Own a piece of the sky.
A LITTLE BEFORE MIDNIGHT ON the evening of March 26th an incoming meteoroid * about the size of a car broke apart in the upper atmosphere, showering debris all over the Chicago suburb of Park Forest and surrounding areas and damaging at least six houses and three cars (July issue, page 25). More than 18 kilograms of meteorite fragments have been recovered so far.
Although the odds of getting hit directly by a meteorite are quite slim, the fact is that an estimated 200 tons of other worlds fall to Earth every single day. Cosmic debris rains down on our planet's surface in the form of interplanetary dust particles, micrometeorites, and chunks of space rock that can be found just about anywhere. Meteoriticists estimate that each year nearly 24,000 stones ranging from a few grams to many kilograms arrive on our planet, a bombardment rate of about 40 meteorites per square kilometer of land surface over a period of a million years. That's a lot of space junk just lying around, waiting to be picked up! If you can find a meteorite or purchase one from people who collect these cosmic flotsam for a living, you'll be the proud owner of a chunk of rock that was once part of an asteroid or even the Moon or Mars.
Thousands of meteorite specimens have been found in Earth's two most productive hunting grounds--the Sahara desert of northern Africa and the ice fields of Antarctica. But meteorites can be found practically anywhere, and anyone with a little training, patience, and desire can collect space rocks in their own localities or by collaborating with other hunters, collectors, and dealers over the Internet.
Meteorites are named for the geographical location nearest to where they are found. Finds are specimens that are discovered unrelated to any specific event, whereas falls are associated with a witnessed fireball or impact. For example, in 1999, of the 22,507 known meteorites listed in the Catalogue of Meteorites (5th edition), 21,502 were finds and 1,005 were falls. Falls are usually favored by collectors because of the documented facts concerning the rocks' arrival on Earth.
One of the most famous falls of recent times occurred in Peekskill, New York, on October 9, 1992, after a widely witnessed (and photographed) fireball broke into fragments. One piece of this stony meteorite hit a parked car and netted the owner a tidy sum for both the car and the 12-kg impactor (S&T: February 1993, page 26). Two other well-known and well-studied events are the June 30, 1908, multimegaton Tunguska blast (S&T: June 1994, page 38) and the February 12, 1947, Sikhote-Alin impact (S&T: February 1997, page 50). These events are still undergoing intense study by researchers around the world who refine and assess the threat of meteorite impacts on Earth.
There are three basic types of meteorites --stones, irons, and stony-irons--names that accurately describe their gross composition. Of the meteorites listed in the Catalogue of Meteorites, 21,514 were stones, 865 irons, 116 stony-irons, and 12 unknowns. Stones are further subdivided into chondrites and achondrites. Chondritic meteorites come in many varieties, including ordinary chondrites (comprising about 85 percent or more of falls), carbonaceous chondrites, enstatite chondrites, and rumuruti chondrites.
The overwhelming majority of chondrites contain spherical structures called chondrules (from the Greek word for "grain") and are thought to have formed around dust remnants from the primordial solar nebula that underwent heating and formed the earliest structures of our solar system. Chondrules are fascinating because of the myriad sizes, colors, and mineral compositions that define their formative environment. Chondrites also have varying amounts of nickel-iron alloy (as much as 23 percent) in their matrices that help in their overall classification.
Achondrites (which lack chondrules) are prized by scientists and collectors alike because they come from the Moon, Mars, the asteroid Vesta, and other geologically differentiated bodies. In differentiation a parent body's magma (molten rock) has stratified or separated into layers to the point of having an iron core, a core-mantle boundary, and a crust--just as Earth does. Over the eons cataclysmic collisions with comets and other asteroids have shattered these parent bodies. Once blasted into interplanetary space, their fragments drifted aimlessly for millions of years before they were swept up by Earth's gravity, entered the atmosphere, and landed on the ground. Thus, iron meteorites represent fragments excavated from the core while stony-irons are believed to have originated from the surrounding core-mantle boundary. Achondrites came from the crust.
Visually, iron meteorites fit a layperson's general perception of what a "space rock" should look like. They are heavy for their size, and they just look alien--large, rounded depressions or "thumbprints," called regmaglypts, can cover a sizable portion of the specimen's surface. Depending on the amount of nickel they contain and the ratio of nickel to iron, iron meteorites consist of three main types: hexahedrites, octahedrites, and ataxites. These properties and the ratios of trace elements determine the internal structure and hence the type of iron meteorite.
Stony-irons have three possible classes: pallasites, mesosiderites, and lodranites. The first two are more prevalent than the last, but pallasites and mesosiderites are not abundant, and there are only two known samples of the latter. Arguably, the most visually stunning meteorites are the pallasite stony-irons, especially the Esquel or Imilac specimens, which have a high ratio of translucent olivine crystals to the surrounding nickel-iron (see page 80).
Collecting Rocks from Space
Unless you're lucky enough to have a meteorite come through your ceiling or fall at your feet (as happened in Park Forest and elsewhere), there are only two ways to acquire them: hunting or buying.
To hunt for meteorites you have to know what you're looking for and have a method for distinguishing "extraterrestrial" rocks from terrestrial ones. Meteorites aren't called such until they have been properly classified and their data published by scientists. This can't be done in the field. The best you can do is identify your candidates to separate the "meteor-rights" from the "meteor-wrongs."
Before starting your hunt, try to view as many meteorites as possible so you can get an idea of what your quarry looks like. Going to museums or surfing the Internet (see the recommended Web sites on page 81) will give you plenty of specimens to view, but there's nothing like holding and feeling meteorites to get familiar with them before heading out in the field. To do this you'll probably have to buy a few relatively inexpensive specimens, unless there's an academic institution nearby with a collection you can view and handle under supervision.
The Internet has revolutionized meteorite collecting because it has made specimens from reputable dealers available to the general public. While buying meteorites from eBay is an option, it's worth getting to know individual dealers, as it's not uncommon for them to give discounts to steady customers and provide requested specimens and weights. As with any sales transaction, buyers must exercise caution as to whom they're dealing with, so here are a few questions to consider when looking for meteorite dealers: Are they a member of the International Meteorite Collectors Association (IMCA)? Do they guarantee their specimens' authenticity and sales? How do their prices compare with those of other dealers? It definitely pays to shop! Preparing the specimen plays a big role in the final cost. A sliced specimen of the Etter stone meteorite may sell for $4 per gram, but give that same specimen a high polish and that will push the price to $8 per gram.
Achondrites are usually the most expensive because of their rarity and place of origin. Their prices can range anywhere from tens to thousands of dollars per gram. Stony-irons come next at about $20 to $50 per gram, followed by stones and irons at about $1 to $10 and $1 to $2 per gram, respectively. For your first acquisitions, I recommend purchasing individual (whole) specimens of an ordinary chondrite and an iron. You can also buy fragments if your budget is limited, but individuals give you the best representation of what you'll be looking for in the field. Fragments can be useful, however, if they show a significant portion of the meteorite's exterior as well as the interior matrix. Over time you can build a nice collection of meteorites that suits your budget and interest.
The Hunt Is On
OK, so you've bought or swapped for some real meteorites, studied them, and done some reading. Where do you go to look for these rocks from space? The best place is where meteorites have been found before--strewnfields. If you're lucky enough to live near such a site or will be traveling near one, it's worth investigating the area. Searching strewnfields gives one a sense of wonder, especially if you know the circumstances surrounding the meteorites' fall. If there are published eyewitness reports, read them and try to imagine what it was like to be there when rocks fell from the sky. Seeing the same landmarks described by eyewitnesses and then carefully walking and searching the ground is a very exciting experience.
Known strewnfields are not common. But dry lakebeds, vast desert expanses, or locations that have few exposed rocks are also good places to look (rock-filled areas or those covered with thick vegetation can hamper efforts significantly). But remember, meteorites fall anywhere and everywhere, so even if you're far from any of the prime areas, it still pays to look wherever you are. Be sure to determine the legal requirements to conduct your search--is it private or public-owned land? You don't want to trespass on private property, and taking meteorites found on such sites has spawned numerous lawsuits.
Before venturing out into the field, be sure to wear comfortable clothing and shoes, and bring plenty of drinking water, a first-aid kit, map, GPS, cell phone, camera, and so forth. You'll also need a small diamond file, a powerful magnet attached to the end of a telescoping rod, and a 10x magnifier. The diamond file makes a small cut into the matrix of your candidate, and the magnifier helps you get a close-up view. The telescoping magnet saves you from back strain when you're checking hard-to-reach rocks for magnetic attraction.
Remember this checklist when identifying possible meteorites: Heavy, Appearance, Magnetic (HAM). Most samples contain iron and nickel, which makes them heavier than terrestrial rocks of the same size. They also can have roughly spherical, elliptical, angular, or cone shapes. Stone meteorites are generally smoother than irons, while stony-irons tend to have the roughest surface of all. After making a few small strokes of the file, examine the sample's exterior and interior for the presence of chondrules or shiny metal grains. Chondrules are not found in terrestrial rocks (as the Earth underwent complete melting and differentiation during its formation), so if they are present in your sample, you're really onto something. Finally, check it for any attraction to your magnet.
Of the three properties in HAM, appearance has more items you can check against, but in each instance all three should be used together to make a final determination. You may have to vary which property to check first because of the circumstances under which you found your rock of interest. In a rocky environment, checking for magnetic attraction first would be a logical step. But beware, as most achondrites and some ordinary chondrites have a weak magnetic attraction at best, whereas their appearance may be more telling. The appearance of rocks can provide vital clues and may be more useful in an area with many candidates. Using a trained eye (as well as binoculars for longer range) one can cover more ground in a given amount of time than by testing each rock for magnetic attraction.
Another Identifying Feature: The Fusion Crust
As meteoroids enter the atmosphere at high speeds, aerodynamic friction causes their outermost layer to melt, forming a fusion crust. This crust is usually black, but it can be light colored or shiny black in rarer types of stone meteorites. The color of most stone fusion crusts is not unlike that of charcoal briquettes, while irons look like freshly welded metal. The crust may not be uniform in appearance due to the formation of a thinner secondary crust caused by the breakup or collision with other meteoroids during flight. The presence of a fresh-looking fusion crust means that the specimen arrived fairly recently on Earth, but a weathered (eroded) fusion crust can still be discernible on older specimens. It may be black to light brown in color and have fine cracks in the crust. Very weathered meteorites will probably have no crust left and may be split by deep cracks.
If you get positive "hits" on all three HAM properties, and especially if there's a fusion crust and if chondrules and/or metal grains are visible, then you have a strong candidate that warrants formal testing in a laboratory. Many meteorite dealers as well as certain academic institutions will test a small sample for you. They may or may not charge a fee for such testing, so to avoid misunderstandings, negotiate any costs up front and get them in writing. If the specimen you submitted turns out to be a genuine meteorite, it will have to be classified and its total known weight determined before its monetary value can be assessed. Again, negotiations should be finalized in writing with any parties you contact.
Collecting meteorites is a unique and personal way to expand your knowledge of the universe. Holding these space rocks in your hands and studying them on a cloudy night or right after gazing at the sky allows you to connect to the cosmos and return to the creation of our solar system itself.
Meteorite Resources on the Web
International Meteorite Collectors Association (www.meteoritecollectors.org)
Natural History Museum's Meteorite Catalogue (http://avalanche.nhm.ac.uk/cgi-bin/earth/metcat/)
New England Meteoritical Services (www.meteorlab.com)
Meteorite magazine (www.meteor.co.nz)
The Meteoritical Society (www.meteoriticalsociety.org)
* A meteoroid is a piece of rock or dust from space that produces a brief, brilliant streak or flash of light, called a meteor, as it enters Earth's upper atmosphere. Meteorites are meteoroid fragments that survived passage through the atmosphere and are recovered on the ground.
A member of the International Meteorite Collectors Association and the Meteoritical Society, GREG REDFERN (email@example.com) is one of this year's NASA Solar System Ambassadors (www.jpl.nasa.gov/ambassador).
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|Publication:||Sky & Telescope|
|Date:||Aug 1, 2003|
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