Scientists drilling into the floor of the Chicxulub impact crater in the Gulf of Mexico are working to increase our understanding of cratering processes on the Moon and other worlds, and to find geological signatures that might aid the search for ancient habitable zones on Mars.
The Chicxulub crater was formed 66 million years ago when a 10-kilometre-wide asteroid or comet smashed into the sea near Mexico’s Yucatan Peninsula.
The impact launched a global catastrophe widely believed to have killed off the dinosaurs. But it is also an excellent earthly opportunity to observe geological processes similar to those that occurred in large impact craters on Mars or the Moon, says Gail Christeson, a geophysicist from the University of Texas-Austin.
Christeson was part of an international ocean-science project called IODP-ICDP Expedition 364, which in the spring of 2016 drilled through seabed sediments to extract an 832-metre core from the crater’s peak ring.
Peak rings are circular mountain ranges formed within the rims of large impact craters. They are created, Chriteson says, when crustal rocks rebound after being pressed deep into the earth by the impact.
In smaller craters the result is a central peak. But in large ones, she says, the peak rises so high that its center collapses inward under its own weight, leaving a ring around what would have been its base.
Such craters are easily visible on the Moon, Mercury, and Mars, but are not as obvious on Earth, where geological processes have hidden them from view.
In the case of Chicxulub, the peak ring is about 80-90 kilometers across (compared to 180 kilometres for the crater itself). One of the purposes of drilling into it, says Christeson, was to study the rock and determine its seismic properties in order to help researchers confirm their seismic maps of the crater as a whole.
But in the process, she says, they also confirmed the theory that the rocks now comprising the peak ring had risen to their present locations from depths of about 10 kilometres via the force of the rebound.
Another finding was that the rocks of the peak ring were very porous. Even though many of them were granite, Christeson says, they, had porosities of eight to 10%.
“That’s really, really high,” she says. “Granite is normally less than 1%.”
Other rocks – a type of conglomerate known as a breccia – had porosities of 30 to 40%.
These impact-shattered items, says Aurial Rae, a geologist from Imperial College London, provided pathways for water to circulate through deep, impact-heated rocks, creating hydrothermal systems much like those found today along mid-ocean ridges.
Proof of this, he says, comes from the existence of hydrothermally altered material throughout the length of the drill core. And while the core represents only a single location in the peak ring, there is no reason to believe it isn’t typical.
“I would expect that there was an active hydrothermal system going throughout the entire peak ring,” Rae says.
In fact, he notes, it probably extended well below the bottom of the core, to a depth of about 3000 metres.
This circulation appears to have gone on for a long time, adds Sonia Tikoo, a planetary scientist from Rutgers University in New Jersey. Tikoo’s specialty is paleomagnetism, the study of the magnetic properties of ancient rocks.
When rocks are formed, she says, they record the direction and strength of the Earth’s magnetic field. But the circulation of chemical-laden hot water in hypothermal systems can alter this, resetting magnetic orientations to that of when the change occurred.
This is useful for geologists, because every few hundred thousand years the Earth’s magnetic field reverses direction. At the time of the Chicxulub impact it was pointing in the opposite direction from today. But 300,000 years later it flipped.
Hydrothermally altered rocks within the core show signs of both orientations, Tikoo says, indicating that the hydrothermal system must have lasted at least long enough to experience the transition.
In fact, she adds, modeling work done by David Kring, a geologist at the Lunar and Planetary Institute in Houston, Texas, has shown that it’s possible that hydrothermal circulation in the peak ring might have continued for one-to-two million years after the impact.
“But there wasn’t any experimental data,” she says. “This provided the first experimental evidence that it lasted at least 300,000 years.”
Understanding these processes, the scientists say, can be helpful in determining if life existed on other worlds, especially Mars. “What we’re looking for is what we can learn that relates to hypothermal systems on other planets where we might expect to find life, says Rae.
Chris McKay a planetary scientist and astrobiologist at NASA Ames Research Center, Moffett Field, California, agrees. Much of Mars is heavily cratered, he says, adding that impact-induced hydrothermal systems in these craters could have been an important habitat for life.
In fact, he notes, “Gale Crater—the site of Curiosity’s roving—is an impact crater, and the sort of hydrothermal circulation they are seeing in Chicxulub would also be expected there. Could be Curiosity will find some evidence for this as it traverses further.”