Where do meteorites come from? A new analysis of 75 fall events suggests that meteorites with different geologies travel from different places in the asteroid belt, which separates Mars and Jupiter. Researchers traced some types of meteorites to particular asteroid families, creating a geologic map of meteorite origins. Most meteorites were generated by just a few recent collisions between asteroids.
“Understanding the asteroid belt is really looking into the past, into the formation of the solar system, and into all the dynamics that happened at that time,” said Peter Jenniskens, coauthor on the new analysis and a meteorite astronomer at the SETI Institute in Mountain View, Calif. Those early interactions and collisions matter because much of the water on Earth and a lot of the organics likely came from primitive asteroids, he added.
Tracking Falls
Spacecraft have returned small volumes of material from the Moon, comets, and asteroids, but meteorites remain the primary way that scientists get their hands on space rocks.
“By reconstructing where specific meteorite types formed, we gain a clearer picture of the compositional and thermal gradients that existed when the solar system was young,” said Michaël Marsset, an astronomer at the European Southern Observatory in Santiago, Chile. “This has major implications for understanding how habitable environments emerge, not just here but potentially in other planetary systems as well.” Marsset studies small solar system objects and Earth impactors and was not involved in the new study.
But matching a meteorite to the asteroid it came from is a tall task.
“Asteroids in space look quite a bit different than the meteorites that we have in our laboratories.”
“Asteroids in space look quite a bit different than the meteorites that we have in our laboratories, because the asteroids in space are covered by regolith and debris and they are exposed to solar radiation and solar wind,” Jenniskens said. A meteorite might come from an asteroid’s interior, which could look entirely different from its surface. That makes it challenging to use astronomical observations alone to match meteorites to their asteroid parents.
When someone witnesses a meteorite falling to Earth, scientists can try to backtrack its orbit to a point of origin. Combining this information with the meteorite’s geochemistry, mineralogical structure, and age, they can then figure out which asteroid or asteroid family—a group of asteroids that originate from the same collision event—sent it hurtling toward Earth.
The trouble is that meteorites fall more or less at random, Jenniskens explained. It has taken a while to document enough falls to spot patterns, he said. Just 6 years ago, there were fewer than 40 meteorite falls with well-measured trajectories.
“The number of falls has doubled since that time,” Jenniskens said.
Meteorite researchers have set up more than 2 dozen global camera networks that have detected many of these recent falls—roughly 14 falls per year. Also, the rising popularity of dash cameras and doorbell cameras has contributed to the surge of recent detections.
In the new analysis, about 36 of the 75 falls were recorded by residential security cameras, Jenniskens said. People report fireball sightings and submit videos for analysis. “We really depend on the citizen science.”
Meteorite Ancestry
Jenniskens and his colleague Hadrien Devillepoix of Curtin University in Perth, Australia, reviewed the trajectories, geochemistry, mineralogy, and size of 75 meteorites. They also looked at the meteorites’ ages, calculated on the basis of how long a rock’s surface has been exposed to cosmic rays.
Though a few asteroids are suspected sources of certain meteorite types, a meteorite’s age was often the key factor in figuring out which asteroid family produced the meteorite. The positions and movements of asteroids within a family evolve in a predictable way over time, and if this so-called dynamical age matched a meteorite’s cosmic ray age, that family was more likely to be the meteorite’s source.
Most of the meteorites originated from a handful of asteroid families, and different classes of meteorites could be traced to different parts of the asteroid belt.
Jenniskens and Devillepoix confirmed that very low iron LL-type meteorites, such as the Chelyabinsk meteorite, originated from the extensive Flora family in the inner asteroid belt. They tracked H-type chondrites to debris clusters in the Koronis, Massalia, and Nele families. They also traced low-iron L chondrites to the Hertha asteroid family, rather than to the previously determined Massalia family.
“Hertha is covered by dark rocks that were shock blackened, indicative of an unusually violent collision,” Jenniskens said. “The L chondrites experienced a very violent origin 468 million years ago when these meteorites showered Earth in such numbers that they can be found in the geologic record.”
“It turns out that, yes, our HED meteorites seem to come from Vesta, not from its family.”
Marsset has also worked to trace meteorites to their asteroid origins, though his team used astronomical observations of asteroids and numeral modeling, rather than meteorite data. “Even with these different approaches, we’re mostly converging on similar conclusions,” Marsset said. “Where we disagree, well, that’s part of the fun! For example, I’d gladly bet a pint with Dr. Jenniskens and Dr. Devillepoix that L chondrites come from the Massalia family, not Hertha,” he joked.
The team also looked at howardite, eucrite, and diogenite (HED) meteorites, achondrites that have long been tied to the Vesta asteroid family. According to the new analysis, the volume of HED material that made its way to Earth must have come from a collision so large that only something as large as Vesta would have survived. (Vesta is the second-largest object in the asteroid belt.) What’s more, the cosmic ray exposure ages of HED meteorites closely match the ages of particular impact craters on Vesta’s surface that were mapped by NASA’s Dawn spacecraft.
“It turns out that, yes, our HED meteorites seem to come from Vesta, not from its family,” Jenniskens said.
Decoding Solar System History
“What’s remarkable about this work is the broader picture it starts to paint,” Marsset said. “We are finally able to map specific classes of meteorites that fall on Earth to distinct regions in the asteroid belt and to specific asteroid families.… That’s a major step toward understanding the compositional structure of the asteroid belt and, ultimately, how our solar system formed and evolved.”
But it’s just as important to understand where meteorites aren’t coming from, he pointed out.
“While one might expect the meteorite flux to represent a broad sampling of material from across the entire asteroid belt, we now know that it is actually dominated by a few recent fragmentation events,” Marsset said. “This insight helps us better understand the natural sampling bias in the meteorites we collect on Earth, and it also highlights which asteroid populations are underrepresented. That, in turn, can guide the targets of future space missions aimed at filling in those missing pieces.”
—Kimberly M. S. Cartier (@astrokimcartier.bsky.social), Staff Writer