From the Author: I’m planning for this post to be part one of a four part series on Meteoritics.
When I began the Geophysics MS degree program at the University of Houston my advisor, Dr. Arch Reid, was the only professor working on space related research. His specialty was Meteoritics – the study of meteorites. He asked me at the time if I was interested in that line of research. I told him no, and that my real interest was the study of Mars, and of course that’s what I went on to do my thesis about. I didn’t give his question another thought for some years. Throughout that same time I was also attending rock and mineral shows whenever I could, because of a long standing interest in geology. I even started taking my kids to “rock” shows from the time they were toddlers. During one such trip I stopped at a booth to look at a sale on meteorite samples. I bought a couple affordable samples and placed them on the shelf in my office at work as conversation pieces. I collected a few more specimens over the course of a few years, still never giving them real serious thought. That was soon to change. In 2014 I attended a show and again ended up at one of the few booths that sells meteorites. I decided that I wanted something different this time. I picked up a small sliver of something that resembled a miniature stained glass window and I asked the dealer about it. His answer floored me. He called it a Pallasite meteorite and went on to state, matter-of-factly, that “people believe that Pallasites originate from the interface between the molten core and the mantle of an asteroid.” Wait. Uh… say that again? “They come from where?!”, I stammered. My fascination with planetary science latched onto that concept and a new hobby was born.
An internet search on Pallasites snowballed into reading articles about all of the different kinds of meteorites. The classifying of meteorites has evolved significantly over the past 150 years. Even today it seems that any two books you pick up will describe the groupings a little differently. An awesome and thorough discussion on the classification of meteorites entitled Systematics and Evaluation of Meteorite Classification was written by Michael Weisberg, Timothy McCoy and Alexander Krot. I also learned a lot from Rocks From Space by O. Richard Norton and Meteorites by Smith, Russell and Benedix. I’ll give a shot at summarizing the main types of meteorites and what makes them so fascinating to study.
The stereotypical asteroid in your minds-eye is a mishapen craggy chunk of rock, orbiting around our sun in places like the Asteroid belt between Mars and Jupiter. The picture is true enough, but how did they come to look like that? It’s a history billions of years long, so let’s start with a simple example. Dust drifting in a lazy orbit about our sun is swirled and heated by the passage of other rocks or comets and starts to clump together. These clumps bang against each other, sometimes breaking apart but sometimes sticking together.When they stick together they form larger and larger asteroids that take on a significant gravity field, further increasing in size as they attract all nearby small asteroids. Once sufficiently large enough the center of an asteroid can experience a heating from the pressure of rock and radioactive material contributed by the smaller rocks now stuck together. The heating can be so intense that the asteroid forms a molten core and a heated mantle. Heavy material like iron will gravitate towards the center and lighter material will be left behind in the mantle and the lightest material in the crust. The asteroid is now described as ‘differentiated’. It can also be called a “planetesimal”. Heat from the core will warm the mantle causing partial melting and subsequent chemical rearrangement of the material there. The surface also experiences change as it is bombarded by more, smaller asteroids, churning and crushing rock together. At some point a cataclysmic event occurs where this planetesimal is torn apart in a violent collision with another large asteroid. The molten core, mantle and surface are once again broken apart and thrown in all directions in space, not unlike the demise of the planet Alderaan in Star Wars, Episode IV.
Figure 1. – Iron Meteorite. Heavy, polished piece of amorphous iron (Photo by Author)
Figure 2. – Pallasite, a Stony-Irion Meteorite NWA 7920. This specimen is rough but you can make out the glassy inclusions of quickly cooled olivine in the matrix of darkened, rusty iron. (Photo by Author)
As awesome as that sounds, remnants of differentiated asteroids actually make up only a small fraction of the meteorites we find. The majority of our meteorites are made of rocky material that never coalesced into a differentiated asteroid. They were simply never invited to that party. These undifferentiated meteorites represent the primordial solar system, unchanged for billions of years except for the occasional collision or close approach to planets or comets that may briefly heat them some. They are samples of the original nebula that formed our solar system, and you can hold…it…in…your…hand! I had failed early on to grasp the magnitude of this fact. When my advisor asked me about studying meteorites I was also ignorant of another amazing fact: Meteorites are not just rock! Their composition is completely alien to anything we find today on Earth.
Figure 3. – Chondrite Meteorites slices from SAU 001 Oman, NWA 8263, and NWA 6338. The chondrule spheres are clearly visible in these specimens. (Photos by Author)
Of all the meteoritic terminology to learn, the term ‘chondrule’ arguably should be one of the first. Chondrules are tiny, glassy spheres believed to have formed in the primordial dust cloud. There are a number of theories about the details, and no one is certain how it occurred. Theories range from lightening in the nebula cloud melting dust clumps to our sun melting and blowing away the dust in the first days after its birth. Chondrules range in size from nearly microscopic to several millimeters across. A majority of our meteorites either contain chondrules, or the mineralogical signature of chondrules, compacted in a matrix of other elements and minerals. This feature has earned them the meteorite classification called the Chondrites. They are so common among all the meteorites finds that many came to be known as “Ordinary Chondrites”. Often flecks of iron and nickel will also be found in the matrix. In fact, so much metal may be present in the matrix that a good check to verify a suspected meteorite is to see if it will be attracted to a magnet. Where did these flecks of iron and nickel come from? They are merely the direct remnants of supernova stars. Nothing special about that, right?
Figure 3. – Achondrite, Eucrite NWA 8555. This type is actually a monomict breccia, the outlines of the components are faintly visible. (Photo by Author)
To discuss the remaining classes of meteorites we return to Leia’s doomed home planet of Alderaan, better known in our discussion as the differentiated planetesimal. These classes consist of the original chondrite material having undergone structural and chemical changes from heating and compression in the planetesimal. The Meteorites that no longer contain chondrules, or even the mineral content of chondrules, are called Achondrites (inventive name, right?). This class includes groups that have nearly 100% iron and nickel content often called Irons, and Stony-Irons which are a mixture or rock and metal. The iron meteorites came from where else, but “Alderaan’s” core! Yes, I said that correctly. The iron meteorites we find on the surface of the Earth originated in the molten core of the infant planetesimal before it was destroyed by the Empire. Stony-Irons originate from the lower mantle that mingled with the core. It is here that the pallasite, which I did end up buying from that dealer at the show, came from. I’m going to hold off on showing a picture of that particular pallasite until my next post in this series where I’ll talk about my favorite meteorites.
Figure 5. – Primitive Achondrite, Winonate NWA 725. Sorry for the dark picture, my specimen is small. (Photo by Author)
If you are starting get a sense that all meteorite classes are related, you’d be right. As you move out from the core to Alderaan’s surface, you encounter chondrite material undergoing various amounts of change. Transitional states between chondrite and achondrite form the meteorite class called Primitive Achondrites (again, quite inventive naming). Primitive Achondrites are rare finds. This group is accepted as chondrites which have undergone chemical changes such that they exhibit properties nether completely like chondrites or achondrites.
The benefits to the field of planetary science from the study of meteorites has been, and continues to be, astounding. If we hadn’t realized that meteorites weren’t just another type of Earth rock, our knowledge of the formation of our solar system would be sorely lacking. The puzzle pieces “fall” into place as you look at the varying meteorite types, and I’ve only scratched the surface in this review. In each of the classes I’ve described there is a family tree of sub groups and types that Meteoriticists use to describe differing elemental and mineralogical content. After buying the pallasite meteorite, and subsequently learning more about its origin, I scratched the surface deeper and the more I read the more exciting the field of meteoritics became. In Part 2 of this series I’m going to talk about some of my favorite meteorites. There I will try to pick up a little more of the details that I didn’t go into here. Hint: Some meteorites carry other ancient nebula material that is believed to be even older than chondrites and some meteorites actually came from another planet!