in

Discovery Buried in 500 Million Years of Meteorite Impacts on Earth

Artist's impression of the asteroid belt. (Pablo Carlos Budassi/Wikimedia Commons/CC BY-SA 4.0)
Artist's impression of the asteroid belt. (Pablo Carlos Budassi/Wikimedia Commons/CC BY-SA 4.0)

It appears that meteorites from space did not fall in the same pattern as we thought over the past 500 million years.

The scientists examined 8,484 kilograms (18,704 pounds) of sedimentary rock from ancient seabeds. They found that major collisions in the asteroid belt did not significantly affect the amounts of meteorites that reached Earth.

Scientists say the discovery could protect Earth from asteroids in the future.

Geologist Birger Schmitz of Lund University in Sweden said previously, “researchers believed meteorite flux to Earth was related to dramatic events in the asteroid belt.” In the new study, the flux, on the other hand, is shown to be very stable.”

It isn’t easy to trace meteorite history on Earth. The amount of space rock falling on Earth is mostly a result of impact events involving smaller bodies, which don’t leave an Earthly crater.

Schmitz and his coworkers have been searching for tiny pieces of debris preserved in sedimentary layers in Earth’s crust: micrometeorites.

Thousands of kilograms of limestone

Chinese, Russian, and Swedish archaeologists harvested thousands of kilograms of limestone representing 15 different Phanerozoic Eon periods from ancient seabeds.

Following the dissolving of limestone chunks in acid, tiny pieces of chromium oxide were extracted from the meteorites to make chrome spinels.

“In total, we have extracted chromium oxide from almost 10,000 different meteorites,” Schmitz said. “Chemical analyses then enabled us to determine which types of meteorites the grains represent.”

Fascinatingly, their results show a stable flux, mostly consisting of chondritic (stony non-metallic) meteorites, similar to the present-day flux. The glaring exception is an increase in this type of meteorite 466 million years ago, associated with the break-up of an L-chondrite parent body, a type of meteorite conspicuously low in iron.

During this time, meteorite flux increased by a factor of up to 300, and 99 percent of the grains were from this one parent body, tailing off after about 40 million years but never quite ceasing. Even today, around one-third of all meteorites falling to Earth is from this parent body.

This suggests that the asteroids that leave the asteroid belt between Mars and Jupiter seem to come from a tiny region.

“We were amazed to learn that only one of the 70 largest asteroid collisions that took place over the past 500 million years resulted in an increased flux of meteorites to Earth,” Schmitz said. “For some reason, most of the rocks stay in the asteroid belt.”

We’re not sure what this reason is yet, but it could help us understand what kinds of objects are likely to collide with Earth and where they come from. That’s if the team’s findings are validated, of course; as they mention in their paper, the sampling might not be comprehensive.

Early Jurassic

There’s a 190-million-year stretch of time from the Carboniferous to the Early Jurassic with no chrome-spinel data, and we know there was an asteroid break-up that affected Earth during that time. An asteroid family that emerged during the Cretaceous – the team’s most densely sampled period – also shows no significant increase in flux for this type of meteorite.

It is possible that future research could clarify the reasons behind these discrepancies. For now, the study represents a new way of understanding meteorite impact history on Earth and what we may expect in the future.

“Future impact from even a small asteroid, for example in the sea close to a populated area, could lead to disastrous outcomes,” Schmitz said. “This study provides an important understanding that we can use to prevent this from happening; for example, by attempting to influence the trajectory of rapidly approaching celestial bodies.”

The research has been published in PNAS.