Two overarching factors that make many meteorites so interesting are Content and Source. What’s in it and where did it come from! A third factor would be appearance. Is it neat or beautiful to look at? In Part 2 of this series I’m going to talk about my top five favorite meteorites. I’ll show pictures of my personal samples, but I would encourage you to look up versions on the internet to get an even better appreciation for them. We’ll need to delve a bit into the family tree below the classes of meteorites we talked about in Part 1 as we look at some of these. To use the terminology described in Systematic and Evaluation of Meteorite Classification, we’ll look at Clans and Groups of meteorites under the Class heading of chondrites, primitive achondrites and achondrites. Also, don’t get wrapped up in the names I use, in Part 3 of this series I will talk about the hobby of collecting meteorites and there I’ll discuss how meteorites are named. For now, just accept them at face value. For fun I attempted to rank the five meteorites and do a countdown to my number one favorite. Here they are:
Fifth Place: Octahedrite Iron
Figure 1. – Etched Partial Slice of Seymchan, an Iron IIE Meteorite. (Photo by author)
You can’t talk about meteorites without showing a slice of an etched Iron. My fifth favorite meteorite is an etched Iron IIE meteorite named Seymchan. It was found in Magadan, Russia. What does “etched” mean? Hold onto your socks as I lay it out for you. Recall that the Iron meteorites are an achondrite-class remnant of the core of a differentiated asteroid. While the core is primarily iron, the element nickel is also present. If a high enough nickel remains mixed with the iron, a mineral named taenite forms when the core cools. On the other hand, if the nickel content is low the mineral formed is called kamacite. What is commonly found is a mixture of these two. There are more than a dozen different iron meteorite groups based on varying amounts of nickel content or other trace elements found with the iron, but they all take one of three forms. The driver for the different forms is the amount of kamacite and taenite present in the meteorite. Pure kamacite meteorites take the crystalline structure of hexahedron and so are called hexahedrites. A mixture of the two will have an octahedron pattern and are known as octahedrites. Finally, if the taenite content is high enough then the meteorite lacks a structure and is known as ataxite.
Why am I going into this much detail about nickel versus iron in an Iron meteorite? Here’s the answer. It turns out that kamacite dissolves easier than taenite. When treated with an acid the kamacite will dissolve just enough to highlight the boundaries between the two minerals. This process is called etching. Hexahedrites will only show a pattern of thin crisscrossing lines, looking almost like scratches, called Neumann lines. Octahedrites will have a pattern of wider bands often broken up by patches of narrower bands, called a Widmanstatten pattern. These bands are the taenite crystals which formed as the metal cooled. The wider the bands, the slower the cooling rate the metal experienced. Ataxites will show no pattern.
Figure 2. – Magnified View of the Seymchan Iron IIE Widmanstatten pattern. (Photo by author)
Having given all that background I can describe the Seymchan Iron meteorite sample as an Octahedrite. Look closely at Figure 2 and you can see the thin Neumann lines on the slightly dissolved kamacite, filling the gaps between within the widmanstatten pattern of taenite crystals. The ‘IIE’ designation comes from the system meteoriticists use to differentiate between types of iron meteorites. It’s a code that relates to the percentage of nickel, and the trace elements Iridium and Gallium, found in the Iron matrix.
Why is all this so interesting that I’ve listed the octahedrite meteorite in my top five favorites? Here’s the clincher. At the time the planetesimal disintegrated (ala Alderaan) its core could have been molten or, in the case of very old asteroid, cool and solidified. Some researchers believe it’s possible to correlate the width of the taenite crystals, to rate at which it cooled. This rate can vary with the size of the asteroid. The basic assumption in the analysis is that the larger the asteroid, the longer it took for it to cool and loose it’s molten core. Using this information we can start to build a picture of the size of the earliest planetesimals in the solar system…just by studying the patterns hidden in these meteorites. This is analogous to learning about the nature of giant dinosaurs by looking at the bits and pieces of fossils, except these are space dinosaurs.
Fourth Place: Pallasite
Figure 3. – Symchan Pallasite (Photo by author)
In fourth place I have a different sample of the Seymchan meteorite that falls within a group called pallasite. Figure 3 is the very pallasite sample I purchased in the story I told in Part 1. Sometimes referred to as a “stony-iron” meteorite type, a pallasite is defined as having a mixture of iron-nickel metal and silicate olivine.
Figure 4. – Magnified view of an Olivine pebble with a Iron-metal matrix. (Photo by author)
As I mentioned in Part 1 of this series, pallasites are believed to have originated deep within a differentiated asteroid at the interface between the core and the mantle. Here the local melting of the mantle causes the mineral olivine to precipitate out, crystallize and fall to towards the mantle. Sort of like chocolate chips in cookie batter the olivine crystals partially melt as they mix with the molten metal, leaving roundish shaped pebbles as a result. Just like the chips in our cookies, when the mixture cools the olivine is left suspended with a matrix of metal. When sliced and polished it looks like a miniature stained glass window, and just as beautiful as one. It’s this amazing beauty, and elegant structure, formed in such an exotic and fierce location, that inspires me to list it in my top five favorites. Still, my sample is small and not very colorful so I encourage you to look up more pictures on the internet.
Third Place: Howardite
Figure 5. –Howardite Achondrite Meteorite, NWA 8559. (Photo by Author)
There are a group of Achondrites I find particularly interesting because we think we know where they came from. Scientists studying the light spectra of some of the largest Asteroids in our solar system have discovered that their surface shares properties with some Achondrites groups. The spectroscopic signature of the asteroid 4 Vesta in particular looks a lot like the Achondrite groups Howardites, Eurcrites and Diogenites (referred together collectively as HED meteorites). The signature is so similar, in fact, that it is believed that our HED meteorites actually came from 4 Vesta – blown off in a chance collision that sent pieces of the surface floating through space for millions of years before finally falling to Earth.
Figure 6. – Magnified View of Howardite NWA 8559. (Photo by Author)
Of the three, I find Howardite the most interesting and fun to look at through a microscope. Howardite is actually a mixture of diogonite crystals in a light gray, pulverized eurcite matrix. Eons of pummeling from other asteroids have been broken up, mixed, fused and broken again the surface of 4 Vesta, resulting in the formation of howardite breccias. If you landed on 4 Vesta today and picked up a rock from the surface, you’d probably be picking up a Howardite.
Second Place: Shergottite
Figure 7.- Magnification of Microgram Specimen of Shergottite (Photo by Author)
If you thought the idea of pieces of 4-Vesta finding their way to Earth was wild, you’ll flip over my second favorite meteorite. Shergottite comes from Mars! Somewhere within the past few million years violent impacts to the red planet ejected pieces of the surface in to space. After wondering the solar system for thousands of years those pieces landed on Earth. Makes you re-think those ‘crazy’ ideas about life from Mars infecting Earth…
There is a long list of reasons that scientists have for justifying the claim of this origin, but I’ll share just one. The energy of the initial impact melted some of the crustal rock into a glass that we find in some of the meteorites. Within this glass are trapped bubbles of gas. When extracted and analyzed the gas turned out to have the same composition as the atmosphere of Mars! Boom. These rocks come from Mars!
Mars meteorites are rare, but shergottite is the least rare of the three known types in collections today. You’ll find better pictures on the internet as Figure 7 is a magnification of a tiny spec smaller than a gnat.
First Place: Carbonaceous Viragano
Figure 8.- Carbonaceous Vigarano (CV3) Meteorite NWA 10223. (Photo by Author)
It might be hard to believe that there is a meteorite more interesting than one from Mars. Quite frankly, you might be right if Mars specimens were more plentiful and inexpensive to obtain. That not being the case, I found it a little difficult to get excited about very tiny gnat-sized crumbs which are difficult even to bring into full focus on a hobbiest digital microscope. Fortunately, there is no lack of other interesting types to fill the void. The carbonaceous class has a unique history among meteorites. As the name indicates they contain carbon, but more that these rocks were once packed with water ice. The water ice was heated at some point causing it to seep into the pore spaces and chemically react with the minerals with it. The result was the formation of hydrocarbon molecules. Hydrocarbons are considered the basic building blocks of life. This begs the question as to whether carbonaceous asteroids contributed to the process that eventually led to the evolution of life on Earth. This theory can never be proven, but the tantalizing clues are there. As if this weren’t interesting enough on its own, there’s more.
Figure 9. – Chondrules, CAIs and AOAs within a CV3 Metorite NWA 8577. (Photo by Author)
Figure 10. – Magnification of a CAI in CV3 Meteorite NWA 10223. (Photo by Author)
The Vigarano (where the ‘V’ in CV3 comes from) group of carbonaceous meteorites are unique even within their class because they contain all the building blocks of our solar system. Researchers say that element-wise they closely match the composition of the sun. The fact that they contain chondrules is not surprising, being a group within the chondrite class, but interspersed among the chondrules are two other peculiar ingredients: Calcium-Aluminum Inclusions (CAIs) and Amoeboid Olivine Aggregates (AOAs). CAIs are usually found as bright white patches while AOAs are smaller and dull white to gray, as highlighted in Figure 9. Why someone couldn’t have given them a more romantic name like “chondrules”, I don’t know. They are more amorphous than the spherical chondrites. They are considered ‘sisters’ in that are chemically related, but differ mineralologically. Some researches use the terms ‘accrete’ and ‘condense’ to describe their formation out of the nebula elements. Maybe someday scientist will devised experiments, to take place in the vacuum and weightlessness of space, to finally test and prove how they formed in the infant solar system but until then the mystery remains.
In 1995 an international team of scientists put their heads together in an attempt to develop an age of the CAIs. Their conclusion, backed by multiple dating methods, is that the CAIs are 4.568 billion years old – about two million years older than chondrules. This age leads to the theory that CAIs and AOAs were the first clumps to aggregate out of the nebula destined to evolve in our solar system. They truly were the first baby steps the nebula took towards the formation of something new and incredible. In my minds-eye I have a picture CAIs and AOAs as the equivalent of dust bunnies in space, eventually mixing with chondrules and free dust to form larger and larger masses.
To me CAIs and AOAs represent the Wild Frontier. They are the ingredients from which all else formed. Whereas on Earth the origins of things even as old as a few thousand years are so often erased by time, weather and biological processes the fact that these chondrites were preserved in space and eventually found intact for us to study is remarkable. We can hold in our hands something that predates Earth itself. These meteorites are the remnants of the family that was the ancestor of our planets, and of us.
Summary
There you have them – my favorite top five meteorites: An octahedrite Iron – a sample of a planetesimal core, a pallasite – nature’s stain glass window, a howardite – a sample of the surface of asteroid 4-Vesta, a shergottite – a sample of the surface of Mars, and finally, coming in at number one, a Carbonaceous Vigarano -a glimpse of the birth of our solar system.
To learn more about these, and other types of meteorites I recommend the books Rocks From Space by O. Richard Norton and Meteorites by Smith, Russell & Benedix. Some really helpful, but quite technical, papers are Nebular History of Amoeboid Olivine Aggregates by Sugiura, et al, Formation of Chondules and CAIs: Theory vs Observation by Jones et al, and Systematics and Evaluation of Meteorite Classification by Weisberg, McCoy & Krot.
Stay tuned for Part 3 of this series on Meteoritics. I’ll be taking a look at the hobby of collecting and studying Meteorites.
Time and Space 