Newsgroups: alt.sci.planetary
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Titan? What do we know about it?
Date: Wed, 7 Aug 1996 14:51:27 GMT

In article <4u7n4p$k77@news.sandia.gov> Kent Budge <kgbudge> writes:
>...Ammonia is a much
>better substitute for water than methane since it is also polar, but I don't
>believe the reaction 2NH3 --> NH2- + NH4+ is nearly as thermodynamically
>favorable as the reaction 2H2O --> OH- + H3O+...

The dissociation constant is rather less, but it's probably enough to be
workable.  You'd need more enthusiastic reactions, because of this and the
lower temperature, but the numbers are not so far away from the water-
based version as to be ridiculous.  (Most of the other non-water solvents
are so different from water that our familiar water-based chemistries just
wouldn't work at all, and we're reduced to handwaving about vague
alternatives that might.)
-- 
 ...the truly fundamental discoveries seldom       |       Henry Spencer
occur where we have decided to look.  --B. Forman  |   henry@zoo.toronto.edu

Newsgroups: alt.sci.planetary,alt.life-mars,sci.geo.geology,sci.bio.misc
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Why all the assumptions?
Date: Sun, 13 Apr 1997 06:40:57 GMT

In article <334D9288.7E2E@geocities.com>,
Kian Maleki  <magus2@geocities.com> wrote:
>Why is it that scientists everywhere have the same idea of life? Must
>breathe oxygen. Needs water to survive. Must be carbon based...

For one thing, as others have pointed out, this is the only form of life
we know anything about.  Discussion about other possibilities tends to
devolve into vague hand-waving.

For another, even more important, there seem to be good reasons why this
is rather the most likely form of life.  Oxygen, water, and carbon have
fundamental advantages that other choices lack.

Hydrogen, carbon, and oxygen are very common elements, for quite
fundamental reasons:  hydrogen is what the universe started with, the next
step in nucleosynthesis is helium (of no biological interest because it's
so inert), and after that we get carbon, and next nitrogen and oxygen.
Everything else comes later, and generally in smaller quantities.

To get complex reactions of large molecules occurring at a rapid rate,
they pretty much have to take place in a liquid.  For complex living
things to survive, the liquid has to stay liquid over a reasonably wide
range of temperatures, because the planet's temperature is going to vary
some.  This is fairly rare; most substances have quite narrow liquid
ranges.  To get a wide liquid range you need hydrogen bonding, and few
stable liquids made of common elements will do that.  Water and ammonia
are about the only possibilities.  Ammonia is not out of the question,
although almost any planet which has ammonia in quantity will have water
in much larger quantity.

To get complex molecules at all, their backbone has to be made of an
element which will bond to itself and to other common elements easily but
with limited enthusiasm -- you want complex molecules to form readily, but
you also want them to react with each other readily, so the bonds can't be
too strong.  Carbon will do this.  Pretty much nothing else will.  In
particular, silicon won't bond with itself at all readily; it's not really
all that similar to carbon.

And to get a concentrated energy source, suitable for animal life, in a
carbon/water environment, reacting carbon/hydrogen compounds with oxygen
is a good bet, and none of the alternatives work at all well.  In
particular, the various alternatives to oxygen are too rare or too
(un)reactive or both.

Life based on some other chemistry is not utterly impossible, but unless
some key issue has been overlooked, it's going to be rare.

>How do we
>know that 30 light-years away, on some planet that's made up of hydrogen
>with a small iron core, there isn't a form of sentient life that can
>breath pure hydrogen, and can float in the atmosphere from the moment it
>is born?

How does it float?  Whatever it's made of is heavier than hydrogen.

What does it use for an energy supply?  Remember, whatever it uses has
to work at night, so direct solar power is out.

What does it use for a solvent?  Water?  Ammonia?

What were the evolutionary pressures that made it intelligent?  Human
intelligence seems to have been driven very strongly by hands and tools.
Without a solid surface, where do the tools come from?

Perhaps it could happen.  But more conventional life is a better bet.
--
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |   henry@zoo.toronto.edu