Tuesday, September 28, 2010
I often wonder why I am interested in so many things, and more importantly how these interests developed. In the case of meteorites, I believe I can trace it to the fascinating meteorite display in the Field Museum of Natural History in Chicago, and some summer nights watching some dramatic meteor showers light up the sky.
The earth is regularly struck by debris from space. It has been hit by about 130,000 meteorites large enough to produce a crater 2/3's of a mile wide. Although this kind of impact site is visible on the moon, earth is so geologically active that evidence of many impacts have disappeared. So far, more than 160 impact sites have been identified worldwide, with new craters discovered every year.
Most meteorites are fragments of asteroids, though some are produced when asteroids hit the surface of the moon or mars, flinging debris into space. When they are outside the atmosphere these rocks are meteoroids, while those that enter the atmosphere are meteors. These are usually so small that they burn up as they plunge toward earth, leaving a bright trail in the sky, often called a shooting star. Meteorites are objects large enough to travel through the atmosphere and hit the ground. If they are the size of a house or larger they blast out a crater.
The first major impact structure to be identified was Meteor Crater in Arizona, with its distinctive bowl shape. It is the result of the most recent significant meteorite impact on earth.
When a meteorite strikes the earth, the impact sends shock waves through the ground, squeezing the surrounding rock to two, or three times its normal density. The compressed rock then springs back, shattering into fragments, hurling chunks upward, and outward, along with pieces of the meteorite that have not yet vaporized. The result is a bowl shaped crater that is much larger than the meteorite.
The potential energy of a significant meteorite impact on the earth is more than one hundred million megatons-more than the world's entire nuclear arsenal.
Buried deep beneath the limestone of Mexico's Yucatan Peninsula, Chixulub is one of the largest meteorite cImage via Wikipediaraters found on earth. The impact was catalclysmic: fires raged over the surface, giant tsunamis radiated across the oceans, and the planet was rocked by massive earthquakes. Many scientists believe that the devastating global effects of the Chixulub impact caused the extinction of the dinosaurs. They believe that debris from the impact was suspended in the atmosphere. This would have blocked out the Sun's rays, turning the world ice cold, and spelling doom for the dinosaurs.
Much of the information for this post comes from the Smithsonian Institution's book called Earth.