Fiery debate over effects

At some point early in the evolution of life on Earth, a meteor measuring at least 1 kilometer across hurtled through the atmosphere and collided with what is currently Western Australia. The impact likely triggered a catastrophe — a fireball estimated to exceed 8 miles in width, with an energy 2,000 times more potent than the largest nuclear explosion.

However, the age and extent of that impact continue to be a contentious subject. Earlier this year, a group of Australian scientists announced that the meteorite struck 3.5 billion years ago — designating it the oldest impact site on Earth — and created a crater as large as 62 miles across.

Now, a recent investigation by Harvard geologists indicates that the event was smaller and more recent. In a paper released on July 9 in Science Advances, the team claims that the meteorite impacted no more than 2.7 billion years ago and created a crater roughly 10 miles wide.

The results will bridge a gap in our comprehension of the planet’s history. Additionally, the Harvard researchers assert that the site, which exhibits some geological features akin to Mars, could provide insights into questions regarding asteroid impacts on the red planet and the possible ramifications on any life that might have existed there.

The Harvard narrative began with a stroke of fortune. In 2023, Alec Brenner, a Ph.D. student in the Department of Earth and Planetary Sciences (EPS) at the Kenneth C. Griffin Graduate School of Arts and Sciences, traveled to Western Australia to carry out paleomagnetic research at the Pilbara Craton.

This formation is of great interest to geologists because it features rocks that are up to 3.6 billion years old from the Archean Eon, a period when Earth was predominantly covered by water, bombarded by asteroids, and witnessing the emergence of early life.

On the initial day of the field season, Brenner and undergraduate scholars Jasmine Palma-Gómez from Harvard and Joanna Li from Smith College (both co-authors of the new paper) drove into the wilderness at North Pole Dome, a geological feature spanning approximately 19 by 25 miles at the heart of the craton. He halted to show the students some rock outcrops. Initially, the rocks appeared to be unremarkable samples of the abundant basalts in the region.

“We were preparing to depart when I spotted peculiar fractures on some of the rock surfaces,” remembered Brenner, who is now a postdoc at Yale. The fissures formed cone-shaped structures, vaguely resembling the tail of a horse — the classic “shatter cones” seen in rocks affected by extreme shocks like meteorite impacts.

A few days later, the team returned and discovered more shatter cones. “They were all oriented in the same direction — and that’s a very intriguing telltale sign,” noted Brenner. “That strongly suggested that these formations might indeed be genuine.”

He sent images of the shatter cones to his adviser, Professor Roger Fu, who typically approaches such news with a cautious skepticism. However, he also found the evidence persuasive.

“I was quite thrilled about it from the very first sight,” stated Fu. “That doesn’t occur every time.”

Over the next few days, Brenner and his colleagues traversed the desert road, visiting additional rock outcrops scattered across several miles and uncovering even more shatter cones.

“They were all directing back toward roughly the same point,” he noted.

Shatter cones — which can vary in size from fractions of an inch to several yards long — often radiate in a spherical configuration from the impact point, with each cone directing towards the center.

“It’s reminiscent of those quirky pirate films where one of the pirates possesses a compass that consistently points toward the treasure,” Brenner remarked.

Ultimately, the team followed the indicators to what Brenner refers to as “ground zero” — the suspected impact location. There, they discovered more cones—pointing straight upwards. These clues implied the meteorite struck the overlaying rocks, which are now more than a mile thicker but have since eroded away.

The team dedicated two years to surveying the rust-tinted hills amidst kangaroos, dingos, and majestic eagles. Their summer breaks coincided with the Australian winter, rendering the desert’s heat bearable.

Eventually, they
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They uncovered thousands of shatter cones dispersed over approximately 4 miles. Their estimation suggested that the original crater would have measured around 10 miles in width, but the formation has vanished due to erosion. They named the impact location “Miralga,” drawing from the indigenous term for a nearby stream and a local family.

A lingering enigma needed addressing: When did the collision transpire? Most of the shatter cones were located within the oldest bedrock in the vicinity, basalts dating back 3.47 billion years, yet the researchers also identified some within younger basalts that emerged 2.77 billion years ago.

Moreover, they discovered a greater number of horsetail-shaped fissures across geological faults dated to 2.71 billion years ago, indicating that the impact must have occurred at some point afterward.

After two years of investigation, the Harvard team presented their findings to a journal earlier this year.

The researchers from Harvard propose that the Australian site serves as a model for examining early Mars.

Both locations exhibit basaltic crust altered by water and oxygen, subjected to meteorite impacts.

The Australian site possesses some of the oldest signs of life on Earth, and researchers are exploring whether Mars might have also supported some form of life.

Shortly afterward, they learned that another set of researchers from Australia had identified the very same impact site in 2021 — and published first.

“That caught me off guard,” Brenner recalled. “I was on my honeymoon. I received favorable revisions for our manuscript, and literally within 24 hours, the other group’s paper was published.”

However, the two teams reached distinctly different conclusions. The Australian group approximated the impact occurred 3.47 billion years ago, creating a crater that could be as much as 62 miles wide. The Harvard geologists contend that the other group miscalculated the impact’s size and overlooked evidence indicating a younger age.

The Harvard team places the impact’s age somewhere between 2.71 billion and 400 million years. Yet, Brenner mentioned the team is undertaking further investigations and will refine the age to a more precise timeframe.

If preliminary findings are confirmed, Miralga may not retain its status as the oldest impact structure on Earth. (Besides Miralga, the oldest verified impact crater is the 2.2-billion-year-old Yarrabubba structure, also located in Western Australia.)

The Harvard researchers assert that this site provides a model for probing early Mars. Both locations possess basaltic crust affected by water and oxygen and exposed to meteorite strikes. The Australian site harbors some of the oldest evidence of life on Earth, and scientists are examining whether the red planet might have hosted any form of life.

“It’s akin to being on a small piece of Mars right here on Earth,” stated Brenner.

Relatively few impact sites from Earth’s early history remain intact, as most ancient rocks have been erased by geological processes. Some researchers have posited that all remaining large impact craters had been identified, but the new impact site suggests that more may exist, even in areas that have already been thoroughly studied by geologists.

“If this structure could evade detection for so many years, that signals there’s much more for us to discover,” Brenner emphasized. “All it requires is inquisitive minds and a bit of luck.”

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