A “Non-Technical” Discussion
Meteorites are some of the most fascinating objects in the world ….. perhaps because they are not of this world! When you hold a meteorite in your hand, you are holding something that is over 4.5 billion years old ….. older than any rock that can be found on Earth and virtually unchanged since the birth of the Solar System. You are holding something that was most likely knocked out of its orbit in the asteroid belt between the orbits of Mars and Jupiter thousands, millions or even billions of years ago ….. that has traveled millions and millions of miles to make its way to Earth ….. that entered our atmosphere at speeds anywhere from 20,000 miles per hour up to in excess of 90,000 thousand miles per hour ….. that was reformed (at least on its exterior) by its passage through our atmosphere ….. that endured unimaginable forces during its short trip through the atmosphere ….. that may have even exploded due to the pressures exerted on it and the remains finally fallen to Earth where, it may have lain undiscovered for hundreds, thousands or even millions of years before it was found. That’s quite a trip!
Part of the delight is holding a meteorite and trying to understand what formed it to be the way it is. You don’t have to cut a meteorite into wafer thin slices and study it under an electron microscope to have the meteorite tell you a story. Just hold it in your hand and you will learn a lot about it. But you do have to know what to look for.
Two of the greatest stories in “Meteoritics” (the scientific name for the study of meteorites) are the 1947 fall of the great Sikhote-Alin meteorite and the impact of the unimaginably massive object that formed Meteor Crater in Arizona some 50,000 years ago. The meteorites from these two events are some of the most sought after by collectors at all levels. It’s fair to say that no collection is complete without at least one meteorite from each of these events. So I have decided to invest my time in specializing in these two collectibles.
So, let’s begin: there are fundamentally three types of meteorites; stony, irons and stony irons (or a combination of the two materials). Each of these three groups is further divided into several subgroups according to their chemical composition and structure but going into that detail is beyond the scope of this primary discussion. Please refer to the reading I have suggested for greater detail on the classification of the three primary groups.
Meteorites do not contain any element that is not found on the Earth. However, the unique combination of various elements in meteorites sets them apart from terrestrial materials. For example, virtually all meteorites contain to some degree an alloy of nickel and iron which does not occur naturally in iron ores on Earth. In fact, this alloy in which nickel is from approximately 5% to 15% is one of the standard ways in which a meteorite can be identified as not being of earthly origin.
Meteorite “falls” are those that are observed to fall and later partially or completely (rarely!) recovered (e.g. the Sikhote-Alin fall of 1947). Meteorite “finds” refer to those that were not seen to fall but later found (e.g. the Canyon Diablo meteorites from the object that formed Meteor Crater 50,000 years ago). We know from keeping track of falls that stony meteorites are by far the most common representing more than 90% of all specimens. However, stony meteorites are much harder to find compared to iron meteorites because in many cases, they look just like terrestrial stones. Irons are much easier to identify because they are sold metal and they just don’t look like terrestrial stones and so can be found by someone not necessarily trained in what to look for. So if you just judge by those “found” meteorites irons would be disproportionately high. They are actually much rarer accounting for less than 10% of falls and stony irons are rarer still.
More definitions: a “meteoroid” is an object in space in size anywhere from a mote of dust to an object up to 100 meters in diameter. Objects much larger than this will fall into the category of being asteroids. A “meteor” is the visual phenomena resulting from the passage of the meteoroid through the atmosphere (more details on this later) and a “meteorite” is any material from the meteoroid that survives the passage through the atmosphere and falls to Earth.
By far, the smallest meteorites are tiny particles called IDPs (for Interplanetary Dust Particles) which probably are left over from the formation of the solar system over 4.5 billion years ago. These particles are micron sized (a micron is 1/1,000 of a millimeter). During its passage around the Sun the Earth encounters (or sweeps up) an estimated 35,000 to 100,000 tons of these dust particles annually. In fact, these particles are continually being swept up by the gravity of the sun and planets so there must be a source for their replenishment. The most likely candidates for replenishment are fragmented asteroids and debris from comets. These particles are too small to make meteors because their size to surface area ratio does not allow them to heat up to the point where they become incandescent. They may float in the upper atmosphere for literally years before finally making their way down through the atmosphere to the Earth’s surface.
When particles are as large as a grain of sand up to the size of a pea, they will become meteors but most objects in this size range will totally burn up in the atmosphere before they reach the ground. It is these kinds of particles that are seen several times during the course of the year when the Earth intersects the debris trail of a comet and we have a meteor shower. As the size of the meteoroid increases so does the chance that parts of it will reach the Earth’s surface and become meteorites.
Larger objects (from the size of a golf ball and up) will potentially form a fireball which is a meteor on steroids! The object may penetrate the Earth’s atmosphere at literally tens of thousands of miles per hour and the friction of the object with the air molecules will make it become incandescent. Further in the fall, the air surrounding the object will become ionized (i.e. it will lose electrons but I don’t want this to get too technical!) and a fireball will be formed. The object itself may be only the size of a basketball but the fireball can be several hundred meters in diameter around the object and can be as bright as the sun. Mind you, because of its velocity, all this may take place in just a few seconds!
Very big meteoroids really put on a show. I’ll use the Sikhote-Alin fall as an example. The original meteoroid (primarily made of solid iron-nickel alloy) probably weighed close to 300 tons! It entered the earth’s atmosphere at an estimated speed of 45,000 miles per hour. The fireball it produced was seen to be as bright as the sun by observers and it actually cast shadows during the morning as it moved through the atmosphere. An enormous dust train formed behind it as the material was “ablated” (or melted) by the friction with the atmosphere. The vaporized material (constituting much of the original mass) in the dust train was estimated to weigh as much as 200 tons! At an estimated altitude of 18,000 feet (about half the cruising altitude of a typical jet) the forces on the mass became so great that it literally exploded into thousands of pieces. At this point, the pieces lost their “cosmic velocity” and fell to the ground from that point just as though they had been dropped from an airplane. The remaining pieces eventually achieved “terminal velocity” (this is the point where the gravity of the Earth and the resistance of the atmosphere are equal) and hit the heavily forested mountainous area at several hundred miles per hour, again shattering into many pieces. It’s estimated that 70 tons of the original mass reached the Earth and some 25 tons of meteorites were collected soon after. Whew! How great that would have been to witness!
The largest meteorite ever found is the Hoba meteorite found on a farm in Namibia. It is a massive piece of iron weighing about 60 tons. It has been left where it was found and is a national monument so you will have to make some travel plans if you ever want to see it.
It is believed that any mass weighing more than 100 tons will hit the Earth with such a tremendous release of energy that it will literally disintegrate or vaporize. I will use the Canyon Diablo object as an example (by the way: meteorites are usually named after a geographical feature in the area of the fall. Sikhote-Alin is named after the mountain rage where it fell; Canyon Diablo is a canyon in the vicinity of the impact point).
Approximately 50,000 years ago, an enormous object made of iron nickel alloy and weighing somewhere between 60,000 and 100,000 tons entered the atmosphere of the Earth over what is now eastern Arizona. The mass may have been as large as 100 feet in diameter and was traveling at a speed of approximately 33,000 miles per hour. Because of its huge mass, it was not as subject to braking from the atmosphere and it retained much of its “hyper” velocity and impacted the earth at a tremendous speed. At impact, the object may have been compressed to one half its original size and the earth and rocks at the impact site were similarly compressed as well. The energy released by the concussion of the impact was equal to that of almost 2 million tons of TNT (or the energy of a very large nuclear bomb) and most of the mass of the object was vaporized instantly into countless droplets of molten metal. Some of these tiny metal spherical objects from the main mass have been found in the desert sand as far as 9 miles away. While most of the mass was destroyed, tens of tons of meteoritic material survived as it was sloughed off during the passage through the atmosphere or survived the explosion itself. Most of the Canyon Diablo meteorites are rough fragment shapes looking much like bomb shrapnel.
The crater resulting from this explosion was almost a mile across and about 1,200 feet deep. Since the event, geological processes such as erosion etc. have half filled the crater so that it is now about 650 feet deep but that’s still deeper than the Washington Monument is tall. Nonetheless, Meteor Crater remains as the best preserved impact crater in the world. I saw Meteor Crater for the first time more than forty years ago and we are planning to visit it again later this year. It’s breathtaking to stand on the edge of the crater and imagine the colossal event that formed it and if you ever get a chance to go there, it is well worth going out of your way to see.
Even larger impacts have occurred such as the well-known event 65 million years ago in which an object estimated to be the size of Mt. Everest hit the Earth at cosmic velocity and the energy released was greater than all of the nuclear weapons presently in the world set off at once. This object hit what is now the Yucatan Peninsula in Mexico with such force that it penetrated the Earth’s crust and the material ejected from the explosion was so great that it essentially permeated the Earth’s atmosphere and blocked out the sun for years. This led to one of the greatest extinction events in the history of the world in which more than three quarters of all existing species were wiped out including the dinosaurs. Not a good day for Mother Earth. The crater created by this impact is some 112 miles in diameter and named the Chicxulub Crater after a local village.
Possibly the greatest impact occurred billions of year ago when a planet the size of Mars impacted the Earth and some of the remnants of this collision formed the Moon. This event is the most widely accepted theory for the origin of the Moon.
When we look at the surface of the Moon, we can see thousands upon thousands of craters and it is only logical to assume that the Earth has been subjected to such bombardment as well. So where are all these craters? Well, the Moon is totally inactive from a geological standpoint while the Earth is very alive with geological processes. Continental drift, volcanism, erosion and other forms of weathering have served to erase the vast majority of this damage caused by thousands of impacts over eons. Because it is buried beneath the Caribbean Sea and so much sediment on land, the location of the Chicxulub crater was only discovered some 50 years ago.
Still, there are approximately 200 impact craters that have been identified around the world (of which Meteor Crater is considered to be the best example) and it is believed that virtually all of them were formed by the collision of iron objects. Stony objects of the size required to make these craters simply don’t have the structural strength to remain intact for the passage through the atmosphere. However, many of these impact craters are very old and iron doesn’t survive well in nature (due to oxidation or simply – rusting). So only a handful of these craters have recovered meteoritic material in their vicinity and Sikote-Alin as well as Canyon Diablo serve as examples. The Sikote-Alin fall happened very recently and about 25 tons of the estimated 70 tons of surviving material was recovered soon after. While the Canyon Diablo event occurred more than 50,000 years ago, it occurred in an arid desert region which is why the meteoritic material has survived the ages.
A few words about the velocity of meteoroids as they enter the atmosphere: we move easily through the air and it is sometimes difficult to imagine the kinds of forces that occur in “hyper velocity” or speeds between 10,000 and 90,000 miles per hour. Hang your arm outside the car at 55 MPH and you can easily feel the wind resistance against your arm as you move through the air at this faster speed. Now, imagine multiplying the force you feel times ten and you would feel what it is like to hang your arm outside the window at the cruising speed of an airliner. You would probably lose your arm in the force of the slipstream (a grim image). Now multiply this force by a factor of 100 and you would be in the mid-range of cosmic velocity or 55,000 MPH. The forces on the leading face of an object entering the atmosphere at this speed would amount to several thousand pounds per square inch and the object would essentially form a total vacuum at the back side. With this tremendous differential in pressure, it’s not so hard to imagine that these forces would cause an object to simply tear itself apart, which is exactly what happens. By the way, the upper speed limit for cosmic velocity in the Solar System is around 94,000 MPH because anything going faster would achieve escape velocity from the Sun and leave the Solar System. The Earth itself is orbiting the Sun at a velocity of about 65,000 MPH and this speed can affect the entry speed of a meteoroid depending on its angle of approach.
So, where do meteoroids come from? The answer is that the vast majority of them come from the Asteroid Belt which is a mass of iron and stony rubble left over from the formation of the Solar System orbiting in a plane almost 100 million miles wide between the orbits of Mars and Jupiter. An Asteroid is a metallic or stony sub-planetary size body orbiting the Sun in the same planetary plane and direction as the planets around the sun. The Asteroid Belt is 100 million miles wide and contains countless millions of objects from as small as dust particles up to asteroids as much as 600 miles in diameter. It is a common misconception that the asteroids are the remains of a planet that was catastrophically disrupted sometime in history. Rather, the asteroids are more accurately described as a planet that never formed probably because the gravity field of the massive Jupiter continually disrupted them and kept them from accreting or joining together to form a planet.
There are estimated to be more than a million asteroids in the Asteroid Belt with a diameter of 1 kilometer (a little more than ½ mile) or more but all of them put together would make a body only a fraction of the size of the Moon. The first asteroid was discovered on January 1, 1801 and, until that time, scientists believed that meteoroids came from the Moon as the result of volcanic activity Of course, we have since learned that the Moon has had no geological activity (including volcanism) for millions, if not billions of years. Despite this, it should be noted that a very small fraction of meteorites found did come from the Moon as well as from Mars not as the result of volcanism, but rather as the result of massive impacts (such as Canyon Diablo) in which the lunar or Martian materials achieved “escape velocity” (in which speed trumps gravity) from the parent body and found their way to Earth. In fact, there may well be meteorites that originated from Earth on the Moon, Mars and other planets that originated in the same manner in a sort of cosmic trade!
Most asteroids are in a proper circular orbit around the Sun. But some of them have achieved elliptical orbits in which they travel from the asteroid belt through the inner Solar System and return to the Asteroid Belt. When these asteroids cross Earth’s orbit around the Sun, they are called NEOs or Near Earth Objects. The number of NEOs number in the thousands and unfortunately for us, not all of them have been found and their orbits identified. So events such as those on the scale of Canyon Diablo occurring again are not just possible, given enough time, they are a certainty. The Sikhote-Alin event was so well witnessed that the direction of approach of the mass was determined. This was then extrapolated to an elliptical orbit that passed through the Asteroid Belt at its furthest from the Sun and passed inside the Earth’s orbit at its closest approach. It was just a matter of time …..
How do asteroids get into an elliptical orbit? The main culprit is probably the planet Jupiter whose massive gravitational field disturbs (or in scientific terms “perturbs”) the orbits of some of the asteroids. These forces may alter the orbit of the affected asteroid or it may cause it to collide with another asteroid resulting in a catastrophic destruction of both bodies and pieces of each going in every direction. When these bodies collide, they don’t do it gently. It is estimated that asteroid collisions may occur at speeds of 11,000 MPH and greater so you can imagine the forces involved!
These collisions may also account for the different kinds of meteorites found. We began with the fact that meteorites were essentially stony, iron or a mixture of the two i.e. stony irons. Some asteroids evolve (over eons of time) into Planetisimals which are like miniature planets complete with a crust, mantle, outer core and inner core just like the Earth. This is a process called differentiation in which the gravity of the object causes the heavier material (the iron and nickel) to migrate towards the center. When these bodies are fragmented as the result of catastrophic collisions, the crust becomes smaller stony meteoroids, the outer core becomes stony irons (of which pallasites are a good example) and the inner core of iron and nickel becomes the iron meteoroids. So, when you are holding an iron meteorite in your hand, you are holding a piece of an asteroid that was part of the inner core of a very ancient miniature planet — literally its heart! Is that a trip or what!
Because meteorites are little changed from the creation of the Solar System some 4.6 billion years ago they provide science with important clues from matters such as how the Solar System was formed to how life arose on Earth. It may sound trite but they are indeed a “window to the past.”
If you are interested in learning more, I would recommend the following books (all available at Amazon.com):
ROCKS FROM SPACE by O. Richard Norton. This is a must-read for anyone interested in meteors; it is filled with great stories of meteorite events such as the Sikhote-Alin Fall and the Canyon Diablo event; it is non-technical and very entertaining.
METEORITES by Caroline Smith, Sara Russell and Gretchen Bendix. This is again a non-technical and beautifully illustrated primer with great photography.
METEORITES A JOURNEY THROUGH SPACE AND TIME by Alex Bevan and John De Laeter. The same as above.
METEORITES AND THEIR PARENT PLANETS by Harry McSween, Jr. This one gets a little more technical but not any more than any good college textbook. It’s packed with lots of great basics as well.
FIELD GUIDE TO METEORS AND METEORITES by O. Richard Norton and Lawrence A Chitwood. This has lots of great basics, excellent illustrations and photos and does tend to be a bit more technical as the book above.
THE BIG SPLAT Or How Our Moon Came To Be by Dana MacKenzie. This is a very readable book about the various theories regarding the origin of the moon and how it came about that one achieved consensus.
If you think you have found a meteorite, please refer to the following website: http://meteorites.wustl.edu/meteorwrongs/meteorwrongs.htm
©AVATAR METEORITES 2011