About 220 million years ago some very large creatures menaced the Earth (and each other!) Humans have been fascinated with dinosaurs for a long time, they have caught the fancy of young and old alike. We have come to learn the these creatures were very good at adapting, and many were quite smart. They ruled the Earth for nearly 200 million years, being second to no other species. These animals evolved until the great Cretaceous extinction event. That  was when the mammals took over.  But, how could the dinosaurs have ruled for so long, and then,  in what is a blink of an eye geologically, die out? The answer, it is believed, is of astronomical origin.

      It is now widely accepted that asteroids and comets may have had a significant impact on life on Earth. What would happen if such a body impacted the Earth? If an object about 5 miles in diameter impacted the Earth, it would strike the surface with the force of 1014  tons of TNT. Temperatures at the impact site would rise to over 20,000 degrees F. Anything within a few miles of the impact would be leveled. Some of the blasted material would be thrown back into space only to rain back down upon the Earth.



      The environment would be affected in extreme ways. The blast would deposit massive amounts of minute dust particles high into the atmosphere where they would be suspended probably causing darkness like night for several months. The dust particles would be circulated globally by winds and would block out a considerable amount of sunlight reaching the Earth. This in turn, would cause a sharp decrease in photosynthesis in plants. It would also cause an overall decrease in temperature, killing all plants and animals that were temperature sensitive. Countless other species would die from starvation, large plant eaters would find it difficult to get enough food, and as they died out, the meat eaters would soon follow.

     The fossil record clearly shows that 65  million years ago there was an abrupt loss of ocean plankton, mollusks, dinosaurs and animals of large mass. Thousands of species of plants died out also. It was clear that something happened very suddenly, but what?  The impact theory was thrown back and forth for some time with much debate. Evidence for it  came from a layer of clay that was deposited at that time. Below the layer, there is the usual range of fossils of dinosaurs and other large animals. Above the layer there is complete absence of many of these fossils. The layer itself has a composition rich in iridium and gold. The overabundance of iridium shows up worldwide from samples of material 65 million years old.

     Iridium is rare here on Earth. Asteroids happen to be rich in iridium. To do as much damage as this object had done, it would have had to be about 7 miles in diameter. And it would have left a crater about 120 miles in diameter. If such a crater could be found with the proper age, astronomers would have the smoking gun to prove their theory. The culprit was indeed found, in the Yucatan of Mexico. It is called Chicxulub crater, and it has the proper diameter and is about 65 million years old. Here, it is believed, lies the gravestone of the dinosaurs.