– Impact reveals how crater ring formed
Scientists drilling deep beneath the ocean pull up clues as to how the dust settled just after impact. BELINDA SMITH and APRIL REESE report.
Scientists have long wondered why craters created by asteroids often have a ring of hills around the centre. These circular ridges, called peak rings, are common in craters throughout the inner solar system. They have been found on the Moon, Mars and Mercury.
There is only one good place to study peak rings on Earth: the Chicxulub crater in what is now the Yucatan Peninsula in Mexico. It formed 66 million years ago when an asteroid slammed into Earth, triggering a series of events that wiped out three-quarters of all species on the planet, including the non-avian dinosaurs. New evidence from the site holds clues to the origins of the crater’s mysterious ring of mountains – and others like them around the solar system.
Though the 180-kilometre-wide crater contains Earth’s best-preserved peak ring, studying it is complicated: the ring, along with much of the rest of the crater, is deep below the sea in the Gulf of Mexico.
Geologists discovered the Chicxulub crater’s peak ring in 2000 using seismic sensing, which measures how seismic waves travel through the Earth’s layers, revealing their composition. Ever since, researchers have wanted to get at the ring and figure out its story.
One explanation for the ring’s formation, known as the “dynamic collapse theory”, holds that the impact caused an eruption of rock from the lower regions of the planet’s crust that then fell back to earth. A competing theory suggests the ridges formed from melting in the upper crust.
In mid-2016, scientists got the chance to settle the debate. An international team led by geophysicists Joanna Morgan of Imperial College London and Sean Gulick of the University of Texas at Austin used a deep-sea drilling rig to burrow into the site and pull up cylindrical sections of rock core. At first, they found layer upon layer of limestone – as would be expected in ocean sediments. But then, about 620 metres down, bits of pinkish-white rock appeared in the cores: granite. It was far closer to the surface than the rock is usually found, meaning it must have got there through some sort of disruption. The granite was also unusually weak, indicating it had undergone significant stress, and the rocks retained the coarse crystalline structure of deeper material.
All of which suggests, the researchers reported in Science in November 2016, that the dynamic collapse theory is correct: the powerful impact blasted granite from
THE “DYNAMIC COLLAPSE THEORY” HOLDS THAT THE IMPACT CAUSED AN ERUPTION OF ROCK FROM THE LOWER REGIONS OF THE PLANET’S CRUST.
deep in Earth’s crust to the surface and made it behave like a liquid, shooting up into the air like a geyser before collapsing into the basin and settling out into a circular ridge around the centre.
The study adds to a small but growing body of evidence in support of the dynamic collapse theory of peak ring formation in craters. Last October, scientists reported that the peak rings within Mare Orientale, a crater on the moon, formed in a similar way.
Geophysicist Penny Barton from the University of Cambridge, who was not involved in the study, writes in a
Science commentary on the discovery that it appears to “validate the collapse models” while also posing “many new questions for further work on these exciting samples”.
THE PEAK RINGS WITHIN MARE ORIENTALE, A CRATER ON THE MOON, FORMED IN A SIMILAR WAY.
One of those questions is how the rocks were weakened enough to behave like a fluid. The team is now attempting to find out.
How the Chicxulub peak ring formed isn’t the only question researchers involved in the US$10 million core drilling project hope to answer. They also aim to glean other insights into Earth’s most famous cataclysm and its effect on the planet.
Four labs are testing core samples for iridium, a metal that asteroids sometimes leave behind. While the space rock would have vaporised on impact, bits of it likely settled into the crater.
So far, the search for iridium in the cores has come up empty, but in December researchers told a meeting of the American Geophysical Union they had the next best thing: nickel, which behaves similarly to iridium and serves as a kind of proxy for the presence of asteroid dust.
Researchers hope the hundreds of metres of core samples from the site will also reveal more about how the crater formed, how the material it churned up was dispersed and how long it took life to rebound at the site.
Impact craters often display ‘peak rings’ – as this artist’s impression of Mexico’s Chicxulub shows. A new study explains.
Chicxulub rock cores suggest the impact turned the rock into a geyser that created the circular ridges.