Yucatan crater holds clues to complex role of impacts

SIGNS OF LIFE: First in a three-day series

This month an international team of researchers, including UA planetary scientist David A. Kring, will begin drilling a hole nearly two miles deep into the northwest tip of Mexico’s Yucatan Peninsula.

The goal of the multimillion-dollar drilling project is to bring up a continuous rock core from a long-buried crater formed 65 million years ago when a 6-mile-wide comet or asteroid slammed the Earth.

The collision created a hole 112 miles wide, about the distance from downtown Tucson to downtown Phoenix. It wiped out the dinosaurs and more than 75 percent of the species on the planet, throwing up a cloud of dust that blocked the sun for months, killing off plants at the base of the food chain and causing ecosystems to collapse worldwide.

It was a global environmental catastrophe.

But a 6-mile-wide rock is puny compared with the titanic hunks of cosmic debris that peppered the Earth and moon shortly after the solar system formed — billions of years before the dinosaurs.

Many scientists believe a Mars-sized object hit Earth 4.5 billion years ago, throwing off a disk of debris that later coalesced into the moon.

Other huge asteroids and comets hit the young Earth regularly, possibly boiling off the early oceans and eliminating nascent life forms.

Kring, who studies meteorites and the environmental effects of asteroid and comet impacts, collaborates with researchers on Arizona State University’s NASA-funded astrobiology team.

The astrobiologists are trying to uncover the origins of life on Earth, and Kring wants to know how asteroid and comet impacts shaped the emerging biosphere.

“It’s certainly possible for large-impact events to completely sterilize the planet. The most vivid case of that is the impact event that we believe created the moon,” he said.

“If there had been any organisms living at that time — and that would have been very early in Earth’s history — there would not have been anything surviving that event.”

Recent studies of ancient rocks have revealed that life got started much earlier than scientists had suspected, pushing biological beginnings back to a time when the Earth was still under heavy bombardment.

The planets formed 4.56 billion years ago from a disk of gas and dust spinning around the newborn sun. Particles of dust collided and stuck together to form comet-sized bodies called planetesimals, which in turn merged to form the planets.

The Earth’s surface solidified about 4.3 billion years ago, and the oceans formed once the surface cooled below the boiling point of water. As the young planet took shape, it continued to be pummeled by debris left over from solar-system formation.

It was during those hot, perilous early times that organic molecules assembled themselves into the first precellular life forms.

Chemical evidence for single-celled microorganisms has been found in 3.86-billion-year-old rocks from Greenland, and fossilized microbes have been recovered from 3.5-billion-year-old Australian formations.

Some time earlier, the very first life forms, which may have been self-replicating entities lacking cell membranes, got started.

“Somewhere between 4.3 and 4 billion years ago — maybe even a little bit earlier — is the period when the Earth’s crust had solidified, the oceans formed, and these chemical experiments could go on in certain environments unmolested,” said University of Arizona planetary scientist Jonathan I. Lunine, author of “Earth: Evolution of a Habitable World.”

“Earlier than 4.3 there just was not an environment on the surface of the Earth amenable to this,” he said, because all known forms of life require liquid water.

The pelting from asteroids and comets tapered off about 4 billion years ago, and the oldest known rocks on Earth date to that period.

Because life began while devastating impacts were reshaping the Earth’s surface, some scientists suspect that biology got started several times, only to be erased again and again.

The repeated snuffing of early life by asteroids and comets is an idea scientists call “impact frustration.”

It’s one of the possibilities being explored by Kring, who models the environmental effects of comet and asteroid impacts.

Real-world data from the Yucatan drilling project will be used to improve computer models that simulate impact effects.

The Yucatan crater is called Chicxulub, named for the Mayan village that sits atop its center. Roughly translated from Mayan, Chicxulub means “tail of the devil” or “tail of the dragon.”

Chicxulub is one of 150 or so known impact craters on our planet, the best preserved of which is Arizona’s Meteor Crater.

Most of the record of Earth’s early violent history has been expunged by ongoing geologic processes that recycle the planet’s crust. But evidence of the battering is etched on the pockmarked face of our moon.

During those violent early days, sea-floor volcanoes may have served as a refuge, sheltering early organisms from the rocky rain.

The idea of a deep-sea haven is favored by origin-of-life researchers like Arizona State geologist Jack Farmer. It is appealing in part because it combines the most attractive elements of the “primordial soup” and “hot origins” models of life’s emergence.

According to this hybrid origins theory, life may have begun in some tepid surface pool, then spread throughout the ocean.

Later, a large asteroid or comet killed everything on the surface, and the only survivors were the hardy inhabitants huddled around the volcanoes at deep-sea hydrothermal vents.

The hybrid model would explain the heat-loving, or thermophilic, microorganisms found at the base of the evolutionary trees created by University of Illinois biologist Carl Woese and his successors.

“All the branches in this lower part of the tree are thermophiles. They are things that live in hot springs or on the deep sea floor,” Farmer said.

“So you look at this tree and say, ‘Wow, well maybe life originated at high temperatures.’ Well, it’s more likely that this is an artifact of a large late impact.”

Impacts extinguish life, but at the same time they create opportunities for other organisms. The asteroid or comet that finished off the dinosaurs allowed mammals, until then an inconspicuous group of mostly rat-sized creatures, to diversify and flourish.

To further complicate the role of impacts, comets delivered water that contributed to Earth’s oceans, and a type of asteroid called a carbonaceous chondrite brought amino acids and other biological building blocks from space.

It’s even possible that life itself was delivered whole from Mars, a prospect being investigated by H. Jay Melosh of the University of Arizona, who also collaborates with the Arizona State astrobiologists.

Scientists have collected more than a dozen pieces of the Martian surface that were blasted into space by impacts, then fell to Earth.

They include the famous Allan Hills 84001 meteorite, which several scientists believe contains evidence of ancient bacteria-like life.

“You have competing effects from impacts,” said Arizona State meteorite expert Laurie A. Leshin.

“You’re supplying parts of your recipe (for life) in some of the smaller stuff, but then occasionally you have a big thing that whacks into you and can manage to sterilize, or frustrate, the origins of life,” she said.

When hot rock comes in contact with water, “hydrothermal systems” such as the Yellowstone hot springs or the deep-sea volcanic vents are formed. These hot-water features would have been much more widespread on the hot, young Earth.

At Yellowstone and on the sea floor, the heat source for hydrothermal systems is molten rock from the Earth’s interior. Ground water is heated by hot rock and forced to the surface as hot springs and geysers, or as hydrothermal vents on the sea floor.

The Chicxulub impact created a lake of molten rock more than a mile thick that remained hot for more than 100,000 years, Kring said. Heat from that “melt sheet” drove the circulation of ground water and probably created hot springs and geysers, he said.

Billions of years ago, impact-generated hydrothermal systems could have created environmental niches on Earth — and perhaps even on Mars — that allowed life to emerge very early, Kring contends.

“It is not clear that you need oceans to have life evolve,” he said.

“Life seems to be rooted in these high-temperature hydrothermal environments, and one could imagine it being, for example, in one of these deep-sea vents that seem to be so exciting, with these black smokers going on.”

Black smokers are undersea geysers that erupt at sea-bottom volcanic zones like the Guaymas Basin in the Sea of Cortez.

“But just as likely, it could be beneath the floor of an impact crater, where the thermal regime created by that impact event is also driving fluids that are already in the crust of the Earth,” he said.

Photos by James S. Wood, The Arizona Daily Star: UA planetary scientist David A. Kring and a rock core from Southern Arizona.

Chart by The Arizona Daily Star: Tree of life

Graphic by David Schlosser; research by: Jim Erickson: Battering from above shaped life on Earth