by Sterling Webb
Chondrules are small, round (more or less) glassy droplets of molten rock that were "flash fried" at high temperatures by... something. What the "something" was, is the cause of long and nasty arguments among cosmologists. But it was;
- Very hot, and
- Very quick, and
- Cooled quickly.
When this happened to the chondrules, they were undoubtedly free-floating in low gravity (because of their generally round shapes) and not in the presence of any larger bodies. It had to have happened BEFORE they became part of anything (like meteorites). This makes them older than meteorites (they were here first). We think they are the oldest things in the solar system.
And sure enough, when you pry them out of the rock matrix they're embedded in and date them separately, they ARE older. Just by a handful of million of years, but older.
Since they are embedded in meteorites in pretty much the same way fruit and nuts are embedded in a fruitcake, this means that the meteorites themselves were assembled in a relatively quiet fashion, around the pre-existing chondrules, not hotly enough nor violently enough to damage the chondrules beyond recognition... usually.
Another clue that the meteorites were not violently assembled is the fact that the rock has varying amounts of metals just dissolved in the rock, or present as small droplets. Nobody "forged" meteorites; they just formed by "hanging out." We assume that almost all the rock materials had chondrules in them, originally.
I said "usually" because in some (but not all) meteorites, chondrules have been damaged, some less, some more, some to the point where they can hardly be recognized, and in some meteorites, they have been completely re-melted and disappeared.
This general scale of "damage to chondrules" is called the meteorite's "metamorphic" stage. "Metamorphosis" means change. If the chondrules are fresh looking, easily recognized little marbles, all different, and still look like they did when they formed independently, the history of that rock has been mild and peaceful (for a rock). The chondrules haven't changed much, if any. The most likely peaceful history for a rock is to have been a smallish rock that never got very hot and was never involved in any colossal collisions.
But if a rock gets swept up into a larger body, it's in for a rough ride, the larger the body, the rougher. Large bodies have gravity that squishes rock to high pressures; large bodies have radioactive elements that heat it up to the point that the rock melts, obliterating the chondrules.
If all the chondrules have vanished, the meteorite isn't a chondrite anymore -- it's an achondrite ("a" is greek for "not"). There is every metamorphic stage from untouched chondrules to partly re-melted chondrules to almost melted "ghost" chondrules to no chondrules at all.
If the body is even larger, it gets hot enough that the iron that is mixed freely in the rock melts and drips out to the center of the body. You end up, in short order, with a "Tootsie Pop" object -- an iron center, a heavy rock body, and a light rock candy coating.
A body big enough for that to happen is called a planet! (Please, don't anybody start "that" argument.) ALL the rock and metals, everything that is part of a planet, has been melted, at least once, and much of it has been melted over and over again. There may have been countless billions of tons of chondrules in some or all of the rocks that went into making a planet, but they (and everything else) get melted. If you could pick the Earth apart one BB-sized grain at a time, you wouldn't find a single chondrule!
The melting and separation of metal and rock is called "differentiation." The "difference" is that iron sinks and rock floats. Some meteorites come from differentiated bodies -- Mars, the Moon, even little Vesta (HED), chips off the old blocks. Some meteorites, the ones with lots of crisp fresh chondrules, must be chips from a very small block that's been hanging around minding its own business for most of the life of the solar system, waiting four and a half billion years to accidently run into the Earth, fall to the ground, be found by some crazy human who will slice it open and say, "Wow, Look at those chondrules!"