Paper: Nevada State Journal City: Reno, Nevada Date: Sunday, November 07, 1965 Page: 34
By Prof. Wendell A. Mordy Director Desert Research Institute, University of Nevada, Reno
Last summer, the Mariner 4 photographs showed the surface of Mars is covered with craters, like those on the moon. The craters apparently are caused by giant meteorites colliding with the planet. Some scientist think this is an indication that meteorites bombard all bodies in the solar system, including the earth. Dr. Frank Dachille, associate professor of Geochemistry at the Pennsylvania State University, thinks that there are many such craters on earth. The outlines of most of them has been softened by erosion, or covered by vegetation or water, according to Dachille. He is co-author of a highly controversial book on the evolution of Earth, called "Target Earth." In that book he lists more than 70 huge depressions on earth which he thinks problably were caused by meteorite impacts. The list includes the entire Michigan Basin, the Wells Creek Basin in Tennessee, and many others, in addition to depressions such as Meteor Crater in Arizona, Deep Bay in Canada, and the tiny Odessa Crater in Texas, which are more obvious and which scientists pretty generally agree were formed by meteorite bombardments.
'Falling Stars'
Small meteorites frequently hit the earth, and are recognized as "falling stars." Occasionally small fragments are found. It takes only about 1,000 years for rain, snow, food, fire, earthquakes, plants, animals and other influences to destroy the traces of the impact of sizeable meteors, according to Dachille. Dachille estimates that the million meteors which have hit the earth during the last 5 billion years, the traces of probably not more than 250 remain sufficiently preserved to be recognized, even by aeriel photographs, or eventually satellite photographs. However, there are other ways to discover the location of hits by meteorites, according to Dachille. Dachille and his colleagues have found minerals from suspected crater sites outwordly appearing as if they were hardly damaged, but with their crystal structure so highly deformed that they were actually powder, rather then crystals as determined by X-ray. Dachille is experimenting to create laboratory conditions approaching those which must occur when a large body crashes into the earth. He says, "Minerals showing severe crystal damage coming from suspected craters, in contrast with those from surrounding rocks, support the hyprothesis that the craters were formed by meteorites." A meteorite which can dig a crater 300 miles in diameter would have to be about 30 or 40 miles in diameter, he estimates. Bodies this size could be quite common in the solar system, and would represent 10 billion times the amount of energy released by the first atomic bombs. Disappears
When such a large meteorite hits the Earth, it is vaporized and completely disappears, he says, just as fragments of superbombs don't survive. Since there is no water or growth on the moon to gradually remove the scars of meteorite impacts, about 60,000 circular scars, some of which are hundreds of miles across, still can be seen on the moon. Dachille says there is no reason to think that the earth could have avoided the kind of bombardment which has occurred on the Moon and Mars. Three large bodies, with energies as large as the Hudson Bay Meteorite have passed within a few miles of Earth since the turn of the century, Dachille points out. There is no reason to think that some of this size wouldn't have struck the earth during its lifetime. He estimates that giant meteorites, like the ones which have formed the Gulf of St. Lawrence or part of Hudson's Bay, only happen once in 10 million years or so. He thinks that lunar explorations will teach us a lot about dating the formation of craters, and the mechanism which created them. In the meantime, he says, "It should be useful as well as instructive at least to consider the possible effects of large collisions - throughout the solar system." Please visit, www.MeteoriteArticles.com, a free on-line archive of meteor and meteorite articles. |
