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. I mentioned chondritic meteorites, or chondrites; a fully melted one is an ‘achondrite’ (Greek for ‘that without chondrules’). Here, the parent body not only got hot enough to melt, but the materials could and did flow. This is ‘differentiation’- a body settling into dense core material, a lightweight crust, and a mantle between. There are, in addition to chondrites and achondrites, primitive achondrites. Their parents warmed, and chondrites got erased. But the body was not hot enough, for long enough, for the materials to separate and flow far. Primitive achondrites still show blended compositions, not like a piece of core, crust, or mantle.

This compositional blend clearly separates chondrites from planets… but not the Sun! The Sun is mostly hydrogen and helium, which cannot form a rock. They can’t even form ice crystals at any temperature around the Solar System. However, the Sun does have various amounts of other elements, which we can measure. We study the light from the Sun. This light has a spectrum based on the elements present, and their levels. (An element’s level in the Sun is its ‘solar abundance’.) Many of these levels can be cross-checked in the Solar wind, an ultrafine exhaust of atoms that the Sun is constantly shedding into space.

Other than hydrogen, helium, and a few other light elements, many elements in chondrite samples have solar-like abundances. In fact, one subcategory, the CI chondrites, is held as the best chemical sample we have of the solar abundances. Abundances in CI chondrites are so close to solar levels it’s eerie. Other chondrites are not that close, but not far either. This is due to being formed in various parts of the Solar System, with various processes occurring. Either way, differentiation was not one of those processes. It’s possible the Earth’s abundances would go sunlike, if we could average together our mantle and core too. Since we’re not going to the core any time soon (bad sci-fi aside), CI meteorites will have to do.

One of these other places/processes included inner vs. outer regions of the Solar System. It appears chondrules were big enough that they didn’t get far from where they formed. That, or they became asteroids soon after forming; neither chondrules nor bigger objects can blow around like gas and dust. The chondrites can then be divided further: ordinary chondrites, enstatite chondrites, and carbonaceous chondrites. Enstatites formed close to the Sun, as we see in their elements and minerals. Carbonaceous ones formed from the current Main Belt’s radius, outward. This cooler zone had some rock, some condensed volatiles like water, some organic chemicals that would fry in an inner zone, etc. And ordinary chondrites (‘OCs’) formed from the Main Belt, inward. This is where Earth is, so OCs are more Earthlike than the others, but that’s not how they got the name. It’s easier for OCs to reach Earth and form meteorites, so they’re the most common kind we collect. They’re still not that Earthlike.

(There’s another effect, we think. Carbonaceous materials are weaker than rock, so they’re more likely to break and scatter while plunging through the atmosphere. They’re more likely to break and erode on the ground, too. So we don’t see many carbonaceous chondrites, even while they’re not rare at all in space. Iron meteorites, meanwhile, are overrepresented in collections. Fewer asteroids differentiated into cores, and got broken into chunks.)

The overall classification scheme of stony meteorites, iron meteorites, and stony-irons has been around, in some form or another, farther than we have recorded. We have found blades made of meteoric iron, by indigenous peoples around the globe, including ancient Egyptians. Their hieroglyphics tell of “heavenly iron” or “iron from the sky”. The chondritic classification scheme is far newer, published from the 1950s on. Both geology and astronomy were making big strides, and as a result meteoritics advanced, too. In the mid-’50s, Urey and Craig made major breakthroughs, followed by Cameron and Mason. By the mid-’60s, van Schmus and Wood pretty much had the meteorite classification system we know today.

In the ’50s view, the three chondrites- ordinary, carbonaceous, and enstatite- could further divide via finer element abundances. OC and enstatite each split into high metal and low, or H and L chondrites, and EH/EL. L was split again, into an LL. van Schmus and Wood 1967 then added numbers- low numbers generally had less alteration by heat, water, etc. Higher numbers got more altered, with fuzzier chondrule borders. (As above, chondrules can soften and flow into the matrix, but the greater body can stay intact. Materials flowed on the scale of the next chondrule, not the radius of the asteroid, or even the meteorite.)

Carbonaceous chondrites are a bit harder… to remember. They have the prefix C, then a letter from a type specimen. CI chondrites are taken to be like the Ivuna meteorite; CM like the Mighei meteorite, and so on. These, too, can get a number suffix. Since they have high water content, numbers 1 and 2 were given to high and low aqueous alteration, respectively. 4 and up indicate rising thermal alteration; it is 3 that is most ancient and pristine.

Speaking of alteration and fuzzing, what’s going on inside even a chondritic, undifferentiated asteroid? Some more background: as chondrules accrete into asteroids, they gather a matrix and other material. Some of it is dust, some is gas and volatiles, some is tiny grains of minerals, too small to melt on their own. And some is prior chondrules, broken to bits. Asteroids consolidate these into a mass. (When forming, the matrix is usually quite similar, since it’s from the same zone in the Solar Nebula. It, too, looks like enstatite, carbonaceous material, or ‘ordinary’ rock.) The act of consolidating is itself warming- as matter falls in, the energy of falling turns into thermal energy. In extreme cases, impact is very consolidating. Some asteroids and meteorites show clear signs of shock, which acts as a temperature spike. And of course, many chondrules would still be warm from their formation. As above, smaller bodies lose heat, while larger ones hold it better. Higher alteration may just be from deeper inside such bigger asteroids. We then see chondrule compositions bleed into their matrix, and vice versa.

In the inner Solar System, it’s just warmer. Yet many carbonaceous chondrites are altered, too. As above, water will be stable at some distance from the Sun (the ‘snow line’). Carbonaceous bodies formed in this zone, gathering much water. Materials then dissolved from chondrules to matrix and vice versa, especially if impacts heat and agitate the water. Another pathway is CAIs (calcium-aluminum-rich inclusions) and other radioactive elements. In the early Solar System, ²⁶Al existed. It’s unstable, and for a brief time gives high heat. CAIs and ²⁶Al were not rare in what is now the Main Belt, and many bodies were irradiated and softened by them, sometimes melted through. There are other short-lived isotopes, but ²⁶Al is the main culprit.

We know some nuclear reactions took place inside bodies, since we see the decay products, at levels that make no sense without those decays. Long-lived isotopes like uranium certainly exist, but are more important in planets. Small bodies cool off too fast for uranium to compete with ²⁶Al. And there are meteorite inclusions called FUNs (fractionated, and unknown nuclear). We know FUNs were nuclear, since they have weird abundances and decay products as the name implies. We’re still debating those pathways, again as the name implies.

At some point- water, nuclides, collisions, or just size- a body may get altered. Alteration with no differentiation produced the acapulcoites, whose type specimen was a meteorite found near, yes, Acapulco. The lodranites are a bit similar; they may be from deep inside the acapulcoite parent bodies. And while we’re on the topic of odd rocks, mesosiderites (one of the two stony-iron meteorites) are stone and iron bits, re-compacted into a new body. Apparently, one or more iron asteroids collided with achondrites, and their rubble fused together and sprayed out. The ‘chondrules’ are actually pebbles and gravel from both asteroid’s exteriors.

It gets weirder. Brachinites are from a melted parent, but the brachinites didn’t really melt. Our assumption is that brachinite meteorites are residues left when a magma in an asteroid didn’t completely liquefy. This residue is not at solar abundances, but not differentiated, either. The deposit was hurled off by an impact, and eventually reached Earth.

It gets even weirder. Though the mesosiderite and brachinite examples may seem like celestial hole-in-one shots, they still used a regulation golf ball, cup, etc. It’s not only possible but likely that some materials simply aren’t in our collections. The Kaidun meteorite for example is, like the mesosiderites, a rock collection (technically called a ‘breccia’). Some of the Kaidun material is achondritic (melted), some is carbonaceous (signs of water, not heat), but none really falls (by element levels) into those classes. Either the collision that formed Kaidun produced unusual gains and losses of elements, or it just involved subtypes we’ve never seen before. In the entire, vast Solar System, those two (or three) just happened to meet, and happened to send a piece toward Kaidun, Yemen, and someone happened to pick it up.

It is naive to think we have specimens like every asteroid- we haven’t even found them all in our telescopes. This, despite well over 20,000 meteorites in collections around the world, and many more found each year. Since the ’70s, Antarctic expeditions, and to a lesser extent in the world’s deserts, have multiplied humanity’s grasp of space several fold. Yet there’s still a real chance that a probe to a small body finds that, to humanity, the body is a new material. For decades, scientists have figured that OCs and irons are overrepresented, and have figured some other varieties which would be underrepresented, or missing completely.

All these, from the humble chondrule. A pebble or very coarse sand grain, preserved from the Solar Nebula. And with presolar grains too- I’ve only touched on them once before.