Roughly 466 million years ago, something huge exploded in space and sent shrapnel down on Earth.
It was the biggest such cataclysm to happen in our celestial neighborhood in some 3 billion years. An asteroid belt body, roughly as large as Connecticut and made of some of the most ancient material in the solar system, collided with another object and splintered into pieces. Those pieces in turn slammed into one another, creating more debris. One by one, the fragments fell toward ancient Earth, where the continents were clumped into a single, gigantic mass called Gondwana and the very first terrestrial plants were just beginning to creep onto land. At the time, those meteorites, called L chondrites, made up 99 percent of all space rocks that landed on our planet.
Millennia passed, the continents broke apart and bunched back together, mountains ranges rose up and eroded away, countless creatures evolved and went extinct. But the debris from that 466 million-year-old breakup continued to fall. And fall. And fall. Even now, they make up the largest group of meteorites that land on Earth.
“That collision cascade” — the series of smaller smashes and crashes that followed the initial breakup — “had consequences that are still felt today,” said Philipp Heck, a cosmochemist at the University of Chicago and curator of for the Field Museum.
The L chondrite meteorites that we find all over Earth aren’t representative of the asteroid belt from which they came. For the past several years, Heck has been working to understand the implications of the “L chondrite parent body breakup” — and it’s become clear to him that the event masks the true diversity of space rocks that bombard our world. Looking at Earth today and assuming that “L chondrites” are common is like looking out the window after a blizzard and assuming snow is the most common weather.
“What has arrived on Earth is definitely not representative of what’s out there,” Heck said. “If we want to understand nature better, especially the asteroid belt, we have to look at other time windows.”
In a new study published Monday in the journal Nature Astronomy, Heck and his colleagues take their first look out a new time window, at the period just before the L chondrite parent body breakup. They report that the meteorite “weather” during that time was dramatically different from what we now see.
Back then, in the early part of the geologic period known as the Ordovician, now-rare meteorites like achondrites (stony meteorites that come from planets and the largest asteroids) were common. Among these were space rocks thought to come from Vesta, a bright protoplanet that is the second-largest object in the asteroid belt. There were also far more “ungrouped” meteorites.
By comparison, L-chondrites (many of which come from the 466 million-year-old breakup) represented a just a small proportion of Earth’s meteorite flux.
“Our main finding was that these primitive achondrites and the ungrouped meteorites … were almost 100 times more abundant than they are today,” Heck said. “That was a big surprise that no one expected.”
“People always ask me, ‘Why is it important to know about the past?'” Heck said. “I answer, ‘Because it’s interesting.’ But also because we need to learn about how nature works if we want to know what’s going to happen in the future.”
