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The Question
(Submitted February 21, 1998)
I am a student currently in the 10th grade. I have been curious
to whether life could exist on Europa (or any other unexplored space
body) in different forms. For example, since Earth is shaped around the
element, carbon, could life exist revolving around a different element?
The Answer
Scientists have occasionally speculated that life could be based on an
element other than carbon. Silicon, being the lightest element with
an electronic structure analogous to that of carbon (having a half-filled
outer shell with 4 unpaired electrons), is the most likely candidate
mentioned. However, carbon's tendency to form the long chains and rings
that form the basis for organic compounds that at some level of complexity
begin to self-replicate is unique. Also, because older stars naturally
produce carbon, along with nitrogen and oxygen (its neighbors on the
periodic table), it is relatively abundant in the universe. Many
astrophysicists who study the spectra of stars believe that complex chains
and even rings of carbon appear in such unlikely places as stellar
envelopes (e.g., in the form of PAHs, polycyclic aromatic hydrocarbons).
When such compounds reach cooler regions of space where they can bond with
readily available hydrogen, organic compounds as we know them are
naturally formed.
Although other elements may form complex, covalently bonded structures,
none has the rich molecular variety of carbon. It is the chemistry of
carbon that allows us to consider the possibility of life "as we know it"
in other parts of the Galaxy and the Universe beyond. We do not know
whether Earth-like conditions exist elsewhere; but if they do, it is
highly likely that life forms, if they exist, will be based on carbon.
One more point: The organic types of structures appearing in stellar
envelopes are very hot and probably stripped of hydrogen, so that they are
not themselves alive; it is only when carried off to a more hospitable
environment, such as a much cooler planet 100 million miles away or so,
that the kind of chemistry required by life becomes possible on a scale
large enough to allow for stable development and replication. Water is
also a factor, causing the hydrophobic proteins to clump together at all,
and serving as a medium a conduit for new material, protection from
temperature changes and harmful stellar radiation, etc.
Mark Kowitt and Damian Audley
for Ask an Astrophysicist.
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