Meteorite

50th Landi-versary: Murchison

1969, in retrospect, was the year of the Allende meteor event and its recovery– overall, 2 tons of primitive material from the Solar System’s birth. It’s the single largest carbonaceous chondrite meteorite, which we classify as a CV chondrite. Another groundbreaking event happened 50 years ago: the Murchison meteorite, the largest of the CI or CM (aqueous) meteorites.

Fragments recovered near Murchison, Australia total far less than Allende- about 100 kg. This is still huge: CV carbonaceous chondrites are actually just ~1 percent carbon compounds, and only a bit of that is native carbon (graphite, tiny diamonds, etc.). We call these meteorites ‘carbonaceous’ since the ‘ordinary chondrites’ have even less- a fraction of a percent carbon. At the birth of the Solar System, carbon tended to form gases (carbon oxides and hydrides, i. e., organics) which tend to blow away instead of forming solid objects. Despite the fact that the forming Solar System had appreciable carbon, a lot dispersed to the galaxy. But CI and CM carbonaceous chondrite meteorites have more carbon- ~2 or so percent. The CI/CMs had never been heated, not to magma temperature, and not to serious oven temperatures, either. They now retain more light chemicals- organics, sulfates/sulfides, and water.

This is why 100 kg is big and important. Allende and similar chondrites are overwhelmingly rock and other durable minerals. They hold up, even when plunging through our atmosphere in a fiery display. CI/CM chondrites aren’t rock so much as clay. Water exposure has broken down most of their rock into layers or particles; when heated and stressed by atmospheric entry, the particles/layers often disperse instead of landing. Landing and Earth weather then keep damaging them. CI/CMs are thus rare, sought-after examples of the early Solar System.

And then came Murchison…

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Meteor-Write: Chondrule Or Not

As we get closer to small bodies (including the first landing), we get deeper into Solar System science. I’m using more and more terms I haven’t defined; let’s remedy that.

The basics include chondrules. Chondrules are literally the basic building blocks of small Solar System bodies, and by extension both planets and meteorites, formed from small bodies. A chondrule (Greek for granule) is a millimeter-scale object; once molten and adrift in space, it is now hard and inside a small body or meteorite. These consist of chondrules, embedded in a matrix. Meteorites which still look like clumps of chondrules are called ‘chondrites’. When we cut open chondrite meteorites, many chondrules are clearly visible.

At the formation of the Solar System, there was just gas and dust- the Solar Nebula. This matter began sticking together- gases condensed to droplets, volatiles into tiny crystal grains, and dust into dust clumps. At some point, heat melted many of these bits, which pulls them denser and rounder. The process of ‘accretion’ continued- gas and dust kept sticking to chondrules, which stuck to each other, which made them better targets for more gas and dust and chondrules. And so on, forming small asteroids and comets, up to planetesimals, then planets. Eventually, most stuff had been consumed; those planetesimals left (now cooled, harder, and further apart) were just as likely to destroy each other in a collision, rather than stick.

In planets and large planetesimals, collisions and radioactive material generate heat. Planets and some asteroids then remelted, losing chondrules. In small bodies, heat is lost more easily, via their closer, relatively larger surface. So most asteroids and comets kept chondrules, a tangible record of the early Solar System and its processes. We value such bodies as time capsules or ‘baby pictures’; we also have samples of gently- and partly-melted asteroids. They, too, look like nothing on Earth. One could say it starts and ends with chondrules.

Chondrules then define our meteorite classification system, and by extension asteroids…

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