Most of us in the group study meteorites, and you can get a bit obsessed with these cool space rocks! It’s easy to get lost in meteorite classifications, so I’ll try to keep things simple in the following. First, we distinguish meteorite finds from meteorite falls. Falls are pieces of rocks found on the ground after having been seen falling through the sky, either by people or cameras, while finds are meteorites that have not been seen falling, and have been lying around on the ground for a while. The Chelyabinsk meteorite, which fell in Russia in 2013, is probably one of the most famous recent fall, having been recorded falling by numerous cameras. Scientifically, falls tend to be more sought after than finds, as terrestrial weathering in finds for thousands to millions of years can modify and/or erase some of their original mineralogical and chemical characteristics.
Meteorites are also compositionally very diverse. Around 90% of meteorites in our collections are what we called chondrites, which are unmelted aggregates of dust and larger solids, such as chondrules and inclusions rich in calcium and aluminium, formed during the birth of our Solar System ~4.56 billion years ago. A bit more than 90% of these chondrites are called ‘ordinary chondrites’ (hence their name…), ~1-2% are ‘enstatite chondrites’ (made up mostly of a mineral called enstatite pyroxene), and ~5% are called ‘carbonaceous chondrites’, the latter typically containing significantly more water- and carbon-rich material compared to any other meteorite types. Among the remaining ~10% of meteorites, about half are what we call achondrites, which comprise rocks that originated from parent bodies that underwent melting and/or differentiation such as the Moon, Mars or large asteroids. The remaining 3-5% in our meteorite collections are iron-rich meteorites, which formed very early in the Solar System history and may have originated from the cores of melted asteroids and planets.
All of this to say that meteorites are rare, and non-ordinary chondrites are even rarer! Yet while scrolling through social media yesterday, I came across photos of nice pieces of a new meteorite fall that occurred a few days ago in Morrocoo near Errachidia. And based on these photos, there is little doubt that this new meteorite is a carbonaceous chondrite, likely belonging to the CM chondrite family. The CM chondrites are particularly interesting because they contain an awful lot of water and carbon-rich species. They also seem to be very similar to rocks observed at the surface of the asteroids Bennu and Ryugu by the NASA OSIRIS-Rex and JAXA Hayabusa2 missions, respectively, and from which these two spacecrafts will bring samples back to Earth in the coming months. But let’s get back to the Errachidia new fall. When I saw the photos I thought “hell, looks like half the falls in recent years are CM chondrites!“. The Mukundpura CM chondrite fell in India in 2017, and ~30 kg of CM chondrite stones fell over Aguas Zarcas in Costa Rica last year. So is it just an impression, or are CM chondrites over-represented in recent meteorite falls?
To check this out, I downloaded the complete list of >60,000 meteorites we have in our collections from the Meteoritical Bulletin Database, and started looking at the data. CM chondrites represent ~1.0% of all the meteorite finds collected worldwide. However, collections from hot deserts such as the Sahara or the Atacama desert might be somewhat biased since not all meteorites are officialy submitted for classification in the Meteoritical Bulletin. Since the 1970’s, systematic meteorite recovery expeditions have taken in place in Antarctica, which resulted in ~40,000 finds (for more details on why we go to Antarctica to collect meteorites, and how we go about to do so, visit the UK Antarctic meteorite website). Every meteorite found in Antarctica is officially classified, so we can assume that Antarctic meteorite collections provide a fairly representative picture of what has fallen to Earth over the past million years or so. For five of the largest icefields where meteorites have been collected (Yamato, Queen Alexandra Range, Miller Range, LaPaz Icefield, and Allan Hills), 1.6% of the specimens are CM chondrites, a tiny bit more than considering all finds. This is consistent with the proportion of CM chondrites among all the ~1,100 falls (2.0%). Comparing all falls to the Antarctic collections, one can see that the HED meteorites (presumably from the asteroid 4-Vesta) and the iron-rich meteorites seem to be under-represented in the Antarctic collections (the UK Antarctic meteorite expeditions were actually set up a few years ago to test one of the hypothesis accounting for the missing iron meteorites in Antarctica). If we limit the dataset to meteorites that have fallen since 1950 and 2000, the proportion of CM chondrites increases to 2.4% and 3.9%, respectively. So not quite half the falls, but nevertheless it looks like the proportion of CM chondrites falling to Earth has increased in the last few decades.
Let’s remove the ordinary chondrites from the statistics to look at this in more detail. CM chondrites now represent 7.3% of all finds, which is pretty consistent with the falls for which CM chondrites represent 10.6%. For Antarctic meteorites, 17.4% are CM chondrites. Now looking at more recent falls, CM chondrites amount to 14.9% of those fallen since 1950, and 24% of all non-ordinary chondrite falls since 2000. The numbers are pretty small, but there does seem to be an increase in the numbers of CM chondrites recovered in the last few decades.
These charts show other interesting observations; enstatite chondrites and non-CM carbonaceous chondrites seem to be over-represented in Antarctic collections compared to falls. On the other, we have never seen a piece of the Moon falling to Earth, while almost 5% of all finds are lunar meteorites.
So has the number of CM chondrite falls really increased dramatically over the past few decades? If correct this could have some important implications in terms of understanding the flux of extraterrestrial objects hitting the Earth, and the dynamical evolution of asteroids in the asteroid belt, from where most of our meteorites originate. Or are variations of these numbers better explained by some other bias? Pairing of meteorite finds (linking together rocks of a single type that originated from a single meteorite fall) is not always straightforward and can affect statistics. The number of falls over the past couple of decades is still pretty tiny, so are statistics reliable with low numbers?
To adress these questions, we thus need to continue doing systematic meteorite recovery expeditions in places such as Antarctica and hot deserts such as the Atacama desert in Chile where I have been lucky to go twice in 2017 and 2019. In parallel, we need to continue developing camera networks that monitor the skies hoping to observe bolides; if several cameras observe the same event we can work out where it came from, and where it landed, roughly, if anything survived entry into the Earth atmosphere. This has allowed Australian colleagues to recover several meteorites already. Such networks are currently being set up in the UK, and we all cross our fingers that the next UK meteorite fall is just around the corner (I highly recommend watching the recent seminar given by Drs Ashley King and Luke Daly on the topic if you want to know more).
Earth & Solar System 
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