Every known form of life on earth, from the largest mammals to the smallest
microbes, relies on water. Why? Because water is an extraordinarily versatile
molecule - it's the perfect liquid medium in which to dissolve nutrients for
ingestion or wastes for excretion, to transport important chemicals or even be
used as one. Water has two particular physical properties that are unique among
natural molecules: it remains liquid over an extremely broad range of
temperatures, and it decreases in density when converted to solid phase
(frozen). While this may seem a relatively minor point, its consequences (that
ice floats) are critical to the evolution of life. If ice were more dense than
water and the earth cooled slightly, ice formed on the oceans would sink and
push the already cold water from the bottom to the surface, where it too would
freeze and sink, repeating the cycle until all water on the planet was frozen.
Not all scientists believe that the presence of water is "concrete" evidence of
life, but liquid water certainly improves the likelihood of life taking hold and
finding a hospitable environment. This should not be confused with ice, however,
which we know is present in many planets and moons in the solar system. Remember
that ice may not be only frozen water, but perhaps vapor from other gases - in
either case not as conducive to life. check out the following websites:
http://spacescience.com/headlines/y2000/ast22jun_2.htm?list (recent news on
Martian water discoveries) http://quest.arc.nasa.gov/mars/ (great site about
Mars) Finally, take a look at Bruce Jakosky’s book The Search for Life on Other
Planets, Cambridge University Press, 1998. He devotes a whole chapter to
understanding how and why scientists believe water may be an essential
requirement for life anywhere.
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