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The Barringer Meteorite Crater (also known as "Meteor Crater") is a gigantic hole in the middle of the arid sandstone of the Arizona desert. A rim of smashed and jumbled boulders, some of them the size of small houses, rises 150 feet above the level of the surrounding plain. The crater itself is nearly a mile wide, and 570 feet deep. When Europeans first discovered the crater, the plain around it was covered with chunks of meteoritic iron - over 30 tons of it, scattered over an area 8 to 10 miles in diameter.

The process of scientific discovery involves the development of hypotheses, tentative explanations which may or may not account for the observable facts. A good scientific hypothesis will generate a number of logical consequences or predictions, which are capable of being tested directly. Ultimately, the hypothesis will be accepted as valid only if: repeated tests of these predictions, by different investigators, tend to confirm it; it is consistent with other well-confirmed hypotheses; it is more useful than other hypotheses in accounting for a broad range of observed facts; and it is more economical or "elegant" than other hypotheses, in the sense of requiring fewer additional assumptions. Although meteorite falls had been reported for thousands of years, until this century no one had ever identified a crater created by such a fall. Even a meteorite as large as the 66-ton Hoba, the largest ever discovered, may be slowed so much by the Earth's atmosphere that it lands without making a significant hole. In 1891 Grove Karl Gilbert, then chief geologist for the U.S. Geological Survey, decided to test two conflicting hypotheses about the crater. The first was that the crater was created by the impact of a giant meteorite; the second, that it was the result of an explosion of superheated steam, caused by volcanic activity far below the surface. If an iron meteorite had created the crater, Gilbert assumed that it would have had to be nearly as big as the crater itself. So what predictions could he test? First, the meteorite should be taking up a lot of space in the hollow of the crater. The volume of the hollow would therefore be less than the volume of the ejected material in the crater rim. Second, the presence of a large mass of buried iron should affect the behavior of magnets and compass needles. Neither prediction was confirmed. Gilbert concluded that a steam explosion was the only surviving hypothesis, in spite of the fact that no volcanic rocks had ever been found in the area. The meteorites around the crater were simply a coincidence.

Ten years later a very different sort of explorer came along. In 1902 Daniel Moreau Barringer, a successful mining engineer, heard about the crater. When he learned that small balls of meteoritic iron were randomly mixed with the ejected rocks of the crater rim, Barringer immediately concluded that the crater had resulted from a meteorite impact. If the meteorites had fallen at a different time from the time at which the crater was formed, they would have appeared in separate layers from the ejected rock. Like Gilbert, Barringer assumed that the meteorite which made the crater would have to be extremely large - large enough, in fact, for a major mining bonanza.

Rather than testing his impact hypothesis, Barringer set out to assemble the evidence in support of it. In 1906, and again in 1909, he presented his arguments for the impact origin of the crater to the Academy of Natural Sciences in Philadelphia. The evidence included: A. The presence of millions of tons of finely pulverized silica, which could only have been created by enormous pressure. B. The large quantities of meteoritic iron, in the form of globular "shale balls", scattered around the rim and surrounding plain. C. The random mixture of meteoritic material and ejected rocks. D. The fact that the different types of rocks in the rim and on the surrounding plain appeared to have been deposited in the opposite order from their order in the underlying rock beds. E. The absence of any naturally occurring volcanic rock in the vicinity of the crater. In 1908, these conclusions were championed by geologist George P. Merrill. Merrill analyzed a new type of rock discovered by Barringer at the crater, which Barringer called "Variety B". He concluded that it was a type of quartz glass which could only be produced by intense heat, similar to the heat generated by a lightning strike on sand. Merrill also pointed to the undisturbed rock beds below the crater, which proved that the force which created the crater did not come from below.

During the same years, a debate was raging among astronomers about the origin of the craters on the moon. As with the Barringer crater, most astronomers initially assumed that those craters were volcanic. Gilbert himself, ironically, was one of the first to argue for an impact origin, in a paper published in 1893. In 1909, a German geologist advanced the same theory, based in part on the evidence presented by Barringer for the Arizona crater. One objection to the idea of an impact origin for the lunar craters was the fact that all lunar craters are round. Astronomers assumed that most meteorites would have struck the moon at oblique angles, producing elongated craters. Barringer, however, had experimented by firing rifle bullets into rocks and mud, and had discovered that a projectile arriving at an oblique angle would nevertheless make a round hole. In 1923, Barringer's 12-year-old son Richard published an article in Popular Astronomy, using his father's rifle experiments to argue for the impact origin of the lunar craters; Barringer himself repeated the arguments a short time later in the Scientific American. The conclusive arguments in the lunar debate were provided by astronomers such as A. C. Gifford, who demonstrated that the force of an impact at astronomical speeds would result in the explosion of the meteorite. Whatever the original angle of impact, the result would be a circular crater.

In 1928, $200,000 was raised for a final assault on the meteorite. Barringer's directors, however, were growing nervous. When the new mine shaft hit water in such great quantities that it could not be pumped out, they consulted the astronomer F. R. Moulton for his opinion on the size of the meteorite. Moulton calculated the amount of energy which would be produced by an impact at the enormous speed typical of a meteorite arriving from space. He concluded that an object big enough to create the crater would probably weigh only 300,000 tons - 3% of the amount estimated by Barringer, and too small to justify any further drilling. In addition, Moulton argued that the explosion caused by the impact would result in the total vaporization of the meteorite. In 1929, work was halted at the crater. By November of that year, it had become clear that other prominent scientists agreed with Moulton. Within weeks, Barringer was dead of a massive heart attack. 

Scientists now believe that the crater was created approximately 50,000 years ago. The meteorite which made it was composed almost entirely of nickel-iron, suggesting that it may have originated in the interior of a small planet. It was 150 feet across, weighed roughly 300,000 tons, and was traveling at a speed of 40,000 miles per hour. The force generated by its impact was equal to the explosion of 20 million tons of TNT. In 1946, meteorite collector Harvey H. Nininger analyzed the tiny metallic particles mixed into the soil around the crater, along with the small "bombs" of melted rock within it. He concluded that both types of particles were solidified droplets, which must have condensed from a cloud of rock and metal vaporized by the impact. Here, he believed, was proof that the crater was created by explosion.

In 1963, geologist Eugene Shoemaker published his landmark paper analyzing the similarities between the Barringer crater and craters created by nuclear test explosions in Nevada. Carefully mapping the sequence of layers of the underlying rock, and the layers of the ejecta blanket, where those rocks were deposited in reverse order, he demonstrated that the nuclear craters and the Barringer crater were structurally similar in nearly all respects. His paper provided the clinching arguments in favor of an impact, finally convincing the last doubters. Three years earlier, Shoemaker, Edward Chao and David Milton had also collaborated in the discovery of a new mineral at the Barringer crater. This mineral, a form of silica called "coesite", was first created in a laboratory in 1953 by chemist Loring Coes. Its formation requires pressures of at least 20,000 atmospheres (20 kilobars) and temperatures of at least 700 degrees Celsius - greater than any occurring naturally on earth. Coesite and a similar material called "stishovite" have since been identified at numerous other suspected impact sites, and are now accepted as indicators for the impact origin of a geologic structure. Another indicator is the presence of rock structures known as "shattercones". These structures, which can be anywhere from less than an inch to more than six feet tall, can only be created by a sudden intense pressure on existing rock. During the 40's and 50's, investigations by Robert S. Dietz and others revealed the existence of shattercones at many suspected impact sites, although not at the Barringer crater. Deitz was able to demonstrate that the apexes of the cones at most of these sites all pointed upwards, indicating that the force which created them had come from above.

Using these methods, meteoriticists have now identified over 150 proven impact sites. Evidence suggests that there have been many thousands of other impacts over the course of the earth's history. Meteorites weighing a quarter of a pound or more hit the earth thousands of times a year. One large enough to form the Barringer crater may arrive as often as once every thousand years. The mysterious Tunguska explosion of 1908, which devastated an area of Siberian forest the size of Rhode Island, may have been our most recent encounter with a visitor of this size. In 1980, a new hypothesis emerged. Scientists Walter and Luis Alvarez discovered that a layer of soil containing extremely high levels of the mineral iridium - rare on earth, but abundant in meteorites - had been deposited all over the earth about 65 million years ago. That date marks the end of the Cretaceous period, a time when not only the dinosaurs, but thousands of other plant and animal species suddenly became extinct. The Alvarezes theorized that the mass extinctions had been caused by the impact of a giant meteorite, perhaps six miles in diameter. Such an impact would throw up a cloud of dust thick enough to obscure the sun for several years, disrupting the planetary food chain and causing the disappearance of vast numbers of species. The recent discovery of two giant craters, roughly 65 million years old, underneath the Yucatan peninsula, plus the worldwide distribution of coesite in the Cretaceous boundary layer, have made the Alvarez hypothesis seem more and more convincing.

Meteoroids, Meteors and Meteorites

When drifting through space, smaller pieces of space rock are called meteoroids.Most of them are the size of sand or gravel. Eventually, some of these meteoroids cross the path of the Earth. They may enter our atmosphere at a velocity of several miles per second.

Because of their tremendous speed, meteoroids are heated by friction with the air to incandescence, which means they glow red hot or even white hot. They are now called Meteors. The air around the meteor also glows, making it possible to see the meteor from the earth. Most small meteors burn up in the atmosphere, leaving only microscopic fragments to drift down to the earth as dust.

Larger meteors are often able to survive their burning path through our atmosphere, however, and strike the earth. We call these meteorites. Because much of the earth is covered by water, most meteorites are never found. When meteorites manage to fall onto land, it is possible for us to find and study them. This is one way to study our solar system.

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