Chondrules

AotHM: Eros Part 4

For as long as there has been an S-type, 433 Eros was called an S-asteroid. See: Chapman Morrison Zellner 1975, Zellner Gradie 1976, Bowell et al. 1978, Tholen 1984, and Bus Binzel 2002. A probe to Eros- the closest, best-studied, big S-asteroid- was sought, to get many answers. What are S-types like, and made of? (Implying their history, and the conditions of the early Solar System.) Do we have samples, via meteorites? Which?

(This would be a good time to review meteorites, per my last post. There are stony, iron, and stony-iron meteorites. Stonys are further divided into achondrites (fully molten, once) and chondrites (never molten). A few are primitive achondrites (barely molten).)

S-type asteroids dominate the inner Main Belt, and the planet-crossing zone (that is, near-Earth objects). We backtrack the trails of meteors; they, like Eros’ orbit, lead out to the Belt. One would expect to find S-pieces on Earth. Of the meteorites, stonys dominate irons and stony-irons. Stonys (technically, ‘ordinary chondrites’, OCs) are ~80% of all falls. Yet, the most common asteroids do not look like the most common falls. S-asteroids’ minerals look like stony-irons, only ~1% of all meteorites. This would imply stony-iron fragments are missing or shy, but there’s a secret pool of stonys, bombarding Earth somehow.

The Galileo mission to Jupiter seemed to find an answer. Passing 951 Gaspra (in 1991) and 243 Ida (1993) on its way, it observed younger areas to be a bit bluer and more ordinary-chondrite-like, than old areas. S-types get ‘space weathering,’ a thin coat hiding the bulk minerals (which are OCs). A flyby of course gives a less than thorough view, while chronic Space Shuttle delays made Galileo fly with late ’70s/early ’80s technology… and a busted antenna. The NEAR mission (Near-Earth Asteroid Rendezvous) would orbit an S-type in 1999, using many, modern instruments, and to be sure a rigid antenna. What did it find?

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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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