All over the solar system, our telescopes, spacecraft, and rovers show us impact craters of all sizes. These craters hold a wealth of information about any given planet or other rocky object’s composition, age, and evolution. Particularly, the central ring of steep peaks typical of an impact crater piques scientists’ interest because it holds secrets to crater formation. But scientists must live with the fact that they don’t get to investigate these key structures with their own hands.
Luckily, though, Earth sports its own crater, albeit buried below 10–30 kilometers of ocean and sediments. This year, scientists from the International Ocean Discovery Program (IODP) finally got a look at the only preserved peak ring on Earth, which lies in the center of this crater offshore of Mexico’s Yucatan Peninsula. The 180-kilometer-wide crater, called Chicxulub, is a remnant of the infamous impact of an asteroid or comet some 65 million years ago that likely killed most of the dinosaurs, paving the way for mammals to rise.
By drilling into the crater and studying core samples, researchers have now finally pinned down how impact craters form and validated a theory that peak rings are made from deep, midcrustal material churned up by the impact. “Because that theory is validated, we can say some fundamental things about the process of impact cratering on Earth and other planets,” said Sean Gulick, a geophysicist at the University of Texas at Austin and coauthor of a new Science paper published today.
Impact Crater Formation
Two theories dominate scientists’ thinking about impact crater formation, with one relying on the notion that when rock is hit at high speed by a large enough object, it behaves like a liquid, said Gulick. This “dynamic collapse” model suggests that in the minutes following the impact, the crater’s sides would collapse inward and the center would rebound, bringing deep material up with it, Gulick said. In this scenario, the peak ring should be composed of originally dense material from the midcrust. Another theory suggests that the target rock near the surface would dominantly melt, impeding rebound of deep material; thus, the peak ring would be made of shallower material that collapsed inward up against the melt, he added.
In the late 1990s and early 2000s, scientists investigated the Chicxulub crater from afar, using sound. They used instruments in the ocean and on land that send sound waves through the crust, traveling at different speeds depending on the composition of the rock. Their findings suggested that the material in the peak rings was much less dense than what would be expected from rocks originating in the midcrust, Gulick said.
“The implication of this finding was that either the rocks in the peak ring were from much nearer the surface than the dynamic collapse crater models infer, suggesting that the models were fundamentally wrong,” or that the deep crustal rocks were so deformed as to be unrecognizable, recalled Penny Barton, a geophysicist at the University of Cambridge in the United Kingdom, in a commentary that published today along with Science paper.
The only way to know for sure was to drill, baby, drill.
Journey to the Center of a Crater
In April and May of this year, the IODP team used a drilling ship off the coast of Mexico to obtain material from more than a thousand meters below the seafloor, where the peak rings reside under layers of limestone and impact-related debris.
When the samples came up, the researchers immediately recognized them as basement granite, which comes from the midcrust, as deep as the dynamic collapse models predict, Gulick said.
In fact, Gulick noted, they were so sure that the rock was deeply sourced granite that the team began writing the new paper that summer, even before the cores had been thoroughly investigated.
Further investigations revealed that although the samples were recognizably granite, the meteorite’s impact deformed the rock enough to alter fundamental properties like its density and enhance its porosity, which explained the unusually slow speed of sound traveling through it.
Solar System Implications
These revelations have implications not only for our own planet but also for our neighbors in space. Studies of the Moon, for example, showed that its crust is much more porous than was originally predicted. The new research now allows researchers to suggest that “cratering over 4.5 billion years has actually enhanced porosity of the lunar crust,” Gulick said.
With the confirmation that peak rings are formed from midcrustal material, the structures become “a window to the crustal compositions of other planets,” Gulick added, where even our most advanced rovers can’t yet penetrate.
“Now that we have verified our impact simulations of Chicxulub, we can be more confident about simulating large craters on other planetary bodies,” said Joanna Morgan, a geophysicist at the Imperial College London and lead author on the paper.
Recovery of Life
The granite’s high porosity could have large implications for life on Earth, as well, Gulick said. How life could recover after such a catastrophic event isn’t well understood, but Chicxulub’s peak rings could illuminate some details. Although mammals on land filled the ecological niche left behind by the demise of most dinosaurs, in the churned-up crust deep below the ocean, simpler life-forms began to flourish.
Gulick suspects that in the mere minutes following the impact—about the length of time it takes to hard-boil an egg—the hydrothermal fluids from the resulting melt flushing through the newly porous granite of the peak ring could have created a habitat ripe for microbial colonization. This research, however, is just in the beginning stages.
“That’s one of the next hot topics we want to investigate as an expedition team,” Gulick said. “What sort of ecosystem evolved in the crater? How did life recover in the oceans?”
—JoAnna Wendel (@JoAnnaScience), Staff Writer