Drilling into a dinosaur killer
The University of Texas at Austin Jackson School of Geosciences
Some 66 million years ago, an asteroid — or perhaps a comet — hit Earth. It struck off the coast of what is now the Yucatán Peninsula of Mexico. There, it formed a giant crater. This impact is thought to be at least partly responsible for a mass extinction that included the dinosaurs.
The site of this impact has been under investigation for a few decades. Its discovery started in 1980. That’s when a research team found lots of iridium in places worldwide. The metal is rare on Earth, but abundant in asteroids and other space rocks. Yet strangely, plenty of iridium shows up along the boundary between rocks from the Cretaceous and Paleogene periods.
It's the first hard evidence for a killer-asteroid impact.
Eleven years later, scientists identified the smoking gun — that hidden crater. It circles the coastal Mexican town of Chicxulub Puerto (CHEEK-shuh-loob PWAIR-toh). Yet questions have remained about how the impact might have caused such global destruction.
For answers, scientists recently returned to the scene of the crime.
Evidence of a geologic upheaval
Reaching the Chicxulub crater takes effort. Tens of millions of years of slowly deposited sediment now enshrouds the crater to a depth of hundreds of meters (yards). Much is below the seafloor. The solution to studying it: the Myrtle drilling vessel.
This ship looks a bit awkward cruising the ocean. Three colossal black cylinders rise from the deck like fat masts lacking sails. That gives the ship a top-heavy look. Once in position, though, the ship transforms. The hulking cylinders drop to the seafloor. Then the ship jacks itself out of the water, standing tall on three legs like an oil rig.
Scientists set up this platform some 30 kilometers (19 miles) off the Yucatán Peninsula. From it, they probed deep underground in April and May 2016. They focused on a halo of hills that tower hundreds of meters (yards) above the crater floor. This peak ring formed in the aftermath of the impact. It is the only one of its kind on Earth.
Getting rocky evidence that had been locked inside the peak ring was key to understanding just how powerful the impact had been. It also helped resolve a mystery about how such large peak rings form.
Computers were used to model what happened. And these simulations now suggest that an impact churns deep rocks to the surface, leaving a circle of peaks. Some scientists had argued that the ring might have had a less violent origin. They said the ring might have formed from near-surface materials as the ground rebounded after the impact.
To confirm whether the computer model reflected reality required some seriously deep drilling, notes Sean Gulick. He is a geophysicist at the University of Texas at Austin. He also co-led the expedition.
His team drilled 1,334.7 meters (4,379 feet) below the seafloor. It took nearly two months at a cost of about $75 per centimeter (0.4 inch) of depth. But it proved worth it once the crater’s first impact-forged rocks were in hand, says Joanna Morgan. She is a geophysicist at Imperial College London. She also co-led the project.
The rocks are “dramatic,” Morgan says. They are a kaleidoscope of black, green, red and white minerals including granite.
“It looks like a fake kitchen countertop,” Gulick says. “No other rock looks like this.”
That granite solved the peak ring formation mystery — even before the ship returned to shore. In that region, minerals such as granite tend to be buried in Earth’s mid-crust, many kilometers (miles) below the surface. (The crust is the topmost layer of Earth.) Finding abundant granite in the relatively shallow peak ring means the models were right. The force of the impact indeed shoved deep material to the surface.
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The researchers reported this November 18 in Science.
The lifted material flowed into the newly opened wound in Earth’s surface. There, it met in a giant column at the crater’s center before collapsing outward and forming a rocky ring roughly 550 meters (1,800 feet) high. All of this occurred within the first 10 minutes after the impact.
Scientists will use the data collected by the Chicxulub drilling team to improve computer models to better estimate how much energy and debris the impact unleashed, says Ross Potter. He is a planetary scientist at Brown University in Providence, R.I. Getting those numbers right is important. That’s because the blast itself wasn’t the big killer in the impact scenario. It was the darkness that followed. Blast debris darkened the skies and chilled the planet for years.
asteroid A rocky object in orbit around the sun. Most orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.
Chicxulub The name given an asteroid (or possibly a comet) that crashed into Earth around 66 million years ago. It left a crater more than 180 kilometers (110 miles) wide near the town of Chicxulub in what is now Mexico. The collision released an immense amount of energy — equivalent to billions of atom bombs the size of those dropped on Japan during World War II. This event changed the planet’s climate and is widely believed to have triggered a mass extinction of species — including the end of the dinosaurs.
comet A celestial object consisting of a nucleus of ice and dust. When a comet passes near the sun, gas and dust vaporize off the comet’s surface, creating its trailing “tail.”
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
crater A large, bowl-shaped cavity in the ground or on the surface of a planet or the moon. They are typically caused by an explosion or the impact of a meteorite or other celestial body. Such an impact is sometimes referred to as a cratering event.
Cretaceous A geologic time period that included the end of the Age of Dinosaurs. It ran from roughly 145.5 million years ago until 65.5 million years ago.
crust (in geology) Earth’s outermost surface, usually made from dense, solid rock.
data Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.
debris Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.
dinosaur A term that means terrible lizard. These ancient reptiles lived from about 250 million years ago to roughly 65 million years ago. All descended from egg-laying reptiles known as archosaurs. Their descendants eventually split into two lines. They are distinguished by their hips. The lizard-hipped line became saurichians, such as two-footed theropods like T. rex and the lumbering four-footed Apatosaurus (once known as brontosaurus). A second line of so-called bird-hipped, or ornithischian dinosaurs, led to a widely differing group of animals that included the stegosaurs and duckbilled dinosaurs.
expedition A journey (usually relatively long or over a great distance) that a group of people take for some defined purpose, such as to map a region’s plant life or to study the local microclimate.
extinction The permanent loss of a species, family or larger group of organisms.
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
granite A type of hard igneous rock, which contains coarse-grained inclusions (essentially mini rocks within a rock) of various minerals, chiefly quartz, feldspar and mica.
iridium Discovered in 1803, its name comes from the Latin for rainbow. It’s a hard, brittle and corrosion-resistant metal in the platinum family. Slightly yellowish, the principle use for this element is as a hardener for platinum. Indeed, its melting point is more than 2,400° Celsius (4,350° Fahrenheit). The element’s atomic number is 77.
mineral The crystal-forming substances, such as quartz, apatite, or various carbonates, that make up rock. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in certain regular three-dimensional patterns).
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.
Paleogene That part of the Tertiary Period that ran from 66 million to 23 million years ago.
scenario An imagined situation of how events or conditions might play out.
sediment Material (such as stones and sand) deposited by water, wind or glaciers.
simulation (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might vary in response to various situations or over time.
Journal: J. Morgan et al. The formation of peak rings in large impact craters. Science. Vol. 354, November 18, 2016, p. 878. doi: 10.1126/science.aah6561.
Journal: L.W. Alvarez et al. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science. Vol. 208, June 6, 1980, p. 1095. doi: 10.1126/science.208.4448.1095.