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Why Study Comets?

Why Study Comets? 
Don Yeomans 
Jet Propulsion Lab
April 1998

Life on Earth began at the end of a period called the late 
heavy bombardment, some 3.8 billion years ago.  Before this 
time, the influx of interplanetary debris that formed the 
Earth was so strong that the proto-Earth was far too hot for 
life to have formed.  Under this heavy bombardment of asteroids
and comets, the early Earth's oceans vaporized and the fragile
carbon-based molecules, upon which life is based, could not 
have survived.  The earliest known fossils on Earth date from 
3.5 billion years ago and there is evidence that biological 
activity took place even earlier - just at the end of the 
period of late heavy bombardment.  So the window when life 
began was very short.  As soon as life could have formed on 
our planet, it did.  But if life formed so quickly on Earth 
and there was little in the way of water and carbon-based 
molecules on the Earth's surface, then how were these 
building blocks of life delivered to the Earth's surface so 
quickly?  The answer may involve the collision of comets 
with the Earth, since comets contain abundant supplies of 
both water and carbon-based molecules. 

As the primitive, leftover building blocks of the outer solar 
system formation process, comets offer clues to the chemical 
mixture from which the giant planets formed some 4.6 billion 
years ago.  If we wish to know the composition of the 
primordial mixture from which the major planets formed, 
then we must determine the chemical constituents of the 
leftover debris from this formation process - the comets.  
Comets are composed of significant fractions of  water ice, 
dust, and  carbon-based compounds.  Since their orbital paths 
often cross that of the Earth, cometary collisions with the 
Earth have occurred in the past and additional collisions are 
forthcoming.  It is not a question of whether a comet will 
strike the Earth, it is a question of when the next one will 
hit.  It now seems likely that a comet struck near the Yucatan 
peninsula in Mexico some 65 million years ago and caused a 
massive extinction of more than 75% of the Earth's living 
organisms, including the dinosaurs.  

Comets have this strange duality whereby they first brought the 
building blocks of life to Earth some 3.8 billion years ago and 
subsequent cometary collisions may have wiped out many of the 
developing life forms, allowing only the most adaptable species 
to evolve further.  Indeed, we may owe our preeminence at the top 
of Earth's food chain to cometary collisions.   A catastrophic 
cometary collision with the Earth is only likely to happen  at 
several million year intervals on average, so we need not be 
overly concerned with a threat of this type.  However, it is 
prudent to mount efforts to discover and study these objects, 
to characterize their sizes, compositions and structures and 
to keep an eye upon their future trajectories.   

As with asteroids, comets are both a potential threat and a 
potential resource for the colonization of the solar system in 
the twenty first century.  Whereas asteroids are rich in the 
mineral raw materials required to build structures in space, 
the comets are rich resources for the water and carbon-based 
molecules necessary to sustain life.  In addition, an abundant 
supply of cometary water ice can provide copious quantities of 
liquid hydrogen and oxygen, the two primary ingredients in 
rocket fuel.  One day soon, comets may serve as fueling stations 
for interplanetary spacecraft.