next intact peak ring would be on the moon,” said Sean P. S. Gulick, a marine geophysicist from the University of Texas at Austin. “It’s ground zero of the Cretaceous extinction event.”

Photo
After collecting core samples filled with limestone and remnants of broken and melted rock, the team suddenly retrieved cores with pink granite. CreditD. Smith/European Consortium for Ocean Research Drilling

Dr. Gulick and his colleague Joanna Morgan, a geophysicist from Imperial College London, led a team of more than 30 researchers representing 12 countries to drill into the Chicxulub crater. By drilling into stone beneath the ocean’s surface, they discovered that the peak rings were made of granite, which is usually found much deeper in Earth’s crust. They concluded that the asteroid impact was so strong it lifted sediment from the basement of Earth’s crust several miles up to its surface.

“These rocks behaved like a fluid for a short period of time, and rocks don’t tend to do that,” said Dr. Morgan. “It’s a very dramatic process when you form a large crater.”

The team’s results, which were published Thursday in the journal Science, may help end a debate over how the Chicxulub crater formed in the minutes following the colossal collision. Their research could lend support to the dynamic collapse model theory, which suggests that the asteroid impact was so powerful it shocked the rocks deep in Earth’s crust and caused them to shoot up before collapsing down to the surface to produce peak rings. Their findings pose a challenge for another model that suggests that the peak rings were formed from the melting of the upper parts of the crust.

Photo
Core samples recovered from the Chicxulub crater showing that its peak rings were made of granite that most likely originated deep in the Earth’s crust. CreditD. Smith/European Consortium for Ocean Research Drilling

“The other model can’t be correct given what we’ve found,” said Dr. Gulick. He said the theory may also explain how large craters found on the moon, Mercury and Venus formed.

The Chicxulub crater is buried beneath 66 million years of sediments, and if you were to look at it today you would see that half of it is underwater and the other half is covered by rain forest. The team conducted their work aboard a boat that was converted into a drilling station that was about 40 feet above the Gulf of Mexico, standing on three pillarlike legs.

In order to get to the peak ring, the team needed to drill through about 60 feet of water and then through about 2,000 feet of limestone and other sediment that had accumulated since the impact. As they dug into the crust they collected drill cores, which were 10-feet-long cylindrical samples of rock pulled up by the drill. For a while, the team kept pulling up drill cores filled with limestone and remnants of broken and melted rocks called breccia.


“It was limestone, limestone, limestone, breccia. And then suddenly pink granite!” said Dr. Gulick. “It was exhilarating, it looks like your classic pink granite countertop.”

They reached the peak ring’s granite around 2,500 feet below sea level, but they think it may have originated from crust that may have been more than 25,000 feet deep before the impact.

“That was the big find because that says that this peak ring didn’t come from something shallow at all,” said Dr. Gulick. “It had to come from deep because it’s made of deeply buried crustal rocks now at the surface.”

The team made another find during their dig. They noticed that the granite samples they recovered were weaker and lighter than normal granite; some even crumbled in their hands. One of the team’s next steps is to figure out how exactly the rocks got to the point where they were so weak they could behave like a fluid.