Since the spectacular impact of comet Shoemaker-Levy 9 on Jupiter in 1994 much interest has been generated in the possibility of comets or asteroids colliding with the Earth. Duncan Steel is an astronomer engaged in the work of discovering such bodies which cross the Earth's orbit and assessing the risks of collision.
Until quite recently the possibility of asteroid and comet impacts on the Earth causing widespread death and destruction was not taken seriously by most scientists. Comets were thought to be too insubstantial to do much harm and it was believed that the asteroids were all safely confined to stable orbits between Mars and Jupiter.
Although all mythologies contained stories of catastrophes and most peoples shared a traditional fear of comets, the prevailing view among scientists during the past few centuries has been a uniformitarian one, the belief that changes on the face of the Earth happened gradually over very long periods. This view was strengthened by Darwin's theory of evolution. It was not until the 1930s that the first discoveries were made of asteroids which crossed the earth's orbit, but this information made little impression on entrenched beliefs of astronomers and geologists.
A good example of this attitude is the case of the Barringer Crater in Arizona .As early as the 1890sis was suspected by some geologists that it could be an impact crater, but it was not until the 1950s that this explanation won general acceptance as so many scientists clung to the idea that it must be volcanic. this was in spite of the fact that the rocks in the area are sandstone and limestone, with no igneous rocks such as granite or basalt which would indicate past volcanic activity.
Once impact craters began to be recognised, many more were discovered and investigated. The discovery in 1990 of the remains of a crater at least 180 kilometres in diameter in Mexico (the Chicxulub Crater) tended to confirm. the theory which had already been advocated by some earth scientists that the extinction of the dinosaurs and many other creatures 65 million years ago, indicated by a discontinuity in rock strata known as the Cretaceous/Tertiary boundary, was caused by an asteroid impact. This was because the Chicxulub Crater was also estimated to have been formed 65 million years ago.
It is calculated that the effect of this impact was to raise the surface temperature of the Earth to over 1000 degrees C. Later the air temperature over the continents would have fallen by up to 40 degrees C below normal for a year or so, as the immense amount of dust in the atmosphere would prevent the sun's heat from [reaching the surface. It is difficult to how any land-dwelling animals could have survived such conditions, but many evidently did. Steel is careful to point out, though, that it is very difficult to make accurate estimates of the precise effects of such impacts.
There is a somewhat speculative chapter on Stonehenge and the Pyramids which the author presents as a basis for discussion and argument. the oldest parts of Stonehenge were constructed about 5000 years ago at the same time that similar structures were independently devised and constructed in other parts of the world. He attributes this to the effects of meteor showers associated with Comet Encke. He thinks that these showers may have been more intense and have contained much larger meteors in those times, and he calculates that these spectacular and alarming events would have occurred every 19 years. The alignments of Stonehenge and similar structures indicate that they could have been used to predict such events by making the appropriate astronomical observations and calculations.
I recommend this work to all those interested in astronomy and earth sciences, and to connoisseurs of actual and potential natural disasters. -- John Harney. First published in Magonia, 1995.