Although the return of Halley's comet is still half a decade away, a news service has sent out a 1910 picture to illustrate an anticipatory story. It shows how romantic we are about a bit of space debris left over from the formation of the solar system.
Indeed comets, which inspire folk tales and prophecy, also tempt scientists to fanciful speculation. Some of the more colorful notions involve head-on collisions or near misses between Earth and comets that bring such catastrophes as the extinction of the dinosaurs and seed our planet with foreign materials, including alien forms of life.
Unfortunately, there is little hard evidence on which the theorists can build. Even the likelihood of a cometary encounter is unclear. Theoretical estimates, such as those of British astrophysicist D. W. Hughes, indicate there should have been at least one massive impact over the past 600 million years. But there is no direct evidence. This is not surprising since the comet probably would have struck the ocean, which covers three-quarters of Earth, where any crater would go unnoticed if it has not already disappeared through geological action.
The Moon might be a better place to look. Earlier this year, P. H. Schultz and L. J. Srnka at the Lunar and Planetary Institute, Houston, suggested that certain exceptionally bright and dark swirls were made by comet impacts within the last 100 million years. L. L. Hood of the University of Arizona recently challenged this, maintaining the markings are more probably linked to meteroid impacts occurring over the entire age of the solar system. However, in a recent exchange of letters in Nature, Schultz and Srnka stood their ground, insisting that the swirls are very young features, probably cometary in origin.
While this is not proof, it is the best direct indication yet that major cometary encounters within the Earth-Moon system can occur, and have occurred, within what geologically speaking is our own era. What makes this a tempting subject for speculation is that a cometary collision would be dramatic.
Reviewing the prospect last May in Nature, Kenneth J. Hsu of the Swiss Federal Institute of Technology pointed out that a "typical" comet -- 30 kilometers in diameter, a trillion metric tons of mass, moving at 45 km a second (160,000 km an hour) -- would hit with an energy equivalent to that of 250,000, 000 one-megaton hydrogen bombs or an earthquake measuring around 11 on the Richter scale, 25,000 times more energetic than the largest quake every recorded (8.5 on the scale). Even a small fraction (say 10 percent) of that enormous energy could cause substantial frictional heating of the air and sea. Sudden heating, Hsu notes, is believed to be implicated in the mass extinctions, including that of the dinosaurs, that occurred some 65 million years ago.
What is more, he suggests, the comet could have poisoned the seas. Comets appear to contain a variety of chemicals including cyanides and probably carbon dioxide or carbon monoxide. Perhaps both cyanide poisoning and an upset in the ocean's carbonate chemistry on which many microscopic but important marine organisms depend contributed to the extinctions in the sea. It is an interesting, but so far unsubstantiated, notion.
This sort of speculation has been carried to an extreme by British cosmologist Sir Fred Hoyle and N. C. Wickramasinghe of University College at Cardiff, Wales. They maintain that comets are cradles for the evolution of life-related chemicals and simple life forms such as viruses. Comets, they say, may once have seeded Earth with life and may still be injecting alien organisms into Earth's atmosphere, perhaps starting epidemics. This is too much to swallow even for scientists who like to speculate about comets. When Hoyle presented the latest version of the theory at a seminar in London in early September, there reportedly were no takers.
Now that the news media are beginning to dust off old pictures of Halley's comet, there probably will be a lot of comment about possible Earth-comet collisions. This can be good intellectual fun. But don't take it too seriously. Nobody knows what did the dinosaurs in. Nobody knows whether or when a comet ever hit Earth.In fact, nobody yet knows in any detail what comets are made of.