The rain of meteorites from space to our planet over the last 500 million years may not have fallen quite as we thought.
After analyzing 8,484 kg (18,704 pounds) of sedimentary rock from ancient seabeds, scientists have found that major collisions in the asteroid belt have not contributed significantly to the number of meteorite impacts on Earth as it had been theorized.
It is a discovery that scientists say could help protect Earth from asteroid effects in the future.
“The research community previously thought that the meteorite flow to Earth was associated with dramatic events in the asteroid belt,” said geologist Birger Schmitz from Lund University in Sweden. “However, the new study shows that the current has been very stable instead.”
Tracing Earth’s meteorite history is not exactly easy. Effect events involving large bodies leaving a significant crater are rare; many space rocks cut apart at atmospheric entrance, leaving only dirt to fall to the ground.
This waste is what Schmitz and his colleagues have been chasing: small fragments of micrometeorite preserved in the sedimentary layers of the earth’s crust.
From ancient seabeds in China, Russia, and Sweden, they extracted thousands of kilograms of limestone, representing 15 different time periods in the Phanerozoic eon.
These limestone pieces were then dissolved in acid, a technique that allows the extraction of chromium spinels – small pieces of chromium oxide, a degradation-resistant mineral found in meteorites.
“In total, we have extracted chromium oxide from nearly 10,000 different meteorites,” Schmitz said. “Chemical analyzes allowed us to determine what types of meteorites the grains represent.”
Fascinatingly, their results show a stable flow, mainly consisting of condondic (rocky non-metallic) meteorites similar to today’s flow. The sharp exception is an increase in this type of meteorite 466 million years ago, associated with the dissolution of an L-chondrite parent body, a type of meteorite that is conspicuously low in iron.
During this time, the meteorite flow increased by a factor of up to 300, and 99 percent of the grains were from this one parent body, after approx. 40 million years, but never completely ceased. Even today, about a third of all meteorites falling to Earth come from this parent body.
This suggests that the asteroids leaving the asteroid belt between Mars and Jupiter appear to come from a very small region.
“We were very surprised to hear that only one of the 70 largest asteroid collisions that took place over the last 500 million years resulted in an increased flow of meteorites to Earth,” Schmitz said. “For some reason, most rocks remain in the asteroid belt.”
We are not yet sure what this reason is, but it may help us understand what kind of objects are likely to collide with Earth and where they come from. That is, of course, if the team’s findings are validated; as they mention in their paper, the sample may not be comprehensive.
There is a period of 190 million years from carbonaceous to early Jurassic without chromium-spinel data, and we know that there was an asteroid eruption that affected the Earth during this time. An asteroid family that emerged during the Cretaceous – the team’s most closely sampled period – also shows no significant increase in current for this type of meteorite.
Future research may help uncover the causes of these discrepancies. For now, the research represents a new way of understanding the Earth’s meteorite impact history and what we could expect going forward.
“Future impacts from even a small asteroid, for example, in the ocean near a populated area could lead to catastrophic results,” Schmitz said. “This study provides an important understanding that we can use to prevent this from happening; for example, by trying to influence the orbit of rapidly approaching celestial bodies.”
The research is published in PNAS.