Dust from disintegrating comets, rich in the molecular building blocks of life, may have rained onto Earth in the distant past, providing a relatively gentle way for such chemicals to reach the surface without being burned up.

How such molecules could have survived the fiery heat of atmospheric entry to reach Earth's surface has been a major stumbling block for those who believe chemicals formed in deep space may have played a role in the origins of life.But Kevin Zahnle and David Grinspoon of NASA's Ames Research Center in California, writing Wednesday in the British journal Nature, argue that the breakup of giant comets in the inner solar system could have produced clouds of dust rich in the molecular building blocks of life.

As Earth sailed through those clouds in its orbit around the sun, amino acids and other chemicals could have made it to the surface without the excessive heating associated with violent impacts.

"If the dust shed by large comets frequently produced rich harvests of interesting chemicals on early Earth, and did so independently of whether the comet hit Earth, the potential for an important . . . contribution to (pre-life) organic chemistry is increased dramatically," Zahnle and Grinspoon wrote.

The researchers arrived at their conclusion after studying the distribution of specific amino acids above and below a layer of sediment associated with the impact of a giant meteorite some 65 million years ago.

Evidence for such an impact, which is associated with the widespread extinctions of numerous plants and animals, is found in a thin layer of sediment that contains larger-than-normal amounts of iridium, a material that is rare in Earth's crust but common in some types of meteorites.

Amino acids, which combine to form proteins, have been found just above and below the layer but not inside it.

Zahnle and Grinspoon argue that a giant comet trapped in the inner solar system may have disintegrated over a long period of time, "raining amino-acid-rich dust onto the planets," according to a summary provided by Nature.

"The body responsible for the impact itself could have been a larger fragment of this disintegrating comet," the summary said. "The Earth would continue to sweep up dust even after the impact, until the giant comet had either disintegrated completely or careened chaotically back to the outer solar system."

Even though amino acids could have made it to Earth's surface in this manner, it is unlikely that such molecules were created by biological processes, according to Christopher Chyba of Cornell University in Ithaca, N.Y.

Writing in Nature, Chyba said it is far more likely that such "biologically interesting" molecules were formed by non-biological space activity.