Geoscience Reference
In-Depth Information
times adhering, the inner planets were born. For hundreds of mil-
lions of years thereafter, impact continued alternately to destroy and
to rebuild their surfaces. One giant collision even carved the moon
from the earth. The early bombardment was so intense that the sur-
faces of the inner planets and their satellites melted completely.
Nothing escaped the inevitability of impact. Those objects that ap-
pear at first glance to have avoided it, for example, certain of Jupiter's
moons, turn out to have had recent volcanic activity or to be covered
with ice, obscuring the underlying craters. Every object in the solar
system has been shaped by myriad collisions. Three decades of re-
search have proven Gene Shoemaker right: Impact is "the most fun-
damental process."
The impact of comets and asteroids on the earth might not only
have destroyed life, it might have delivered it. The K-T boundary
clay contains amino acids not found elsewhere on our planet; per-
haps the early impacting comets brought with them other building
blocks of life that then combined and evolved to colonize Earth. Or,
perhaps life developed first on Mars and was brought to Earth by a
chunk of rock blasted off the red planet by impact. These are among
the exciting possibilities that scientists will be studying over the
next few years.
If impact is fundamental in the solar system taken as a whole,
Earth could not have escaped. We have discovered about 160 impact
craters, of which one—Chicxulub—was formed in the most energetic
event in the last billion years of earth history. Even though it is coun-
terintuitive, our intellect requires that we recognize that Earth has
been struck many more times than 160; it must have been hit thou-
sands, indeed tens of thousands of times. But where is the evidence of
these collisions and their effect on Earth and on life? Could 50 mil-
lion bombs the size of the one dropped on Hiroshima exploding
every 7 million years, and larger events less often, have had no effect
on life? So far, the evidence is insufficient to answer this question.
This contradiction between reason and observation could have one of
three explanations, or, more likely, a combination of all of them: First,
most of the evidence of impact may have been removed by erosion;
second, our methods of detecting impact may be inadequate; or
third, we may not have looked systematically enough. This third pos-
sibility represents an opportunity.
So far in this story, advances have come about in the traditional
way: through the efforts of scientists working alone or in small
groups, each following their intellectual curiosity, without an overall
strategy. Though many scientists would agree with Al Fischer, who
does not like "science by committee," one can still ask whether this
