Like Dinosaurs, Meteorites Could Have Been Very Different Millions of Years Ago

Geologists discovered a new kind of meteorite they reckon could now be "extinct."

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Jun 14 2016, 3:00pm

Image: Shutterstock

Geologists have found a new type of meteorite which smashed into the planet around 470 million years ago, and they reckon it could be the first example of an "extinct" meteorite—a kind that no longer falls to Earth today.

The meteorite, discovered fossilized in a limestone bed in Sweden, has a different composition to all of the known meteorite types. This suggests that the meteorites that fell in the past may have been different to those we see today, and offers potential new insight into the formation and composition of the Solar System.

"We know absolutely nothing about what types of meteorites fell on Earth in the distant past, before one or two million years [ago]," said Birger Schmitz, a professor in geology at Sweden's Lund University and the lead author of the paper outlining the discovery, which was published on Tuesday in Nature Communications.

The meteorite fossil. Image: Birger et al

He explained that the majority of meteorites discovered from the past few million years fall into the same category ("ordinary chondrites"), but we don't know what came before, hundreds of millions of years ago.

"We know that we had other animals, we had other plants, we had other climate—but if you ask what type of meteorites fell on Earth in the geologically distant past, no one will be able to give you any answer," he said.

It's easy to assume that the meteorites that fell in the past were just the same as those seen today, but the discovery of this new meteorite suggests they could have been very different. If that's the case, then our current understanding of the materials in our Solar System—which is largely based on what we can tell from meteorites that make it to Earth—could need revising.

Meteorites can be classified by analysing their chromium and oxygen isotopes; the makeup of the new meteorite, known as Österplana 65 (Öst 65), falls outside of existing types. It was found in a quarry alongside more than 100 fossil meteorites known as "L chondrites," which are dated to the same 470-million-year-old time-frame. The L chondrites are believed to have resulted from a massive collision that broke up their parent body.

The quarry where the meteorites were found. Image: Birger et al

However, because these L chondrites all come from this specific event, they're not necessarily representative of the meteorites of their time, even though there's a lot of them. That's why Birger and his colleagues were excited to find Öst 65, which isn't associated with the same parent body. "It's the first meteorite we have found that represents the background flux," he said.

Based on the meteorite's cosmic-ray exposure age, the team found that it broke off its parent body around the same time as the L chondrites. It's therefore possible that it's a remnant of the asteroid that collided with the source of the L chondrites. But wherever Öst 65 came from, the simple fact that it exists suggests that meteorites in the past weren't the same as those we see today.

Caution is necessary before drawing broader conclusions, however: There is always the possibility that Birger and his team chanced upon an anomaly that was actually very rare even hundreds of millions of years ago. We may also eventually find other similar meteorites still falling to Earth.

Noting these caveats, Birger was confident in saying that the finding at least "hints" that there are, or have been, more kinds of space rock out there than we know of.

"I would say [the finding] hints that the meteorites that fall today are not representative of the types of parent bodies that exist between Mars and Jupiter in the asteroid belt, and that's the most important," said Birger.