Most of Earth’s meteorites can be traced back to just a few collisions within the asteroid belt between Mars and Jupiter, two new studies report, including a particularly cataclysmic event about 470 million years ago.
The positive side of this discovery, published on October 16 in Natureis that it provides researchers with vital context: By knowing the return address of meteorites, scientists can more easily understand how and where the building blocks of the planets came together to create the solar system we see today. The downside is that it can mean that researchers have an extremely biased collection of meteorites that can only tell a sliver of the story.
Meteorites record the tumultuous history of the solar system’s formative years, but the origin of these ancient space rocks is often unknown (SN: 18.4.18). “It’s absolutely like a pot of gold at the end of a rainbow for a meteorologist to know where this asteroid comes from,” says Sara Russell, a planetary scientist at London’s Natural History Museum, who was not involved in either study. Without this information, a meteorite is like a piece of a puzzle without a picture of the complete puzzle to accompany it.
Most meteorites on Earth are rocks called ordinary chondrites. Two classes of these chondrites, known as H and L, account for 70 percent of all meteorite falls.
Scientists had suspected that L chondrites originated from an asteroid with a single parent. Many have mineralogical features that indicate they were severely shocked, burned and degassed before gradually cooling, implying that they were released from a giant asteroid – at least 100 kilometers long – through a supersonic collision.
Using radioactive decay elements to determine the age of meteorites has revealed that they first emerged from an impact that occurred 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 rocky-type asteroids, comparing the mineral signatures of each to those of L chondrites.
The best fit was a group of asteroids called the Massalia family. Their scattered presence and current orbits could effectively be traced back by scientists – and it looked like the asteroids all formed about 500 million years ago after breaking up from an older, larger asteroid. This 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 come from the family, as do the orbits of L-chondrite meteors that burn across Earth’s skies. before leaving behind telltale meteorites.
“They all point to the same thing. There is no doubt,” says Michaël Marsset, an astronomer at the European Southern Observatory in Santiago, Chile, and author of both studies.
That ancient impact also set the stage for a more recent bombardment, sending streams of L-chondrite material crashing back into the larger asteroid remnant. Another impact no more than 40 million years ago sent that wreckage to Earth.
Is H going against it? Many are 5 million to 8 million years old, so they came from a different impact event—or two events, it seems. Reconstructing past orbits of mineralogically compatible Koronis2 family of asteroids, the team found that many of those asteroids existed unified as a single asteroid 7.6 million years ago.
Previous research had already applied the same time-rewinding technique to another group of asteroids, known as the Karin family, and found that many of them had also coalesced as a single asteroid 5.8 million years ago, shortly before an asteroid another to hit him. Since both families cover each end of the date range for H chondrites, the team concluded that they are the source of this class of meteorite.
The fact that Earth’s meteorite collection could be so biased toward just a few asteroids is troubling, Russell says. The asteroid belt is home to a variety of rocks, stones and even dwarf planets, each revealing something unique about the solar system (SN: 8/3/16). “Maybe we’re only seeing a small fraction of them” through our meteorites, she says.
There is a solution, albeit a more expensive one than scouring the Earth for more meteors. “We have to have space missions to go out there,” she says, and hunt down these ancient rock archives ourselves (SN: 15.2.24).
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