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Comets as cradles for organic life

Within six years Halley's comet will again blaze across the sky. Its apparitions have been taken as a variety of omens in centuries past. This time will it perhaps be seen as a herald of life?

W. M. Irvine, S. B. Leschine, and F. P. Schloerb of the University of Massachusetts point out that comets may provide a more hospitable environment for organic life, or at least its chemical precursors, than may have been thought.

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The notion that comets can be vats for the formation of biologically important chemicals -- if not indeed sites for the evolution of primitive living organisms themselves -- has been advanced by Sir Fred Hoyle and N. Chandra Wickramasinghe at University College in Cardiff, Wales. They have little more to go on than imaginative (some critics would say naive) speculation. Most scientists interested in life's origins have scoffed at the suggestion or ignored it entirely.

However, instead of rejecting the proposition out of hand, Irvine, Leschine, and Schloerb have used what facts are known about comets and some simple thermodynamic reasoning to infer the kind of environment that might exist inside a typical comet. Their conclusion, as stated in a letter in Nature, is that "comets may contain quite complex molecules, and that comets may have played a role in the origin and conceivably even in the subsequent evolution of terrestrial life.

Pointing out that comets are thought to have formed out of the original dust and gas of the nebula that evolved into the solar system, the three scientists note that there would have been enough radioactive material to heat a comet interior significantly. They infer that environments with liquid water present and with temperatures suitable for chemical evolution could exist within comets for hundreds of millions of years or, as they put it, "over geologically and biologically significant times." The temperatures involved could range from above water's normal boiling point down to about freezing. Under such conditions and given the makeup of a comet, biochemicals could be expected to evolve. This lends some theoretical support to the Hoyle-Wickramasinghe hypothesis, although it doesn't, of course, constitute any proof of it.

Hoyle and Wickramasinghe have suggested, among other things, that the fallout from comet tails may even now sometimes seed Earth with organic chemicals or alien living viruses. This notion has had few takers. As biologist Lynn Margulis of Boston University said in a review of their book "Lifecloud: the Origin of Life in the Universe," this and other aspects of their thesis "is entirely contrary to the considered opinion of most workers in the field."

Now the three University of Massachusetts scientists have joined Hoyle and Wickramasinghe in the minority at least to the extent of saying that their comet study makes the Hoyle-Wickramasinghe view seem a little more plausible.