Millennia passed, the continents broke apart and bunched back together, mountain ranges rose up and eroded away, countless creatures — trilobites, dinosaurs, woolly mammoths — 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 meteorites 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” (astronomers clearly weren't feeling very creative when they named that one) — 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 big blizzard and assuming that snow is the most common type of 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 — the designation given to rocks too weird to fit into any of the established categories.
By comparison, L-chondrites (many of which, but not all, come from the 466-million-year-old breakup) represented just a small proportion of Earth's meteorite flux (the quantity and type of meteorites that rained down).
“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.”
Uncovering this fact was no easy task. Meteorites are hard to spot under any circumstances. Finding ones that have survived half a billion years without being eroded into dust or subsumed into the Earth by weather and plate tectonics is even harder.
Instead of looking for whole meteorites, Heck and his colleagues sought chrome spinels, hardy black minerals found in space rocks in rock formations from China, Sweden and Russia. Though the sediments are now on land, they once formed the bottom of ancient seas, where the minerals were most likely to survive. The team dug up nearly 600 pounds of rock in search of mineral grains that can barely be seen without a microscope.
“It's a needle in the haystack problem,” Heck acknowledged. “So we have to take a brute force approach: We burn away the haystack to find the needles.”
The scientists didn't actually light the rock on fire. Instead, they used acid to dissolve the sedimentary rock. This left them with 41 extraterrestrial chrome spinels, most of them the diameter of a human hair. By analyzing the chemical composition of the minerals — particularly the varying ratios of oxygen isotopes — they were able to develop a chemical “fingerprint” for each one, giving at least a rough understanding of what kind of meteorite it came from.
Those fingerprints can also be compared to spectroscopic analysis of bodies out in the solar system, offering important clues about bodies scientists will never get to see up close. “We can do essentially space exploration by finding rock fragments on Earth,” Heck said.
Heck said his latest findings — along with future looks through other “time windows” on the meteorite record — will help astronomers understand the collision history of the asteroid belt (which circles the sun between the orbits of Mars and Jupiter) and its influence on our planet. He pointed out that scientists are just beginning to understand the behavior of near-Earth objects — asteroids and other bodies in space that could one day strike Earth. Understanding which meteorites fell in the past could answer questions like, “How long does it take [after a collision] until the fragments arrive on Earth, and when are they all used up?” he said. “How important is the collision cascade in generating fragments?”
“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.”