Pre-biotic organic matter from comets and asteroids.

Several authors have suggested that comets or carbonaceous asteroids contributed large amounts of organic matter to the primitive Earth, and thus possibly played a vital role in the origin of life. But organic matter cannot survive the extremely high temperatures (>10(4) K) reached on impact, which atomize the projectile and break all chemical bonds. Only fragments small enough to be gently decelerated by the atmosphere--principally meteors of 10(-12)-10(-6) g--can deliver their organic matter intact. The amount of such 'soft-landed' organic carbon can be estimated from data for the infall rate of meteoritic matter. At present rates, only approximately 0.006 g cm-2 intact organic carbon would accumulate in 10(8) yr, but at the higher rates of approximately 4 x 10(9) yr ago, about 20 g cm-2 may have accumulated in the few hundred million years between the last cataclysmic impact and the beginning of life. It may have included some biologically important compounds that did not form by abiotic synthesis on Earth.