Today, more than 200 years after the publication of Chladni's 'infamous' book, we know that meteorites (from the Greek word meteoros, meaning 'high in the air') are chunks of extra-terrestrial matter, remnants of geological processes that formed our solar system 4,600 million years ago. When these chunks enter Earth's atmosphere they shine brightly because of the heat produced by friction with the air. Most chunks are too small - usually the size of a grain of sand, but no larger than a pea - to survive the trip, and are called meteors (or falling stars or shooting stars because they leave momentary streaks of light in the sky).
Very rarely, a large chunk, which flashes like a fireball in the sky, survives its journey through the air to hit the ground. The falling object - a solid piece of stone or iron, often weighing many kilograms - is known as a meteorite. (Until a meteor or a meteorite enters Earth's atmosphere it is known as a meteoroid.) Asteroids (also called planetoids or minor planets), on the other hand, are small bodies orbiting the Sun, mostly in between Mars and Jupiter. Most meteorites are pieces of rock and/or metal from asteroids; most meteors are produced when comets disintegrate (comets are independent masses of ice and dust that orbit the Sun).
The fall of a large meteorite is a rare but spectacular event. The meteoroid enters Earth's atmosphere at a very high speed, ranging from 40,000 to 250,000 kilometres per hour. The friction against air not only decelerates it, but also raises its temperature. At a height of about 100 kilometres the meteoroid is so hot that it shines like a fireball. Its outer layer is continuously vaporised and ejected, leaving a trail of dust and smoke. At a height of about 20 kilometres, its speed has been so much reduced - it is about 10,000 kilometres per hour - that it no longer glows. It decelerates further to a free-fall speed of somewhere between 320 and 640 kilometres per hour until it reaches the ground. Because of high speed and high temperature, most large meteoroids break into several or sometimes thousands of fragments at a height of between 11 and 27 kilometres. The fragments strike the Earth in a roughly elliptical pattern a few kilometres long. The 'shower of stones' at Siena in 1794 was this type of fall.
A meteoroid's spectacular entry into the atmosphere is accompanied by an equally spectacular sonic boom. Because sound travels quite slowly, at only about 1,200 kilometres per hour, it takes from 30 seconds to several minutes after the appearance of the fireball before any sonic boom can be heard. However, many witnesses have claimed that they heard strange noises as a meteorite streaked across the sky. Known as electrophonic sounds, these range from hissing static to the sound of an express train travelling at high speed. Electrophonic sounds have not yet been validated scientifically, but scientists suspect that light given off by a meteorite must be accompanied by invisible electromagnetic radiation in the form of VLF (very low frequency) radio waves at frequencies from 10 hertz to 30 kilohertz. These waves could reach the observer as soon as the meteorite approached, but you wouldn't hear them. Often, the witness of such sound is located near metal objects. It is possible that such objects act like a transducer, converting inaudible electromagnetic waves into audible sound vibrations.
Meteorites contain various proportions of metals (iron and nickel) and stones (silicates). Thus, meteorites can be divided into three simple categories: irons consist mainly of metals; stones consist of silicates with little metal; and stony-irons contain abundant metals and silicates.
The largest known meteorite is still lying where it fell in prehistoric times in Hoba, Namibia. This room-sized meteorite is one metre high and weighs 60 tonnes. It is mostly iron. The most famous sacred meteorite is the black stone of the Ka'bah, which now lies in the Great
Mosque in Mecca, Saudi Arabia, towards which Muslims pray five times daily. Islamic tradition has it that the stone came from heaven and was originally hyacinth in colour before it turned black because of humanity's sins.
A large meteorite or asteroid hits the ground with such an enormous force that it shatters into pieces and leaves a big hole - a crater. How would you recognise a meteorite crater if you fell into one? When the meteorite shatters at the moment of impact, the pulverised earth and meteorite fragments are hurled out of the crater and scattered around it, but a considerable part falls back into it. This causes craters to display raised and overturned rims. But most of the time this above-surface evidence is erased as the Earth's surface is always changing. After thousands of years of weathering and erosion by wind, rain, ice, changes in temperature, gravity and activities of animals and plants, a crater may not look like a crater.
Perhaps the most important characteristic of a crater is the presence of meteorite fragments in the vicinity of it. Only small craters are expected to have meteorite fragments. If a meteorite explodes the moment it strikes the ground, most of it is changed into gas. The main feature of craters produced by such meteorites is the complete absence of fragments. Another feature is that these craters have diameters of more than 1 kilometre. Thus the absence of meteorite material is not evidence against a meteorite impact.
Even if a meteorite does not leave any fragments, it leaves some evidence of impact. Geologists look for three types of impact evidence:
■ Impactites. The impact produces so much heat that the rocks melt and splatter into the air. As the drops of melted rock cool they turn into glassy globules, called impactites. They often contain iron-nickel grains, remnants of the meteorites. Depending upon how much glass and other minerals they contain, these globules are sometimes given specific names such as tektites, krystites and suevites.
■ Shocked quartz. The impact also shatters the rocks, throwing tiny grains of quartz into the air. The shattering is so violent that it leaves patterns on these grains, known as shocked quartz.
■ Rare elements. Certain elements such as iridium are rare in rocks in the Earth's crust. A gigantic impact can scatter these elements all over the world.
Since the late 1950s geologists have confirmed only 160 impact craters. They all were formed by metal meteorites, and their diameters range from 10 metres to 200 kilometres. No craters associated with stony meteorites have been found. The reason perhaps is that stony meteorites disintegrate in the atmosphere. Even if some fragments have survived the journey, they may not have survived the terrestrial weathering and erosion.
The world's first authenticated and best-preserved impact crater is in Arizona. Known simply as the Meteor Crater, its rim-to-rim diameter is 1.2 kilometres and its circumference is nearly 5 kilometres. Its depth below the surrounding plain is about 175 metres, with a 45-metre-high rim rising above the plain. It was gouged in about 50,000 years ago by a meteorite with the diameter about the width of a football field. The original meteorite, packed with more than 300,000 tonnes of iron and nickel, was travelling with a speed of about 64,000 kilometres per hour. It was strong enough to pass through the atmosphere without breaking into pieces.
The Meteor Crater has also found a place in the annals of foolhardy investments. In 1902 Daniel Moreau Barringer, a successful mining engineer from Philadelphia, heard about the crater and the small balls of iron scattered around it. He rejected the prevalent idea that the crater was formed by a volcano and formed the view that it was a meteorite crater. He estimated that an iron-nickel body weighing between 5 and 15 million tonnes lay beneath the surface. In 1903, without ever having seen the crater, he formed the Standard Iron Company and applied for and received from the United States government a 199-year lease (signed by President Theodore Roosevelt himself) on two square miles of land around the crater. Over the next 26 years, until Barringer's death in 1929, the Standard Iron Company spent more that $600,000 (a considerable fortune in those days) drilling scores of holes - the deepest reached 412 metres - but produced nothing except tiny samples of meteorite material which contained 93 per cent iron and 7 per cent nickel and traces of other elements, including precious platinum and iridium.
The 'fool's iron' probably gleamed in the eyes of Russian authorities when in 1921 they decided to find meteorite falls which had been recorded in Russia during and after the war years. The new regime formed after the October Revolution of 1917 (the Union of Soviet
Socialist Republics was born in 1922) was reeling economically when the news of the iron bonanza of the Meteor Crater reached Moscow. Authorities dreamed about discovering iron worth a fortune that had fallen within Russia. A bright 38-year-old scientist was put in charge of finding these 'treasures from space'.
Was this article helpful?