Residing at the farthest reaches of the Sun's influence, comets did not undergo the
same heating as the rest of the objects in the solar system, so they retain, largely
unchanged, the original composition of solar system materials. As the preserved
building blocks of the outer solar system, comets offer clues to the chemical mixture from
which the planets formed some 4.6 billion years ago.
The geologic record of the planets shows that, about 3.9 billion years ago, a period of
heavy cometary and asteroidal bombardment tapered off. The earliest evidence of life
on Earth dates from just after the end of this heavy bombardment. The constant barrage
of debris had vaporized any water on Earth, leaving the planet too hot for the survival
of the fragile carbon-based molecules upon which life is based. Scientists therefore
wonder: How could life form so quickly when there was so little liquid water or
carbon-based molecules on Earth's surface? The answer may be that comets, which
are abundant in both water and carbon-based molecules, delivered essential ingredients
for life to begin.
Comets are also at least partially responsible for the replenishment of Earth's ocean
after the vaporization of an early ocean during the late heavy bombardment. While
Earth has long been regarded as the "water planet," it and the other terrestrial planets
(Mercury, Venus and Mars) are actually poor in the percentage of water and in carbonbased
molecules they contain when compared to objects that reside in the outer solar
system at Jupiter's orbit or beyond. Comets are about 50 percent water by weight and
about 10 to 20 percent carbon by weight. It has long been suspected that what little
carbon and water there is on Earth was delivered here by objects such as comets that
came from a more water-rich part of the solar system.
While comets are a likely source for life's building blocks, they have also played a devastating
role in altering life on our planet. A comet or asteroid is credited as the likely
source of the impact that changed Earth's climate, wiped out the dinosaurs and gave
rise to the age of mammals 65 million years ago.
Read more about comets in general.
This text is extracted from the NASA Deep Impact Launch Press Kit. More information on the Deep Impact mission is available
on the project home page.