Meteors appear to rain from the sky during the Geminid meteor shower in 2020. Credit: Jeff Sullivan (Flickr, CC BY-NC-ND 2.0)
A couple of articles published today in Nature look at the origins of many meteorites that fall to Earth. By examining the detailed composition of the rocks, ascertaining the time since they broke off from larger bodies and comparing them to asteroids in space, the researchers found evidence that the vast majority of meteorites falling to Earth may have come from three sources. They are the Massalia, Karin and Koronis2 asteroid families.
Fingerprinting of suspects
About 70% of meteorites on Earth are classified as H or L chondrites. H chondrites are generally rich in iron, while L chondrites are rich in the mineral olivine and have significantly less elemental iron. The authors of the papers searched the main belt for asteroids that matched the elemental composition of H and L chondrites, using spectroscopy, which breaks light down into its constituent wavelengths to look for patterns that match certain elements. They also had to look for objects in the right locations in the main belt to deliver the debris to Earth.
They found the right chemical abundance for L chondrites in 20 Massalia, an asteroid with a diameter of about 90 miles (140 kilometers), and its family of nearby asteroids, all of which have essentially the same composition. This shows that the smaller asteroids likely came from Massalia, created in an ancient collision 470 million years ago that destroyed the original asteroid Massalia. But it also indicates that some fragments may have arrived on Earth in a steady stream of rainballs practically as long as vertebrates have existed.
Most H chondrites, on the other hand, came from more recent asteroid collisions on the planets Karin and Koronis.2 families, as well as another more recent impact on Massalia. These events occurred approximately 5.8 million, 7.6 million, and 40 million years ago and represent the formation of many H-class and similar meteors that fall on our planet today.
Ticket to Earth
Asteroids orbit in the main belt, a region between the orbits of Mars and Jupiter. So how did their pieces get here?
“Those collisional families… are very efficient at producing small fragments that can be easily transported into our solar system through non-gravitational forces, like what we call the Yarkovsky effect, for example, and can be delivered to Earth,” says Michaël Marsset. of the European Southern Observatory, author of both studies. The Yarkovsky effect is a phenomenon caused by photons pushing on an object in space, acting almost like thrust from a mini rocket. It is enough to send an object from the main belt to the inner solar system, where it could potentially encounter Earth.
Furthermore, Massalia and her “children” have a resonant orbit with the Earth. For every three orbits around the Sun that Earth completes, Massalia completes one, and these resonant orbits provide a way for debris to arrive at Earth at a constant rate. L chondrites account for “about 37% of falls at the moment,” Marsset says.
In search of origins
The genesis of the study, according to Marsset, stems from a number of L chondrite meteorites that fell into what is now a limestone quarry in Sweden 466 million years ago. This led the team to search for a common ancestor of these objects and other meteorites of the same class.
“The fossil meteorites that were found in this quarry in Sweden […] they are the same meteorites that fall on our planet today,” Marsset says, with the second impact on Massalia 40 million years ago kicking up more debris.
Once a collision occurs, families of meteorites cascade from there. For example, Massalia’s initial impact would have created a series of large objects. Over time, these objects collided with each other to create medium-sized rocks, and so on until some debris became as small as simple grains of sand.
There are other sources of meteorites on Earth, to be sure. For example, about 6% come from the Moon, Mars, or asteroid 4 Vesta, a strange and surprisingly large object in the main belt that was visited by NASA’s Dawn mission.
These two studies lay the foundation for understanding the event that brought so many space rocks to our planet, including those we still see in the night sky. But more data will be needed both to reconstruct the event and to better understand the transport mechanism that brought its debris to Earth.
However, these are fascinating feats of planetary science that help reconstruct the history of our planet and the broader solar system.
