A lively debate
Sure asteroids can be destructive. But as junk left over from the formation of the four inner planets-- Mercury, Venus, Earth and Mars -- they also deserve study in their own right. Asteroids escaped the erosion, heat and chemicals that have altered ancient rocks on Earth, and represent our best chance for glimpsing the primordial solar system. "Scientifically, it's very important to study the chemistry and structure of the objects that formed the Earth," says Donald Yeomans, a Jet Propulsion Laboratory expert on the sub-planetary orbiters.
The oldest known fossils are cyanobacteria like this filamentous Palaeolyngbya from Archaean rocks of western Australia, dated
3.5 billion years old. ©1994 by The Museum of Paleontology of The University of California at Berkeley and the Regents of the University of California.
Only after the bombardment ended about 3.8 billion years ago did the first fossil evidence of life -- in the form of bacteria -- appear.
The hot-and-heavy bombardment may have played a dual role, he adds, not only suppressing life, but also supplying the water and carbon molecules that later permitted life to begin. And it's even possible that one of those meteorites brought some stray DNA to the planet -- thus seeding a barren planet with life.
Are asteroids rocks, grit or what?
Yeomans is one of a growing number of scientists who believe asteroids and comets played a crucial role in the origin of life. During the first billion years of the solar system, asteroids and comets blasted Earth so regularly that "the building blocks of life [water and complex organic molecules] could not survive," says Yeomans.
