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8.16 Introduction to Meteorites



The Black Stone, surrounded by its silver frame and the black cloth?kiswahon the?Kaaba?in?Mecca. This stone is believed to be a meteorite. Click here for original source URL.

Long ago, stones from the sky were a source of awe. Meteorites have been found buried with American Indian artifacts in Mexico and revered as a sacred possession by a tribe in Alaska. A stone worshipped in the Temple of Diana at Ephesus (one of the seven wonders of the ancient world) reportedly fell from the sky. The "Black Stone," enshrined around 600 A.D. or earlier in the sacred Muslim shrine in Mecca, is also believed to be a meteorite. They have even had a deep influence in history; in 1492, a meteorite fell at Ensisheim, France. Emperor Maximilian was in residence nearby, and he took the fall as a sign that he should go on a Crusade.

Even when the origins of meteors is unknown, it is often clear that they are something unusual. These half-melted, often mostly metal rocks are typically significantly more dense than their Earth-formed rocky cousins, and this high density means that meteors consistently outweigh same-size Earth cousins. It took a long time for a scientific explanation of these strange rocks to emerge. In the 1700s, many naturalists still felt that the idea of stones falling from the sky was no more than superstition. One scientist, however, decided to do a careful investigation rather than just dismiss this idea. In 1794, German physicist named E.F.F. Chladni reported that the Ensisheim stone and other supposed celestial stones seemed similar to each other, and different from normal terrestrial stones. He concluded that these "meteorites did indeed fall from the sky."

This conclusion was controversial, and many people still refused to accept a celestial origin for these stones. Upon hearing that a meteorite had fallen in Connecticut, Thomas Jefferson, himself an accomplished naturalist, is supposed to have joked, "It is easier to believe that Yankee professors would lie than that stones would fall from heaven." The French Academy — that bastion of rational thinking — dismissed meteorites as superstition. But when a meteorite exploded over a French town in 1803, pelting the area with stones, the Academy sent the noted physicist J.B. Biot to investigate. His report became one of the historically important documents of science. Biot constructed an irrefutable chain of evidence from eyewitness accounts, measurements of the 12 square kilometer area of impacts, and specimens of the meteorites themselves. This report was instrumental in establishing that stones do indeed fall from the sky.

Rocks in space may seem remote from human affairs, and generally they are, but they can have a rather strong influence when they collide with the Earth. Although the Earth shows evidence of the destruction meteorite impacts can cause (in the form of impact crater scars), human encounters with falling meteorites are extremely rare. Nobody has been killed by a meteorite in recorded history, although a dog was killed in Nakhla, Egypt in 1911. In the last hundred years, a few people in the United States have experienced close encounters. In 1938, an Illinois woman heard a crash in her garage, and found a meteorite lying on her car seat. Some years later, an Alabama woman was seriously injured by a ricocheting meteorite that hit her in the hip. A house in Wethersfield, Connecticut was hit in 1971, and then another house just a mile away was hit 11 years later! In 1991, two boys in Noblesville, Indiana heard a whistle and a thud, and looked down to see a small 4-inch meteorite lying in a crater in the sidewalk. One year later, a woman found a 30-pound meteorite that had smashed through the trunk of her 1980 Chevy Malibu, fusing to the metal of the car. She was offered $69,000 for the wreck, which was far more than the car was worth before it was damaged! Don't let this summary worry you, though. Your chances of being hit by a meteorite are tiny — statistically speaking, your bathtub is far more dangerous.

Large meteorites are rare. Only a few boulder-sized meteorites are recovered each year. In 1972, an object weighing about 1,000 tons just missed the Earth, skipping off the outer atmosphere like a stone off a pond. It was filmed from the ground and detected by Air Force reconnaissance satellites. Had it fallen through the atmosphere instead of skipping back into space, it would have caused a bomb-sized explosion in Canada. Objects weighing 10,000 tons, like the object that caused the Tunguska explosion of 1908, are large enough to cause nuclear-scale blasts (although they don't leave behind the high radiation). This occurs every few centuries, on average. The Chelyabinsk airburst in Russia in 2014 was the most recent example. Larger blasts, thousands of years apart, may form craters many kilometers across.

Many big meteorites have been identified, but scientists are always looking for more. A fine rain of meteorites, ranging from the size of dust grains to pebbles, reaches the surface of the Earth all the time. They are difficult to notice on most terrain, since they often look like regular terrestrial rocks, but they stand out on the blue-white surface of the Antarctic icepack. Scientists regularly travel there and sweep the surface with sticky rollers, to gather more of this valuable evidence from space. Another popular meteorite-hunting area is the Saharan desert in northwest Africa, which has revealed many interesting meteorites on it's ever changing sandy surface in the past few decades.

E. F. F. Chladni. Click here for original source URL.

Much of what we know about asteroids comes from meteorites. Modern evidence suggests that most of them are fragments of Earth-crossing asteroids that were ejected from the main asteroid belt. So by a fortunate chance, we get "free samples" of cosmic debris left over from the formation of the Solar System, and occasionally even samples of other planets! They allow us to reconstruct many of the processes that led to the formation of the Earth and the other planets.

J. B. Biot. Click here for original source URL.