Three new studies have revealed the common origin of most of the meteorites that hit Earth, linking them to collisions in the relatively recent past.
Though the Earth doesn’t make a fuss about it, every day it is bombarded with around 44,000 kilograms (48.5 tons) of meteoric material. Most of it burns up harmlessly in our atmosphere, but the few that do make it to the surface can be studied, to take a look at their composition. This can tell us if they came from the same family of asteroids, for instance, if a large asteroid broke apart, with some of those fragments eventually hitting Earth.
Until now, only around 6 percent of meteorites had been traced back to their source. These were meteorites that came from Vesta (the second largest asteroid in the Solar System), the Moon, and Mars, and are known as achondrites.
“Meteorites fall into two broad categories, chondrites, which are almost pristine pieces of the early solar system before planets formed, and achondrites, which come from differentiated bodies; i.e., planets or asteroids with layers of core, mantle, and crust,” a paper on the topic explains.
Chondrites, which make up the vast majority of the remaining meteorites, may not look as exciting as achondrites, but are just as interesting scientifically. An international team, led by the French National Centre for Scientific Research (CNRS), the European Southern Observatory (ESO), and Charles University, has now studied this class of meteorites in detail.
The team found that 70 percent of all the meteors that fall to Earth come from three families of asteroids, known as Karin, Koronis, and Massalia. These smaller meteorite families were likely created by relatively recent – in astronomical terms – collisions, occurring around 5.8 million, 7.5 million, and 40 million years ago.
“These break-ups, including the well-known Karin family, took place in the prominent yet old Koronis and Massalia families and are at the origin of the dominance of H and L ordinary chondrites among meteorite falls,” another team explains in their paper.
“These young families are distinguished among all main belt asteroids by having a uniquely high abundance of small fragments. Their size–frequency distribution remained steep for a few tens of millions of years, exceeding temporarily the production of metre-sized fragments by the largest old asteroid families (for example, Flora and Vesta).”
Of particular interest is the Massalia family of asteroids, which accounts for over 20 percent of meteorite falls today.
“Studies of micrometeorites in mid-Ordovician limestones and impact craters on Earth indicate that our planet witnessed a massive infall of ordinary L chondrite material about 466 million years ago that may have been at the origin of an Ordovician ice age and major turnover in biodiversity,” one team explains in their paper. “The breakup of a large asteroid in the main belt is the likely cause of this massive infall.”
While Karin, Koronis, and Massalia make up most of the numbers of meteorites, larger, more intimidating near-earth objects (NEOs) were found to have their origins elsewhere.
“For kilometre-sized NEOs, our model indicates that most of them originate from just two families: Polana and Euphrosyne,” one team explained in their paper. “On the other hand, the flux of metre-sized carbonaceous chondrites is dominated by just one family: Veritas.”
Studying these families of asteroids will help us figure out, among other things, the likelihood of Earth being bombarded by them. Large asteroid collisions that took place more recently, for instance, may pose more of a threat, prompting more collisions and chaos that could allow fragments to escape from the asteroid belt and put them on a collision course with Earth.
Though jumping from only 7 percent of meteorites being traced to their origin to 90 percent is an impressive leap forward, 10 percent remain unidentified. The team plans to continue their work, focusing on young families of asteroids formed less than 50 million years ago.
The studies are published in Astronomy & Astrophysics, Nature, and Nature.
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