Meteorites - Ambassadors from Space

"Meteorites - Ambassadors from Space" by Michael Gilmer.

 
Summary - this brief article explains what a meteorite is, where they come from, the major types of meteorites, and how to collect them.

    It was by chance that I acquired my first meteorite. It was around Christmastime of 2006 and I was shopping for last-minute stocking stuffers for my wife and kids. Even though I am the only hardcore astronomy nut in the bunch, the rest of my immediate family has always been interested in space-related subjects. So, when I ran across an online ad for cheap meteorites, I took notice. These meteorites were hardly impressive to look at. They were tiny pebbles about the size of a dime, but they were priced just right at five bucks each. At first, I couldn't believe that one could own a meteorite so cheaply and easily. So using Google, I did a little homework on meteorites and the seller - to make sure the offer was legit. I soon discovered that although meteorites are considered quite rare, they are actually more available than most people think. The Earth is a large target whipping around the Sun in space, and it gets struck by meteorites at a constant and steady rate - like bugs hitting the windshield of a moving car. Most of these meteorites fall into the oceans which cover the bulk of the Earth's surface. Many others fall into inhospitable areas where they are unlikely to be found - jungles, mountaintops, forests, etc. However, a good number of meteorites land in areas where they are easily identified and collected - snowfields, deserts, dry lake beds, and plowed fields. In areas such as those, a strange looking rock tends to stand out like the proverbial sore thumb, and someone with the right knowledge can easily spot the meteorite and collect it. Not only do meteorites fall regularly (and get found regularly), but there are also dozens of types of meteorites with some more rare and interesting than others. Just like collecting conventional Earth-rocks, there is a thriving culture of hobbyists who collect and enjoy meteorites.

    Collecting meteorites is related to the field of meteoritics (the study of meteorites), and it is a branch of planetary science that is not only accessible to the layman, but the layman is actually encouraged to participate. While I was waiting for my first meteorites to arrive in the mail, I did a lot of reading up on them and by the time the mailman finally brought my space rocks, I was excited like a kid in anticipation of Christmas morning. When the package arrived, I eagerly tore it open and saw a handful of small brown pebbles that resembled pieces of dog food. They were heavy for their small size, very smooth, and strongly attracted to a magnet due to their high metal content. But the most fascinating aspect of these unassuming little stones was their origin. In my hand, I was holding a rock from outer space that was as old as the Earth itself. These meteorites had formed out of the solar nebula around the same time that the planets had formed - over 4.5 billion years ago. Long before the first single-celled organisms had developed on an infant Earth, these stones had coalesced in the hard vacuum of space, and there they had circled our young sun as small members of our planetary family.

    So, how did this chunk of space rock end up in my hand? At some point in the evolution of our solar system, a number of large asteroid-type bodies coexisted with our adolescent planets. Circling around the sun in a chaotic dance of creation, these bodies would cross each other's orbits and eventually some of them collided. These collisions were violent and they tore apart the parent bodies of the meteorites, smashing them to bits. These tiny bits (meteoroids) of debris continued on in their own orbits until a combination of fate and physics led them to cross paths with Earth. They were captured by Earth's gravity and pulled down through our atmosphere for a fiery descent. Many meteors will burn up completely in the violent passage through our dense atmosphere. But some were large enough and hardy enough to survive the trip and they landed on the ground intact.

    Once a meteoroid enters the atmosphere, it becomes a meteor - that familiar shooting star streak of light that kids make wishes on and adult stargazers delight in seeing. And once that meteor hits the ground, it becomes a meteorite. This rain of falling meteorites has continued, unabated, for billions (and billions) of years. The deserts and snowfields of the world are literally peppered with meteorites that have fallen to Earth. Many oxidize away and are never found because the Earth is not a meteorite-friendly environment. In space, there is no oxygen, moisture, or wind, so the meteorites exist in a nearly-perfect state of "deep freeze" where they are protected from the effects of weathering. Once the meteorites are exposed to Earth's air, wind and water, they begin to break down. Stony-type meteorites will simply weather away into their base components and will eventually lose their identity as meteorites. Iron-type meteorites will likewise rot into a pile of rusty iron shale fragments. If the meteorites are not collected and preserved, they are simply lost forever.

    Any article that would attempt to explain all the various types of meteorites, how they were formed, and what they are made up of, would entail far more space than I have here. Indeed, entire libraries of books have been written on the subject, and much is still unknown or under debate about meteorites. Here I will attempt to briefly detail the major types of meteorites and what makes them interesting to science and the layman collector.

    There are 3 main types of meteorites - stony, iron, and stony-iron. Stony meteorites are exactly what they sound like; they are rocky combinations of minerals that could be easily mistaken for an Earth rock if not for their peculiar properties.


    Iron meteorites are pieces of solid metal that are very dense and contain high concentrations of the element nickel. Imagine something as dense as a piece of a railroad track and you get some idea how heavy iron meteorites are for their size.

 


    Stony-irons are a combination of the two types - part stone and part iron, mixed together into a single mass.

 

    While stony meteorites are more common, iron meteorites are more resilient, tend to resist weathering, and survive impact more often than some stony types. In addition to these 3 major types of meteorites, there are many sub-types and variations which would fill up an encyclopedia (see Richard Norton’s Encyclopedia of Meteorites for a complete treatment of all types). Some of the most interesting of the rare types are the so-called "planetary" meteorites. Planetary meteorites have been determined to originate from large familiar parent bodies, like the Moon and Mars. Yes, you read correctly - some meteorites are actually fragments of our Moon and the planet Mars! How is it possible that a piece of the Moon or Mars ended up on Earth? In fact, most of the craters you can observe on the Moon were caused by meteorite impacts!   A long time ago, a large meteorite that was a mile across (or bigger) slammed into the surface of the Moon or Mars. The resulting catastrophic impact and explosion hurled large amounts of material upwards which escaped the feeble gravity of these planets and escaped into space. These fragments then floated around in the solar system until they crossed paths with Earth's orbit and were captured by our gravity. In this manner, small pieces of other worlds can actually land on Earth! These planetary meteorites are the rarest of the rare and are highly desired by collectors and scientists alike for what they can teach us about the evolution of planets, including our own.

    Now I would like to address the obvious question - how do we really know it's a meteorite? I get that question all the time, mostly from people who are curious about meteorites, but have no prior experience with them. This is a relatively easy question to answer because meteorites possess some peculiar properties that are not present in Earth materials. First and foremost is the content of elemental nickel. Pure nickel is exceedingly rare in the Earth's crust and the vast majority of the Earth's nickel is found deep down in the core of our planet. The nickel that is present on the surface of the Earth (or near the surface) is contained in combined ores and not in a pure elemental form. Pure nickel must be extensively processed out of the raw ore, and one just doesn't find Earth rocks or minerals with pure nickel in them. Any "rock" that has high nickel content is automatically a meteorite suspect. One can easily determine a sample's nickel content by doing a simple chemical reaction test. Another hallmark of meteorites are "chondrules" - which are small spheres of various minerals which are embedded in the body (or matrix) of the meteorite. These chondrules are distinctive and unlike any Earthly formation of minerals. On a sliced meteorite, chondrules will appear as little circles embedded in the rocky matrix. This is the result of the saw cutting through the spherical chondrules - revealing them in cross section. Chondrules are the product of processes that are not present on Earth, so if one has a rock with chondrules in it, then it's a meteorite. There are other litmus tests as well, including specific gravity, and the degree of magnetic attraction.

    As recently as the 1950’s, the exact origin and nature of meteorites was under hot debate, but after decades of scientific study, meteorites are now well-understood and can be easily distinguished from earthly materials. But how do we know a Martian meteorite is really from Mars or that a lunar meteorite is really from the Moon? The answer is again found in chemical and physical analysis. Thanks to the astronauts of the Apollo program, we have actual Moon rocks that were brought back from the surface of the Moon. Comparing these known Moon rocks against suspected lunar meteorites allows us to determine not only that a meteorite came from the Moon, but we can also determine if the meteorite came from the low-lying lunar basins or the mountains of the lunar highlands - because each have their own chemical and physical signatures. In the case of Martian meteorites, we can compare their chemistries against the known chemistries of Martian materials that we have analyzed with remote Martian rovers like Sojourner. There is now no doubt such meteorites are truly pieces of other worlds.

    Now that we have established what meteorites are and how we determine their origins, let's examine how the meteorite finds its way onto the open marketplace. Most of the meteorites on the collector market come from the deserts of North West Africa. These meteorites are called "NWA" meteorites and the bulk of them come from the Saharan desert. Any rock lying out in the middle of the open desert by itself is automatically suspect. In areas such as the Saharan, where native rock is either absent or highly distinctive, a curious looking rock that is out of place must have fallen from the sky. In some cases, the meteorite is actually seen to fall to Earth by eyewitnesses - these meteorites are called "witnessed falls". Conversely, when someone stumbles across a meteorite, recognizes it as unusual and picks it up, that is called a “find”. This is mostly done by Bedouin nomads and other desert dwellers that are intimately familiar with the sand dunes and know something special when they see it. They collect these stones from the open desert and bring them back to marketplaces in Morocco or large rock shows like the one in Tucson every year. In these marketplaces, Western collectors and dealers comb through the offerings and purchase promising specimens to bring back to the states or Europe. Sometimes these meteorites are relatively-common types and they are sold as unclassified NWA meteorites (UNWA) - this means they have been identified as meteorites due to their properties, but they have not been formally analyzed by a laboratory. If a particular meteorite seems promising as a potentially rare type (like a Lunar or Martian), then a piece of it is sent off to a respected laboratory for analysis. This analysis is intensive and can takes months of study, which includes thin-sectioning and study with scanning electron microscopes and other instruments to determine its exact chemical composition. Once analyzed, the meteorite can be compared to other known meteorites and then it is officially "classified". A classified meteorite is given a name and/or number in the official catalogue of the Meteoritical Society. Classified meteorites typically have names like "Bassikounou" which means that particular meteorite was found near Bassikounou Mauritania. Meteorites, whose exact places of discovery are known, are given place names such as the nearest post office or permanent landmark. Meteorites whose exact point of discovery cannot be determined (because they were moved and then sold at a marketplace) are given catalogue numbers like NWA 869. Generally speaking, since meteorites which are seen to fall (witnessed falls) are rarer than found meteorites (finds), witnessed falls are more valuable to the collector.

    At some point, the meteorite is offered for sale on the marketplace by the owner, finder, or dealer. Of course, the internet is a great boon to collectors and there are a wide variety of meteorites available from dealer websites, eBay, and collector groups. There are some frauds (like most hobbies that involve valuable collectibles) and there will always be someone trying to pass off a fake or misrepresented meteorite. Since meteorites are rarer than diamonds or gold, one must be a savvy buyer to avoid possible scams. The best way to ensure that a meteorite being offered for sale is the real thing is to check the credentials of the seller. Credentials to look for include membership in the Meteoritical Society and the IMCA (International Meteorite Collectors Association). The IMCA is group of collectors and dealers who maintain a code of standards and ethics for selling or trading meteorites. The Meteoritical Society is the official governing body of meteorite study and it is comprised of scientists, collectors and dealers. It is the Meteoritical Society that publishes the official classifications of all meteorites. Both of these organizations are generally a safe bet to buy legitimate meteorites from members. If purchasing from a marketplace like eBay, use a good measure of common sense and examine the seller's feedback ratings. Prices for meteorites on the open market can vary widely from dealer to dealer. Unlike gold or diamonds which have a regulated market with going rates, the price of meteorites is dictated solely by supply and demand. One dealer may charge $20 for a given meteorite and another dealer may be selling an identical meteorite for $100, so it pays to shop around and compare prices. Assuming one has purchased a meteorite, what does one do with it?

    Meteorites are fascinating for what they can teach us about the solar system because they are 4+ billion year old ambassadors of outer space. It's a thrill to hold something older than the Earth in our hands. Meteorites can be displayed, stored, and shared like any other valuable collectible. What's more, meteorites make great outreach props for astronomy. Children absolutely love meteorites! Give a child a meteorite to hold and tell him/her it's a piece of a shooting star and watch his/her eyes light up with wonder and delight! And at this point, I think I should warn our readers that collecting meteorites is much like eating potato chips - you can't have just one. It's a very addictive hobby that meshes well with astronomy, telescopes and stargazing. After all, one can look at the planets through a telescope, and then actually hold a piece of one!

    Meteorites should be cared for like any other rare collectible - they should be handled with care and properly maintained. This means keeping them away from moisture and sources of oxidation which might damage or degrade the meteorite. After handling a meteorite, it should be wiped clean with a dry cloth and stored in a dry area. A Tupperware container with a packet of desiccant to absorb moisture is an ideal way to store meteorites. With proper care, a meteorite will last for many generations to come.


    So, do you want to learn more about meteorites and how to collect them? I strongly recommend two books and one magazine for those who want to do their homework on meteorites. The first is Rocks from Space by O. Richard Norton. This is the definitive book in the field of meteorites and it covers every fundamental aspect of meteorites and collecting. It is understandable to the layman and is not laden with dense math and complex terminology. The second book is Cosmic Debris by John G. Burke. This is a more serious work for the advanced enthusiast and it covers the history of meteoritics from its very beginnings hundreds of years ago to the latest theories today. Cosmic Debris also includes a fascinating chapter on the folklore of meteorites and describes how meteorites were viewed by primitive people, including the legends that surrounded these objects hurled from space. Unfortunately, Cosmic Debris is no longer in print, but used copies can be found from online book dealers. Last but not least, there is Meteorite Magazine, which is the only periodical dedicated solely to meteorites and their study. It is a quarterly publication of the Arkansas Center for Space and Planetary Sciences and it is loaded with informative articles and beautiful photos. The interested reader will find that meteorites are much like astronomy in general - the people who are interested in these fields are very friendly and approachable folks who love to share their knowledge about space.   If after reading this, you still have questions about meteorites, feel free to contact me via email and I will do my best to answer them for you. :) 

(c) Copyright 2008, Michael W. Gilmer