Most of Earth’s meteorites can be linked to just a few collisions within the asteroid belt between Mars and Jupiter, two new studies report, including a particularly cataclysmic impact event around 470 million years ago.
The upside to this discovery, published October 16 in Nature, is that it provides researchers with vital context: By knowing the return address of meteorites, scientists can more easily work out how and where the building blocks of planets came together to create the solar system we see today. The downside is that it may mean researchers have an extremely biased meteorite collection that can tell only a sliver of the story.
Meteorites record the tumultuous history of the solar system’s formative years, but the origins of these ancient space rocks are often unknown (SN: 4/18/18). “It’s absolutely like a pot of gold at the end of a rainbow for a meteoriticist to know what asteroid the sample’s come from,” says Sara Russell, a planetary scientist at London’s Natural History Museum who wasn’t involved with either study. Without that information, a meteorite is like a piece of a jigsaw puzzle without a picture of the full puzzle to accompany it.
Most of the meteorites on Earth are stony ones named ordinary chondrites. Two classes of these chondrites, known as H and L, make up 70 percent of all meteorite falls.
Scientists had suspected that the L chondrites originated from a single parent asteroid. Many have mineralogical features indicating they were heavily shocked, scorched and degassed before gradually cooling, implying they were liberated from a giant asteroid — at least 100 kilometers long — via a supersonic collision.
Using radioactively decaying elements to determine the age of the meteorites has revealed that they first emerged from a collision that happened 470 million years ago. To search for the site of that destruction derby in the asteroid belt, researchers used NASA’s Infrared Telescope Facility in Hawaii to scan many prominent stony-type asteroids, comparing each one’s mineral signatures to those of L chondrites.
The best fit was a group of asteroids named the Massalia family. Their scattered presence and current orbits could effectively be rewound by the scientists — and it looked like the asteroids all formed around 500 million years ago after splitting from an older, larger asteroid. That timing suggested that the impact that created the L chondrites also created the Massalia family. One of the asteroids in that family is about 140 kilometers long, a perfect fit for the estimated size range of the L chondrite parent body.
Other independent lines of data also point to the Massalia family, including the fact that near-Earth asteroids with L chondrite–like signatures have orbits that trace back to the family, as do the orbits of the L chondrite meteors that burn through Earth’s skies, before leaving telltale meteorites behind.
“All point at the same thing. There’s no doubt,” says Michaël Marsset, an astronomer at the European Southern Observatory in Santiago, Chile, and an author of both studies.
That ancient impact also set the stage for a more recent bombardment, sending streams of L chondrite material tumbling back onto the largest asteroid remnant. Another impact no more than 40 million years ago then sent that rubble Earth’s way.
What of the H chondrites? Many are 5 million to 8 million years old, so came from a different impact event — or two events, it seems. By reconstructing the past orbits of the mineralogically matching Koronis2 asteroid family, the team found that many of those asteroids existed unified as a single asteroid 7.6 million years ago.
Prior research had already applied the same time-rewinding technique to another asteroid group, known as the Karin family, and found many of those were also united as a solitary asteroid 5.8 million years ago, just before another asteroid struck it. As both families cover each end of the date range for the H chondrites, the team concluded that they are the source of this meteorite class.
That Earth’s meteorite collection could be highly biased to just a few asteroids is distressing, Russell says. The asteroid belt is home to a dizzying array of rocks, boulders and even dwarf planets, each revealing something unique about the solar system (SN: 8/3/16). “Maybe we’re only just seeing a tiny fraction of them” through our meteorites, she says.
There is a solution, though more costly than scouring Earth for more meteorites. “We’ve got to have space missions to go out there,” she says, and hunt these ancient rocky archives down ourselves (SN: 2/15/24).
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