Life's Far-Flung Raw Materials; July 1999; Scientific American Magazine; by Bernstein, Sandford, Allamandola; 8 Page(s)

For centuries, comets have imprinted disaster on the human mind. By 400 B.C. Chinese astronomers had sketched 29 varieties of comets, many foretelling calamity. Aristotle's assumption that comets were a warning from the gods gripped Western civilization for two millennia after the heyday of the ancient Greeks. Even at the close of the 20th century, comets and meteors play starring roles in cinematic tales of doom and destruction. The comet threat, it turns out, is not merely mythological. Modern science has revealed that a giant collision probably did in the dinosaurs, and in 1994 human beings nervously watched Comet Shoemaker-Levy 9 smash into Jupiter.

In light of their ominous reputation, it is ironic to consider that such far-flung space debris might be responsible for making Earth the pleasant, life-covered planet it is today. Since the early 1960s, space scientists have speculated that comets and other remnants of solar system formation hauled in gas and water molecules and that these components provided the atmosphere and oceans that made the planet habitable. A growing number of investigators, including our team at the Astrochemistry Laboratory at the National Aeronautics and Space Administration Ames Research Center, now believe that some important raw materials needed to build life also hitched a ride from space. Some of these extraterrestrial organic molecules formed leaky capsules that could have housed the first cellular processes. Other molecules could have absorbed part of the sun's ultraviolet radiation, thereby sheltering less hardy molecules, and could have helped convert that light energy into chemical food.