For many millions of years, the Earth has been subjected to constant bombardment, a ‘stone rain’ that never stops. Every day, in fact, about 50 meteors They manage to survive their fiery descent through the Earth’s atmosphere to reach the surface in the form of meteorites. Nearly 50,000 of these space rocks have already been recovered, but until now scientists have only been able to determine the origin of a small percentage of them.
However, that has just changed. Now, and after long years of work, an international team of researchers has managed to track up to 70% of the meteorites that reach Earth. And it turns out that all of them are part of only three bodies in the Solar System, which broke up in different impacts to give rise to the different families of asteroids in the belt between Mars and Jupiter, and also to the countless smaller fragments that so often arrive so far.
The advantage of this discovery, recently published in two studies in Nature (here and here) and in a third in ‘Astronomy & Astrophysics‘ is that it provides researchers with vital context: by knowing the return address of meteorites, scientists not only now know where more ‘dangerous visitors’ are likely to arrive from, but they can also more easily determine how and where The basic components of the planets came together to create the Solar System we see today. However, it is not all advantages: since the origin of most meteorites is so specific, it is possible that the collections managed by researchers are extremely biased and therefore only tell part of the story.
‘H’ and ‘L’ chondrites
Most meteorites that fall to Earth are ‘ordinary’ chondrites, molten rocks with little metallic content. Scientists divide these chondrites into two classes, ‘H’ and ‘L’ (‘High’ and ‘Low’, high and low iron content). And it turns out that, together, the two categories account for more than 70% of all the meteorites that fall on our planet.
Until now, says Miroslav Brož, an astrophysicist at Charles University who led one of the three studies, “only 6% of the meteorites were known to come from; Now, more than 70% have a known origin. “It is the discovery of a lifetime.”
For some time, the mineralogical characteristics of the ‘L’ chondrites suggested that they all came from a single, huge asteroid, at least 100 km in diameter, which, about 470 million years ago, suffered an impact that shattered it into pieces. . Fragments that today form a group of asteroids known as the ‘Massalia family’ and whose largest member is about 140 km long.
«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 co-author of the two Nature studies. That ancient impact also paved the way for much later, about 40 million years ago, another collision to launch a new barrage of rocks toward Earth.
But what about H chondrites? Many of them are between 5 and 8 million years old, so it is clear that they come from a different impact event, or perhaps two. By reconstructing the past orbits of the Koronis 2 family of asteroids, whose compositions match, the team discovered that many of those asteroids were part of a much larger one 7.6 million years ago.
Other researchers had already done the same with another group of asteroids (the Karin family) that were also united into a single asteroid 5.8 million years ago, when it was hit and fragmented by another rock. The Koronis 2 and Karin families are therefore the source of the H chondrites.
The vast majority of meteorites that reach Earth, then, come from just a few asteroids. Which is still worrying for researchers. The asteroid belt, in fact, is home to a huge variety of rocks, boulders and even dwarf planets, and each of them reveals something unique about the Solar System and its history. A variety that is not reflected if we limit ourselves to studying only the rocks that reach here.
There is a solution, although more expensive than continuing to explore the Earth in search of more meteorites: go look for them there. That may be the only way to access all the information that these rocks hold about our distant origins.
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