By:Kelly Snyder
If you have ever walked through an open field, down a dirt road, or in the mountains, chances are that you stepped over a meteorite. But it is not likely that you would have taken notice because most meteorites, upon first glance, look like ordinary Earth rocks. Generally, a close and careful inspection is required to show that a meteorite is different from any rock that was formed on Earth.
What exactly is a meteorite?
A meteorite is a body of solid material that has fallen to Earth from space. It is usually in the shape of a stone or large rock. The size of a meteorite may range anywhere from that of a small pebble to that of a gigantic boulder. Since the time the Earth was formed, many millions of meteorites have landed on its surface. And since the time that humans have occupied the Earth, many hundreds have been seen to fall to the ground and were later recovered.
What are meteorites made of?
It is not always easy to distinguish meteorites from Earth rocks. It sometimes requires special equipment, such as powerful microscopes, to do so. One big difference between meteorites and Earth rocks that you can usually recognize is that nearly all meteorites contain metal. Only rarely is it present in Earth rocks. The metal in meteorites consists mostly of iron with smaller amounts of nickel and cobalt.
There are three main groups of meteorites, distinguished by their different composition.
Stones (stony meteorites) are silica rich; a large proportion of them contain chondrules, sphere-shaped material not found on Earth. Commonly about a millimeter in diameter, they are formed from an aggregate of the silicates olivine and pyroxene. The stony meteorites are the most common group, forming about 69 percent of all known meteorites (including all 'falls', seen to fall to Earth, and 'finds', picked up at a later date.)
Irons are metallic meteorites made chiefly of iron and nickel. They represent about 28 percent of known meteorites; because they are both tough and distinctive, they are well represented in collections, yet are only 7 percent of observed falls.
The last three percent consists of the stony-irons, which are half metallic and half silicate.
Most meteorites come from material which condensed or crystallized at the birth of the Solar System. However, a large carbonaceous chondrite, the Allende meteorite, which shattered when it landed in Mexico in 1969, has been a remarkable source of so-called presolar grains, which appear to have originated in the outer shells of stars which exploded before the formation of the Sun!
Space invaders?
As far back as the year 476 B.C., ancient Greeks recorded that 'a stone the size of a chariot' fell from the sky. Other ancient societies, Roman, Egyptian, Chinese and Japanese, also recorded similar meteorite events. People have used iron meteorites to make weapons, such as swords and knives. Others used them to make nails and anvils. There were also superstitions about meteorites. It has been said that the Chinese buried meteorites shortly after they were recovered so that their crops would not be ruined. Meteorites have also been thought to be messengers from the gods.
How fast does a meteorite fall?
Meteorites enter the Earth's atmosphere at a speed between 11 and 40 kilometers per second. The intrinsic velocities of Earth and meteorite can sum, so speed of entry depends greatly on the direction from which they came. The Earth moves around the sun at about 30 kilometers per second. If a meteorite comes from the opposite direction that the Earth is moving, the overall relative velocity can be as high as 73 km/s.
As a meteorite gets closer to the Earth's surface, the speed of entry is rapidly reduced by wind resistance. This effect is noticed when riding a bicycle into a strong wind. The air around us can do much to slow down a moving object. The final speed of a meteorite just before touching down may be only 1.5 kilometers per second or less.
What happens when it hits?
A meteorite will usually leave a hole, or crater, in the ground. The size of the hole will depend on the size, weight and speed of the meteorite. The depth of the hole depends on the softness of the ground.
One of the best examples of a large crater carved out by a meteorite is Meteor Crater in Arizona. The crater is nearly 1,300 meters or one mile across and about 180 meters or 600 feet deep. The impact occurred some 50,000 years ago.
Nearly all of the iron meteorite that produced the crater was vaporized. However, thousands of small meteorite fragments have been found throughout the area. Some 30 tonnes of iron have been recovered. These iron fragments have been named the Canyon Diablo meteorite. An excavation in the centre of the crater did not find a big mass, though a large amount of material has since been found embedded in the southern rim of the crater.
Where do meteorites come from?
Scientists have determined that meteorites come from within our Solar System. Roughly a dozen are thought to have been brought by impacts from the Moon to Earth, and a dozen more from Mars! The likely source of most of the remainder is small bodies called asteroids, in the asteroid belt. This is located between the orbits or paths of the planets Mars and Jupiter around the sun.
The asteroids in this belt vary considerably in size. The largest, named Ceres, is 1,025 km in diameter, about the size of the province of Ontario. It is possible that asteroids represent what was once a larger planet that broke apart.
Death of the dinosaurs
Many scientists believe that an asteroid that fell to Earth 65,000,000 years ago may have been responsible for the extinction of many animal and plant species. The incoming body, which has been calculated to have had a diameter of 10 kilometers, struck the planet at the end of the Cretaceous Period. At this time the dinosaurs suddenly became extinct. After dominating the Earth for millions of years, these great reptiles gave place to the tiny mammals that gained ascendancy during the Tertiary Period that followed.
Deadly cataclysm
The giant meteorite could have caused the extinction of species in a number of ways. If it landed in the ocean, it could have caused a tsunami (a giant tidal wave) as high as 100 meters. Some studies have revealed layers of marine debris consistent with the passage of a giant wave at this time.
The impact would also have thrown a vast amount of material into the atmosphere. It could have blocked out the light of the Sun long enough to disturb other forms dependent on these plants. Scientists know that 70 percent of all the life on Earth was extinguished at about this time. The Cretaceous-Tertiary boundary also shows widespread traces of soot, intense shock fractures in mineral grains, an iridium anomaly (a chemical signature of the impactor) and spherules of once molten rock. The crater of a meteorite on this scale would have been 200 kilometers wide.
Volcanic trigger
Both the Sudbury, Canada and Vredefort, South Africa meteorite craters show evidence of having triggered volcanic eruptions. Large-scale volcanic activity would have contributed significantly to the extinction of many species. Widespread eruptions would have added to atmospheric dust, which might have led first to a period of permanent winter and then, to an equally traumatic global warming, complete with deadly acid rain.
Meteors on schedule
Sometimes, Earth experiences the cosmic fireworks display of a meteor shower. Many meteors, associated with comets, may be composed largely of ice and dust. Minor meteor showers occur every year, reappearing regularly on particular dates. Every August, for example, peaking between the 9th and the 13th, the Perseid shower makes its display in the northern night sky. The Perseids are named after the constellation of Perseus, as they appear to come from that direction. Other regular meteor showers include the Draconids, which peak in intensity on October 9; the Orionids, on October 20; the Leonids, on November 16; and the Geminids on December 13.
Did you know?
The only living creature thought to have been killed by a meteorite in the 20th century was a dog, struck dead at Nakhla, Egypt in 1911. Reports of possible human fatalities come from ancient China, and from 17th century Dutch Travellers.
Meteors burn up in the atmosphere and fall down to Earth as dust. The dust may add roughly 100 tonnes per day to the mass of the Earth.
On August 10, 1972, a meteorite estimated to be 80 meters across entered the Earth's atmosphere. It came within 58 kilometers of the planet's surface and then left the atmosphere again, heading out into a solar orbit at an increased speed.
Meteorites provided some of the first readily accessible iron used by man. This iron may have been an important contribution to the transition from the Bronze Age to the Iron Age.
Meteorite impacts recorded in Egyptian papyrus manuscripts go back to 2,000 B.C. Scientists did not accept that extraterrestrial material could reach Earth until the early nineteenth century. The Sacred Stone of Mecca, one of Islam's most holy objects, is a meteorite.