Chicxulub crater

• 1. Introduction

  ›

  The Chicxulub crater lies hidden beneath the Yucatán Peninsula in Mexico, and the asteroid that carved it measured about 10 km across. Planted at sea level, that rock would have towered higher than Mount Everest. Today almost nothing of the wound shows at the surface. Younger sediment has buried it, and its centre sits offshore under the Gulf of Mexico. The people who first saw its outline were not hunting for a catastrophe. They were looking for oil. Why did their find drift past the scientific world for more than a decade? What did the strike do to the planet in its first hours, and in the years of dim light that followed? And where in the Solar System did a body this large come from?

  11923253

• 2. A Layer of Clay

  ›

  At Gubbio, in Italy, a thin band of clay marks the line between the Cretaceous and Paleogene periods. In the late 1970s the geologist Walter Alvarez and his father, the Nobel Prize-winning physicist Luis Walter Alvarez, studied that band and found something odd. It held far too much iridium, a metal that is scarce in Earth's crust but common in asteroids. Levels ran as much as 160 times above the background. Their explanation was dramatic. An impactor had vaporized, spread iridium through the atmosphere, and let it settle across the globe with other debris.

  No one agreed on what had caused the extinction at that boundary. Proposed culprits included a nearby supernova, climate change and a reversal of Earth's magnetic field. Many paleontologists rejected the impact idea outright. Fossils thin out well below the boundary, a gap nicknamed the "three-meter problem", and they read that as a slow die-off. Still, the case spread fast. The Alvarezes, working with Frank Asaro and Helen Michel at the University of California, Berkeley, published in Science in June 1980. Jan Smit and Jan Hertogen had reported iridium from Caravaca, Spain, in Nature just a month earlier, in May 1980.

  Similar iridium spikes then turned up around the world. More than 2,000 papers on the extinction appeared during the 1980s. Yet no known crater matched the right age and size, so the hunt for one began. Lee Hunt and Lee Silver gathered specialists from many fields at Snowbird, Utah, in 1981. Unknown to them, evidence for the very crater they wanted was being presented elsewhere that same week.

  46792313

• 3. The Oilmen's Bullseye

  ›

  In 1978, the geophysicists Glen Penfield and Antonio Camargo were flying an airborne magnetic survey for Petróleos Mexicanos, the Mexican state oil company known as Pemex. The survey covered the Gulf of Mexico north of the Yucatán Peninsula. Penfield's task was to read geophysical data and suggest where to drill. He mapped anomalies in the offshore magnetic readings and estimated their depth. Then he pulled out onshore gravity maps made in the 1940s and laid the two side by side. A shallow "bullseye" 180 km wide stood out against otherwise flat, uniform surroundings. To Penfield it looked like an impact scar.

  He was not the first to think so. A decade earlier, a contractor named Robert Baltosser had drawn the same conclusion from the same gravity map. Pemex policy kept him from announcing it. Penfield's bosses also rejected the crater idea and preferred an older explanation of volcanic activity. They withheld the specific data but allowed Penfield and Camargo to speak at the 1981 Society of Exploration Geophysicists conference. Attendance was thin, because many impact and boundary experts were at Snowbird. Carlos Byars, a Houston Chronicle journalist who had studied the data himself, put the claim on the front page. The story went nowhere.

  Penfield had maps but no rocks. A Pemex well drilled in 1951 had hit a thick layer of andesite about 1.3 km down, which crews had written off as a lava dome. William C. Phinney, curator of lunar rocks at the Johnson Space Center, urged Penfield to track those cores down. He was told they were lost or destroyed, and visits to the drill sites turned up nothing. Penfield gave up, published his results and went back to his Pemex duties. After reading the 1980 Science paper he wrote to Walter Alvarez about the Yucatán structure and never heard back. Meanwhile the Alvarez circle kept searching the open ocean.

  89776523

• 4. Proof and a Name

  ›

  The ocean search rested on a misreading of glassy spherules from the boundary layer, which seemed to say the impactor had landed in deep water. Unaware of Penfield, Alan R. Hildebrand, a graduate student at the University of Arizona, and his adviser William V. Boynton looked near the Brazos River in Texas. They found greenish-brown clay rich in iridium, shocked quartz grains and tiny weathered glass beads that looked like tektites. Thick, jumbled beds of coarse rock fragments also appeared, apparently scoured from one place and dumped in another. These beds seemed to cluster around the Caribbean Basin.

  Then the Haitian professor Florentine Morás found what he took for traces of an ancient volcano on Haiti. Hildebrand suggested it could instead be a sign of a nearby impact. Samples from the boundary there held more tektite glass, a material that forms only in asteroid impacts and high-yield nuclear blasts.

  In 1990, Carlos Byars told Hildebrand about Penfield's old claim. Hildebrand called Penfield, and together they located two drill samples that had sat in storage in New Orleans for decades. Hildebrand's team tested them and found clear signs of shock metamorphism. California researchers studying satellite images spotted a ring of cenotes, or sinkholes, centred on the town of Chicxulub Pueblo, matching the ring Penfield had seen. A paper identifying the crater appeared in 1991, with Hildebrand, Penfield, Boynton and Camargo among the authors.

  The crater took its name from the inland community, not the larger coastal town of Chicxulub Puerto. Later evidence suggests the crater is 300 km wide and that the 180 km ring is only its inner wall. Penfield recalled one more motive for the name: he wanted "to give the academics and NASA naysayers a challenging time pronouncing it".

  778536545

• 5. Hours After Impact

  ›

  The best dates for the strike come from argon-argon dating, which reads the age of volcanic and melted glass. A 2013 study in Science, using tektites from Haiti and ash-bearing clay layers in Montana, gave 66,043,000 years ago, give or take 11,000. A 2018 study of spherules from Gorgonilla Island, Colombia, reached 66,051,000, give or take 31,000. The season is known too. Annual growth patterns in sturgeon and paddlefish bones at the Tanis site in North Dakota point to spring in the Northern Hemisphere. The rock layer holding those bones formed within hours of the strike.

  The target was a shallow marine carbonate platform. Water ran about 100 m deep at the western edge of the future crater and over 1200 m at the northeastern edge, with roughly 650 m at the centre. Beneath lay about 3 km of Jurassic and Cretaceous sediment, mostly dolomite, limestone and anhydrite. A 2021 study estimated the asteroid arrived from the northeast at 20 km/s, inclined 45-60 degrees to the horizontal.

  Estimates of the energy released vary widely, from tens of millions to billions of megatons of TNT. Winds near the blast exceeded 1000 km/h. The impact opened a transient cavity 100 km wide and 30 km deep, which then collapsed. Megatsunamis more than 100 m tall swept outward, and one simulation suggests the first waves reached 1.5 km. They left ripples in the sea floor under what is now Louisiana, with wavelengths near 600 m and heights near 16 m, the largest ever documented. The shaking was equivalent to an earthquake of magnitude 9-11. Sediment was disturbed as far as 6000 km from the site.

  217243536

• 6. Years Without Sun

  ›

  As much as 25 trillion metric tons of rock were blasted skyward. Some escaped into the Solar System, while some fell back and vaporized during re-entry, heating the ground below. Wildfires are estimated to have engulfed nearly 70% of the planet's forests. In New Jersey, 2500 km away, a soil layer only 10 cm thick records a sudden mass death and burial of many kinds of animals. Fieldwork in the Hell Creek Formation of North Dakota, published in 2019, found the same simultaneous loss of countless species.

  The sea floor at the site held gypsum, a sulfur-rich mineral, and the blast injected its vapour into the air. Seismic images from 2008 showed the impactor struck deeper water than once assumed. More water vapour could have reacted with vaporized anhydrite, producing more sulfate aerosols, sharp cooling and acid rain. Dust may have veiled the whole planet for years, perhaps up to a decade. Photosynthesis stalled, and the food chain collapsed with it. Burning carbonate rock also released carbon dioxide, which brought a sudden greenhouse effect. A 2001 model by Lomax and colleagues suggests plant productivity might eventually have risen above pre-impact levels because of all that carbon dioxide.

  The toll was 75% of plant and animal species, including every non-avian dinosaur. In March 2010, forty-one experts from many countries reviewed twenty years of data and concluded the Chicxulub strike triggered the extinction. Gerta Keller of Princeton University dissents. She and others point to the Deccan Traps, a vast volcanic province in what is now the Indian subcontinent. Their studies place the worst eruptions before the impact, with the volcanism mainly shaping which survivors evolved afterward. A 2013 study compared isotopes in Chicxulub impact glass with isotopes in boundary ash. The two dated almost exactly the same.

  4329131446

• 7. Inside the Crater

  ›

  In October 1996, the BIRPS group shot three long seismic lines across the offshore crater, totalling 650 km. The longest, Chicx-A, ran parallel to the coast. A 2005 survey lifted the total of deep 2D seismic lines to 2,470 km and added ocean bottom seismometers, plus gravity readings along 7638 km of profiles. Funding came from the National Science Foundation and the Natural Environment Research Council, with help from Mexico's national university, UNAM, and the Centro de Investigación Científica de Yucatán.

  Drilling added direct evidence. UNAM cored eight boreholes in 1995, and three of them, UNAM-5, 6 and 7, reached ejecta beyond the crater rim. Between 2001 and 2002 the scientific hole Yaxcopoil-1 went down 1511 m and cut through 100 m of impact rock. In 2016 a British and American team drilled borehole M0077A, 1335 m below the sea floor, for the first offshore cores from the peak ring.

  The structure is a set of concentric rings. The outermost is a ring of faults up to 130 km from the centre. Inside it, the main rim, with a radius of 70 to 85 km, lines up with the onshore cenotes. A terrace zone of slumped fault blocks follows, then a peak ring about 80 km across. Together with the much older Sudbury and Vredefort structures, Chicxulub ranks among Earth's largest impact sites. It is the only one whose peak ring is intact and open to direct study. About 1 km of younger sediment covers it.

  The peak ring cores revealed pink granite dated to 326 million years ago, give or take 5 million. The rock is cracked and sheared by the violence of the crater's formation. The cores also showed a hydrothermal system that altered roughly 1.4 × 10 to the power 5 km3 of crust and lasted for hundreds of thousands of years. Researchers call it an oasis where life could recover. The impact also left behind the Yucatán sedimentary basin, which, in the words of the researchers, "ultimately produced favorable conditions for human settlement in a region where surface water is scarce".

  1950576178

• 8. Tracing the Asteroid

  ›

  In 1998 scientists described a meteorite fragment about 2.5 mm across, pulled from a North Pacific sea-floor core. It sat at the base of the boundary iridium spike, and some suggested it came from the impactor itself. It best matched certain carbonaceous chondrites, a primitive stony type. A 2021 paper looked at chromium-54 and platinum group metals in marine impact layers and favoured CM or CR carbonaceous chondrites. There is broad agreement that the body was a C-type asteroid, a class that formed beyond the orbit of Jupiter.

  In 2007, William F. Bottke, David Vokrouhlický and David Nesvorný published a specific origin in Nature. They linked the impactor to the Baptistina family, born when a 170 km parent body collided with a 60 km body 160 million years ago. The largest surviving member is 298 Baptistina. The idea collapsed. A 2009 spectrographic study showed 298 Baptistina is an S-type asteroid, not a carbonaceous one. In 2011, data from the Wide-field Infrared Survey Explorer moved the Baptistina collision to about 80 million years ago, leaving only 15 million years for orbits to evolve and reach Earth, far too little. In 2010 another candidate emerged, asteroid 354P/LINEAR of the Flora family. A 2021 simulation instead pointed to the outer part of the main asteroid belt.

  Some researchers argue for a comet. Two papers in 1984 proposed a comet from the Oort cloud, and a 1992 proposal held that tidal breakup of comets could raise impact rates. In 2021 Avi Loeb and a colleague suggested in Scientific Reports that the culprit was a fragment of a disrupted comet. A rebuttal in Astronomy & Geophysics replied that the worldwide iridium total was too large for a comet of that size. It also said comet impact rates had been overestimated. Ruthenium isotope ratios in the impact layers back the asteroid case as well.

  6770717222