by Robert Beauford
What are Meteorites?
'Meteorites' are rocks or bits of metal that fall from space. We see these objects frequently as meteors burning up as they enter the atmosphere. Meteors are only called meteorites if they actually fall to the ground. Far less than 1 in 10000 visible meteors, or shooting stars, results in a meteorite being left on the ground. All of the rest completely burn up in the atmosphere before they can reach the ground. Many of those that do reach the ground, however, break up in the air, and leave many fragments scattered over areas ranging from a few yards to a few square miles. The area over which the pieces from a single meteorite are scattered is called a 'strewnfield'.
Meteorites are the oldest objects on earth. Most of them originated shortly after the beginnings of the solar system, about 4.52 to 4.567 billion years ago, and have remained essentially unaltered since then. As such, they tell us a great deal about how our planet, the sun, asteroids, and all of the other matter in our solar system came into existence. Meteorites have recently acquired additional importance to science as we have begun to establish our initial footholds in space. These fragments of remote celestial bodies are our best opportunities to study future sources of oxygen, water, minerals, and metals necessary for space travel and colonization. Meteorites are also some of the rarest objects on earth. There is far less total known meteorite material on earth, counting all known varieties, than there is gold or platinum, or emeralds or rubies, or sapphires
Most meteorites originate as the bodies of asteroids, or large pieces of rock and metal, that orbit the sun primarily between Mars and Jupiter. The sun and planets are made up of the same materials as the asteroids, but the minerals that make these larger bodies up have been completely altered by heat, pressure, and atmospheric weathering.
Before there was a solar system containing the earth (about 5 billion years ago) this part of space was a nebula, or cloud of hot dust and gasses resulting from the decay and nova explosions of earlier suns. Gravity and turbulence caused clumps of this gas and dust to draw together forming new stars. Heat and violent winds, blowing out from our newly forming sun, caused droplets of matter to form among the dusts and gasses, much like raindrops form in earth's water clouds. We call these droplets chondrules. Metals, mostly iron and nickel, also condensed in this cloud as small flakes, much like our snowflakes. These small rocky droplets known as chondrules, the metal flakes, and bits of dust were attracted to each other, by mutual gravity, in larger and larger groupings until they began to form masses meters or even kilometers across. The largest of these masses, known as proto-planets fell into relatively steady orbits around the sun due to their size. Smaller masses continued to be blown and hurled around by the influences of impacts, solar winds, and gravities of these larger masses. Each of these protoplanets continued to sweep up dust and smaller objects around them, and occasionally, they even ran into each other, creating larger and larger masses.
Within a billion years of the suns formation, over 99.9 percent of the matter in the solar system had fallen into the sun or onto the surface of these largest proto-planets. These few largest survivors are known as our planets and moons. Between Mars and Jupiter there is a large space, roughly equivalent in width to twice the distance between the earth and the sun. In this space, and to a lesser extent in the spaces between the other planets, there were no remaining bodies large enough to sweep up the many smaller rocks that formed and settled into stable orbits. Neither Mars, nor Jupiter has a powerful enough gravity to pluck these rocks from that vast gap. Most of our meteorites come from this area, known as the asteroid belt.
The process of the largest objects in the solar system sweeping up the smaller objects and becoming larger and larger continues today, although on a dramatically smaller scale than happened early in solar history. The Earth increases in mass by about 35,000 to 100,000 tons a year due to a steady rain of this captured matter. This is roughly the equivalent of a small mountain every 100 million years. Most of this matter is in the form of dust and micrometeorites, however, and it is difficult to capture and study. Very rarely, an object large enough to see with the unaided eye, a meteorite, is swept up with these dust-sized particles.
The frequency with which meteorites fall decreases strikingly as the size of the meteorites increase. Dust sized particles fall regularly on every home in the world. Particles of a gram (about the size of a small pea) or more in weight, however, are estimated to fall at a rate of less than 8 per square mile per year. Similarly, objects over 10 grams (about the size of a quarter) fall at something less than the rate of 1 per 1000 square miles per year. As the size of the objects get larger, the rate of fall becomes exponentially smaller, so that we can expect that an object over 1 kilo (2.2 lbs) might fall in a given 1 square mile piece of land only once in every 50,000 to 100,000 years. Estimates of fall rates vary widely, and the above numbers may be overly generous.
Types of Meteorites
All of the colliding, along with the process known as radioactive decay, generated a lot of heat in the proto-planets and planets of the early solar system. Our moon is an example of the types of heat that can be generated by collisions. Earth was struck by a proto planet at one point in its early history, and much of it became molten from the energy of the impact. The moon was flung, as melted matter, from the surface of the earth during this violent impact. This occurred about 4.45 billion years ago, when the earth was a mere 50 million years old. The planetoid that caused this catastrophic impact was about the size of Mars, that is, about 1/2 earths diameter, and weighing only 1/10'th as much as the earth.
Although a lot of heat can be generated by impacts, the heat generated from radioactive decay is much more important in understanding meteorite types. Heavy radioactive elements can only be made, in nature, by the action of a super nova or nova type explosion of a star. These elements were abundant in the nebular cloud from which our solar system condensed. Over time, all radioactive elements decay into inert stable elements the way uranium turns to lead. This happens at a predictable rate, but the process can drag out for billions of years before all of the radioactives finish decaying. Today, we still have some radioactive elements on earth, such as Uranium, but in the early history of our planet, before it had time to decay, there was much more of this type of material. The decay of radioactive matter creates heat. In the early days of the solar system, when there was a lot more radioactive matter, it created a lot of heat. In small bodies, the heat can radiate out into space as fast as it is generated. In larger dense masses that are many miles in diameter, however, such as large asteroids, planetoids, and planets, the heat builds up faster than it can escape. The size of the objects insulates their interior so that they just get hotter and hotter. It can take 1000's or billions of years for an object of this size to cool significantly, even after major impacts have ceased and the majority of the radioactive material has decayed and is no longer producing heat.
In the early solar system, as bodies swept up more and more matter, and began to grow in size to planetoids and planets, the heat of radioactive decay inside them built up until they began to cook, or in larger examples, until they completely melted. When these large objects melted, the metal flakes combined and ran to the center because they were heavier, while the silicates, the minerals that made up the dust and most of the chondrules, floated on the surface. This process of separation is known as differentiation. We can see the effect today in our own planet, where the center is a core of dense iron and nickel, and the crust is mostly dirt and rock, and contains very little metal except for that which is thrust up from the inside by volcanoes. The exact same thing happened in protoplanets, large asteroids, and moons. This melting and separation process gives us 3 of the 4 major types of meteorites: 1) achondrites, a fancy word for the crusts of ancient asteroids, 2) mesosiderites and pallasites (the stony-irons), which are a mix of metal and pieces of silicates from where the lighter and heavier elements didn't completely separate, and 3) irons, pieces of the iron-nickel cores of these giants early moons and planetoids.
The 4'th major type of meteorite is known as chondrites, which are simply pieces of objects that were too small to build up enough heat to melt. Chondrites are so named because of the presence of visible circular chondrules of about 1mm to 1cm in diameter that can be seen in their interior. These chondrules are important because there are few other objects that have not been melted or otherwise destroyed since they formed in the early solar system. Only in the remnants of very small asteroids are these original components of all matter in the solar system still visible. Chondrites tell us the most about how our solar system started out because, of all of the meteorite types, they are the least altered by heat. Because the matter in the early solar system was sorted to some extent by gravity, chondrites that formed from material that was located closer to the sun tend to be made of heavier elements than those that formed farthest from the sun.
Each of these types of meteorites is of coarse subdivided into many different sub types or classes. Chondrites differ according to how far from the sun they formed (ordinary, carbonaceous) and by how hot they got (petrologic grade 3 - coolest, to 6 hottest) which is related to how big a thing they were a part of, as we explained above, and by how much of certain types of metal they contain (LL, L, H, etc), which relates both to how hot they got and how far from the sun they formed. Irons and mesosiderites are subdivided by how much iron and nickel they contain, as well as by what other non-metallic minerals they might contain, and achondrites, which are much like earth rocks, are subdivided by what types of rocks they are and how they formed.
There is a 5'th major class of meteoritic material that isn't usually counted when we are considering meteorite types because it is made up of particles too small to see. While meteorites of visible size are quite rare, and masses of over a gram are not likely to fall on your house in a lifetime, microscopic particles from space strike with amazing frequency. Millions of these microscopic particles, made up of burnt space debris, the tiny remains of burnt up meteors, and space dust, rain down on our homes, streets, and on us each day.
Meteorites From Other Planets
A very small percentage of meteorites originate, not among the asteroids, but on other planets. The only planetary bodies from which there are known meteorites are the earth and the moon. It is difficult to keep up with the number of known Martian and Lunar meteorites on Earth, since new ones are discovered by scientists every year or so, but at last count, there were between 20 and 25 of each. These are some of the most expensive and actively sought after, as well as some of the most scientifically important of the meteorite types. Planetary meteorites are extremely difficult to find on earth unless they are very fresh or in a selective environment such as the Antarctic ice sheets where they can be recognized. The reason they are so difficult to find, apart from the fact that they are incredibly few and far between, is that they look a lot like normal earth rocks. We can generally only tell for certain if a meteorite is from one of these 2 sources by using detailed chemical and isotopic testing.
Humans have been to the moon and have carried back over 800lbs or well over 350 kilos of moon rock for study. We have only sent remote probes to the planet Mars, however, not people, and nothing has ever been brought back. Martian meteorites are the only samples of Mars that we have on this planet. As such, they are very important to science.
The most famous of the Martian meteorites is Alan Hills 84001. This is the meteorite on which some scientists believe they might have found fossils of ancient Martian life of a type similar to some bacteria that can be found in earth soils. The question of whether these are real fossils or not is still being actively debated among meteoriticists and planetary scientists today, and it is far from resolved. It is already widely accepted, however, that there was once water flowing and standing on the surface of Mars, and that some or all of the conditions necessary to support life were present. Mars is now a place with very little atmosphere, very little surface water, and extremely high and low temperatures that vary from night to day. Even if we do find that there was life on Mars 1 or 2 billion years ago, it is extremely unlikely that it will have survived the changes on the planet that have occurred since then.
Any time the subject of Martian and Lunar meteorites comes up, the question 'How did they get to Earth?' follows immediately. Because they are smaller, and their gravities are much less than earth's, very large asteroid impacts on the surface of the Moon and Mars can throw bits of these bodies in to space. Once a piece of Mars is in space, the only direction it can go is towards the sun, due to it's gravity. Earth intercepts a few of these bits during their long slow spiral in towards the sun. Pieces knocked form the surface of the Moon would need a much greater energy to escape Earth's gravity as well, so most of them fall in a slowly decaying orbit towards our planet.
How do you recognize a meteorite?
In order to recognize all possible meteorites, you would need a tremendous amount of specialized knowledge. There are a few characteristics, however, that might help you to recognize well over 90% of them. Most (virtually all) meteorites can be attracted by a strong magnet since the vast majority contain between about 5% and about 95% of iron. Because of the iron content, most meteorites will be slightly to significantly heavier than earth rocks. The surface of any fresh meteorite, unless it is broken, will have a dark colored burnt crust known as fusion crust. If the meteorite is an iron, it may be covered with rust. The interior of a meteorite will most often be a different color than the fusion crust, usually lighter. The fusion crust may show thumbprint like depressions known as regmaglypts or flow lines, both of which originate from the uneven burning away of the surface by Earth's atmosphere. Most meteorites, if cut or polished, will show some amount of shiny silvery colored metal. This can vary from flakes about 1mm across or less, to the entire body of the specimen. The interior, if cut, may also show round chondrules.
No known meteorites contain quartz crystals or mica flakes, and very very few contain any significant number of gas bubbles of the sort found in basalt. The presence of any noticeable fossils is also a quick eliminator.
If after examining a specimen and taking into account the above factors, you think you may have a meteorite, you can send a specimen to any of the following institutions for validation:
The American Museum of Natural History
Central Park West at 79'th Street
New York, NY 10024
National Museum of Natural History
Department of Mineral Sciences
Washington, DC 20560
The Field Museum of Natural History
S. Lake Shore Drive
Chicago, IL 60605
Of the thousands and thousands of possible meteorites that are sent in to these institutions, perhaps 1 a year or fewer turns out to be a new meteorite find. There are many types of earth minerals and objects that look somewhat like meteorites, and can be easily confused. The most common of these are magnetite, basalt, nodular iron rich accretions, furnace slag, and scrap iron. Anyone wishing to take up meteorite hunting or collecting as a serious hobby should not only familiarize themself with the appearance of the various types of meteorites, but also with these commonly mistaken items.
In addition to a solid familiarity with the objects that are frequently mistaken for meteorites, there are a few other items of understanding that help meteorite dealers and researchers in distinguishing real finds from cases of mistaken identity. First, 99.9% of all meteorites are cold or only very slightly warm when they reach the surface of the earth. This is because the part that we see burning as they fall is blown away as quickly as it is heated by friction and compression in the atmosphere. The remaining body of the meteorite will still retain something close to the temperature it had in space, or below -150 degrees Fahrenheit. Any meteor that burns all the way to the ground, thankfully less than 1 in 100,000 meteorites, will, for the most part, be going so fast that it will strike the ground with destructively explosive force capable of killing anyone nearby. Thus, virtually any news report that involves a flaming object striking the ground, a meteorite being hot to the touch, or fires resulting from a meteorite impact, is most likely the result of falsehood or a mistake on the part of the observer.
Secondly, and closely related to the concept explained above, any report of a witness seeing a shooting star fall to the earth, is almost uniformly a false report. The streak of light is frequently reported as disappearing as the object landed 'just over the hill', 'on the other side of the field', 'out at the edge of town', or 'in that bunch of trees'. This is an easy mistake to make, as the shooting stars appear very bright, making it virtually impossible to visually judge their distance. Most of these reported falls turn out to be shooting stars that fell or burned away in the atmosphere dozens or hundreds of miles away. The object appears to fall as it passes the horizon or the limits of the observer’s field of view. A real meteorite fall usually consists of a rock thudding unexpectedly to the earth with no flash, or fanfare, but occasionally a little thunder or rumbling. The streak of light and occasional rumbling sounds that accompany the fall of a meteor usually cease when the object is slowed by the atmosphere below a certain speed. This usually happens about 8 to 16 miles up, and the meteorite, during the the subsequent miles of it's travel, is quiet and gives off no light.
Most of the meteorites that fall on the earth will be destroyed by moisture, weathering, or by some other force within a few thousand years. This leaves very few meteorites around to be found in a given location at any given time. And in order to find those few that remain, we must somehow manage to recognize them as different from the surrounding rocks and gravel. This can be a formidable task.
In a few rare locations such as hot deserts in the Sahara and on deep Antarctic ice sheets, meteorites do not decay as rapidly, and are somewhat easier to tell apart from the surrounding environment since there are conditions such as few other rocks, little soil, or very little covering vegetation, that aid their recovery. We have only recently realized this, and in the last few years, there has been a meteorite rush to these locations similar to the famous Gold Rush of California in the 1800's.
The deep ice sheets of Antarctica are probably the most ideal setting for meteorite recovery. A rock sitting in the middle of big sheet of ice is obviously unusual and worth taking a look at, especially if, as in Antarctica, there are no people around to carry it there. In the last 20 years, almost 20000 meteorites have been recovered by expeditions to favorable locations on the Antarctic ice sheets. This represents over 90% of all of the known meteorites ever found on earth. All of these Antarctic meteorites are reserved for study by scientists, and are shared among countries around the world.
The second group of targets for this meteorite rush, the hot dry desert regions of North Africa, are easier to access than the Antarctic, but there are a lot more rocks to confuse with meteorites. Because of this, search in these areas depends on the knowledge of local nomads. Since one good find can earn them more money than they otherwise might make in a year, many of the nomads of these regions have learned to recognize meteorites when they see them. They gather these rocks throughout the Sahara and sell them to westerners in towns such as Erfoud, Morocco. The large numbers of meteorites that have come into the market this way have driven current prices of many meteorite varieties to record lows. Many of the meteorites of this area have names that begin with Sahara or NWA, followed by a 5-digit number. The first 2 digits of this number are the year in which the meteorite was found, and the next 3 refer to the specific meteorite. These names are assigned by the Nomenclature Committee of the International Meteoritical Society, the organization responsible for naming and tracking information about all known meteorites.
Due to sheer volume of recently recovered material, as well as to lack of interest among scientific institutions who find themselves with a surplus of specimens, many of these hot desert meteorites simply go unnamed and unclassified by science. Many dealers and scientists feel that this meteorite surplus may only last a few years before these areas have been scoured and these precious specimens become harder to find.
For those who want to search for meteorites a little closer to home than the Sahara Desert or Antarctica, the best bets are known meteorite strewn fields, dry lake beds, and deflated desert areas. By far the easiest places to find meteorites are the known strewn fields. Strewn fields are the areas over which the pieces of a meteorite that blew up, or broke apart while falling, landed. Seldom if ever are all of the pieces recovered from a strewnfield. In the case of the Holbrook strewnfield in Arizona, 10's of thousands of tiny pieces of stone fell. Over 16000 have been recovered thus far! Most of the Holbrook stones were smaller than a pea. Many stones are still on the ground at Allende in Mexico, Imilac in Chile, Odessa in Texas, Sikhote Alin in Russia, and Gold Basin in Arizona. This is only a tiny sampling of the known strewnfields. In many of these places, however, prospecting is not allowed, is severely limited, or requires special permission.
Meteorite hunters who wish to add to science by locating brand new meteors can do so by chasing new reported falls, a process that is detailed in the book "Find a Falling Star" by Harvey Nininger, or by prospecting dry lake beds and deflated desert areas. Deflated desert areas are expanses in which the wind has blown away most or all of the soil, leaving many years accumulation of rocks, and thus meteorites as well, exposed on the surface of the ground. Dry ancient lake beds and deflated desert areas allow efficient searching because the topsoil will be thin if present at all, and there will be little vegetation. The technique is pretty much to just sweep the area with a metal detector that will detect iron, and examine any rock that creates a signal. This can be a long, arduous, and fruitless task, but the result of finding a brand new meteorite can be worth it.
Two of the most widely recommended metal detectors for meteorite hunting are the Fisher Gold Bug 1 and 2 and the White's Gold Master II and III.
Myths and Misconceptions
Because of early movies depicting meteorites as poisonous, radioactive, monster-carrying harbingers of doom, there are a number of public misconceptions that need to be dispelled. Meteorites, while they do carry traces of radioactivity from the interaction of their atoms with cosmic rays, are no more dangerous than earth rocks, plants, or the food you eat. They may in fact be less so, as the amounts of radiation they emit are slightly less than what is given off by earth objects, and are of less hazardous wavelengths. The slight radioactivity that these objects do give off can be used to our benefit in dating their origins as well as the amount of time that has passed since they were separated from their parent bodies and began their journey towards earth.
Meteorites are not in any way toxic, and as far as monsters are concerned, not a chance. Although there are a number of scientists that have presented the idea that earth may have exchanged microorganisms with Mars via meteorites several billion years ago during early periods of intense impacts, the circumstances today are quite different. If any microorganisms still exist elsewhere in the solar system, and if they by some miracle managed to reach earth, they would most likely be fried by our high temperatures, cooked chemically by our oxygen rich atmosphere, or eaten by the first one or two of the billions of happy Earth organisms waiting here for their lunch.
In summary, meteorites are quite harmless. They contain no unusual poisons, radiation, or toxic elements of any sort in any quantities to be worried about. You could eat them if not for the fact that this would be hard on your teeth and on the meteorites. We, with our moisture, body oils, and salts, along with the harsh oxygen rich atmosphere in which we live present much more of a hazard to meteorites than they do to us, unless of coarse we are standing under them when they fall.
Where can I learn more about meteorites?
Scientists that study meteorites are known as meteoriticists, and their field of study is known as meteoritics, and sometimes planetary science. Meteoriticists study a wide range of topics including craters, impact catastrophes, the origins of life, the formation of planets and stars, high speed impacts, orbital mechanics, minerals and metals, and many, many other interesting topics. Meteorites provide us with glimpses of parts of space that we may not be able to reach directly for many decades. By combining a study of meteorites with remote observation through telescopes, meteoriticists and planetary scientists can learn a great deal about objects that are millions of miles away. In a few rare meteorites, scientists have even found ancient grains of dust from ancient stars located billions of miles outside of our solar system. The efforts of today’s meteoriticists, combined with the studies of our other space scientists and explorers, are paving the way for humanities' gradual push towards colonizing our solar system and surrounding space.
Because meteorites are unique and scientifically important objects that provide a significant key to human kind's future, care should be taken that they are never carelessly disposed of or destroyed. For the sake of science, keep them dry, document what you know about the history of any that you own, and see that they reach the hands of scientists or a University if you ever grow tired of them.
Most colleges and universities in America offer the opportunity to study astronomy and the fields related to meteoritics and planetary science such as geology, chemistry, and physics. A few universities, including the University of Arizona in Tucson and the
University of New Mexico in Albuquerque offer complete specialized meteoritics and planetary science classes and degree programs.
If you want to begin learning about meteorites, but you aren't quite ready to spend the time necessary to enroll at a University in order to study them, there are a number of very good books and publications available on the subject.
The first books to read:
-Rocks from Space: Meteorites and Meteorite Hunters; O. Richard Norton, Dorothy S. Norton; Second Edition, 1998.
-Find A Falling Star; Nininger, Harvey; 1972.
-Meteorites and Their Parent Planets; Harry Y., Jr. McSween; 2'nd Edition, 2000.
Other very good references on the subject include:
-Meteorites from A to Z; Anne Black, Michael Jensen, and William Jensen; 2001.
-Catalogue of Meteorites; Monica Grady; 5'th Edition, 2000.
Meteorite Related Publications:
-'Meteorite' magazine is a quarterly journal of meteorite related news and science published out of Union City, California, and currently edited by Derek Sears and Robert Beauford. To subscribe, visit: http://meteoritemag.org/
-meteorite-list mail list: This is an active email discussion list for meteorite collectors, dealers, researchers, and enthusiasts at all levels. Off topic posts and meteorites sale announcements are kept to a minimum. Visit http://www.pairlist.net/mailman/listinfo/meteorite-list to subscribe or unsubscribe.
I would like to thank the authors of the above references, as well as the many members of the meteorite-list for their assistance gathering the information contained in this article.
This articles is from a short booklet I produced to introduce customers to the subject of meteorites and the science of meteoritics. It was originally published to the web back in the 90s, and I had lost track of it until a customer kindly reposted and sent me back a copy. (Thanks again for doing so.) I hope you enjoy it and that it is useful to you. I will eventually update this article.
I wrote this one pretty well straight through, so I haven't put as much time into refining it as I would like. Please feel free to send me both editing suggestions and content ideas.