A piece of Henbury meteorite
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ABOUT METEORITES
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METEORITES - witnesses of exploded worlds
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Today, we are continually hearing about the space flights (spacecraft, satellites, etc.) of objects which are sent into space from the Earth, and we can all too easily forget that "flying objects" from space (meteoroids) have been landing here on earth since the beginning of time. Many of these are very solid spacecraft indeed, and their landing is sometimes so powerful that it exceeds the effects of an atomic bomb. Meteoroids entering
our atmosphere are commonly called "shooting stars" or meteors.
They are arriving all the time, but we usually only see them as a
beautiful frictional firework display in the sky of a clear night.
Bigger, but rarer, meteoroids may penetrate through to the surface
of the earth, shedding droplets of molten skin along the way, and
sometimes exploding either at impact or along the way, but finally
coming to rest in the soil, where in time they will merge with earthly
rocks through weathering. Those that reach the surface are called
meteorites. In Canyon Diablo in Arizona, USA, you can visit the best preserved meteorite crater on earth. It was formed about 22,000 years ago by the impact of a giant metallic nickel-iron meteorite, which arrived from space at a speed of about 50,000 kilometers per hour, and weighed many hundreds of tonnes. Friction with the atmosphere does not appreciably slow such a mighty mass, which smashed into the solid earth, punching away 300 million tonnes of earth rocks in a mighty blast which left a crater which even today is still 1.5 kilometers in diameter and 170 meters deep. Many of the nickel-iron meteorites being offered for sale today, are from this or quite rarer crater sites in various parts of the world. The price of such specimens is much less than the price of gold, although they are in reality much rarer to find than gold. In the whole history of the world the total amount of meteorite material ever discovered is only about 300 000 tonnes. This includes big lumbs like the single 60 t Hoba Iron Meteorite in Namibia. Although stony meteorites are the most common kind which fall, they also are the most difficult to find as they tend to look quite like normal earth rocks. Nickel-iron meteorites, due to their distinctive apperance, and very considerable density, are the most commonly available meteorites of all. The discovery rate for stony meteorites has been, until recent years, about 10%, compared to iron meteorites at 86.5% and stony-iron meteorites at 3.5%. Starting with the end of the 90th, when nomads in the Sahara Desert started looking for this 'rocks from heaven', a considerable amount of new meteorite finds, mainly Chondrites but also some Lunar and Martian Meteorites, have entered the market.
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Not quite as rare as meteorites, but even more mysterious are the objects called 'tektites'. These small, dark, glassy objects often have local names such as the 'moldavites' from the Moldau River in the Czech Republic, or the 'australites' which are found in Australia. The ultimate origin of the peanut- to walnut-sized fragments of glass is still not well understood, but their range of special shapes, including spheres, teardrops, dumbbells, and discs, often with pitted and etched surfaces, are very distinctive. No identical naturally occurring glass is found anywhere on earth. Geologically they are not particularly old, being from hundreds of thousands of years, up to 40 million years old, and with the timing of the last fall perhaps only 500,000 years ago. Theories of their origin range from being blown out of volcanic eruptions on the Moon, to flash melting of earthly rocks by the impact of giant meteorites, which is the most accepted theory. It is not surprising that in ancient times meteorites were attributed to have come from the Gods. Moslems from all over the world still make pilgrimages to Mecca, where within the Kaaba at the centre of their worship, there is a meteorite which quite believably did fall from the heavens. Meteorites have apparently been falling upon our planet since the beginning of time. One theory is that they are fragments of a shattered planet which formerly orbited between Mars and Jupiter. Another one is that they ere fragments of a not finally formed planet there in the Asteroid Belt between Mars and Jupiter. They are evidence of interplanetary history, and they help us to see how human history interconnects with the cosmos. For many billions of years they have circled through the Solar System until they encountered our Earth. They are the oldest 'space craft' of all, and are believed to have formed at the very beginnings of our Solar System, about 4.5 billion years ago, and so are even older than our Earth itself. A piece of meteorite is the oldest solid piece of matter any of us could ever own, far and away the most ancient rock we could ever have in our rock collection. |
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