Re-examining the role of extraterrestrial organic matter in the origin of life
by John L. Roeder
"We are living on an insignificant speck of rock going around an undistinguished star in a low-rent section of the galaxy. . . .only on this insignificant speck of rock have beings evolved who can look at the universe and ask the question, 'Why?'" So wrote James Trefil in a book he co-authored with Robert Rood, are we alone? The Possibility of Extraterrestrial Civilizations (Scribner's, New York, 1981). The likelihood of extraterrestrial intelligent life is usually assessed by SETI (Search for ExtraTerrestrial Intelligence) enthusiasts with the so-called "Green Bank" equation (so named after the site of the conference at which it was first posited). I was so taken aback by Rood and Trefil's conservative estimates for the terms in this equation when I reviewed their book for The Physics Teacher (20, 190 (1982)) that I felt compelled to read Walter Sullivan's We Are Not Alone, published 15 years earlier (McGraw-Hill, New York, 1966 (rev. ed.)).
But Sullivan agreed with Rood and Trefil on one thing: the search for extraterrestrial intelligence would be conducted more efficiently by conducting earthbound observations than by venturing into outer space. This was the case even as recently as 1992, when Frank Drake spoke at the 1992 NSTA Area Convention in New York City (reported in our Winter 1993 issue) and wrote (with Dava Sobel) Is Anyone Out There? The Scientific Search for Extraterrestrial Intelligence (Delacorte, New York, 1992), which I reviewed in the same issue.
Then, with the claim by McKay, et al., of the possible existence of life on early Mars (Science, 273, 924-930 (16 Aug 96)) the outlook changed. Maybe it wasn't intelligent life; but if it was life of any kind, the Earth is no longer a unique site for generating living organisms. Donald Goldsmith's The Hunt for Life on Mars (Dutton, New York, 1997), reviewed in this issue, not only details the work of McKay, et al., and their objectors, but also indicates other sites being searched in our solar system for evidence of living organisms, most notably the Jovian satellite Europa. Interestingly, one of Sullivan's arguments for the likelihood of extraterrestrial life was the profusion of organic molecules in outer space, especially in our own solar system's comets and asteroids.
More recently, Chemical & Engineering News, 75(48) 20 (1 Dec 97) reports a symposium at the Fifth Chemical Congress of North America, held at Cancœn, Mexico (near the site of the impact crater believed to be responsible for the demise of the dinosaurs 65 million years ago). The subject was the origins of organic matter on early Earth, and two sources weighing very heavily in the symposium's deliberations were asteroids and comets, either of which probably made the Cancun crater. Noting that "300 tons of organic matter reach Earth every year in the form of interplanetary dust," William Irvine (University of Massachusetts, Amherst, Five College Radio Astronomy Observatory) estimated that "comets, meteorites, and dust could easily have brought in enough organic molecules and water to make up the raw materials for the Earth's hydrosphere and biosphere." Whether this actually happened is another story. And studies of isotopic distributions show that comets could not have been the source of all the noble gases and water found on earth. But the presence of extraterrestrial sources of organic matter is clearly prompting a reconsideration of their role in the origin of life.
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