Life's Little Essential by Peter Tyson
Everybody knows that liquid water is necessary
for life, at least as we know
it. But just why
exactly?
A friend of mine once had a poster on his office wall that asked at the top in
big letters "WHY IS THE SKY BLUE?" I first saw the poster from a distance, and
my initial reaction was to snicker slightly, thinking "Everybody knows why the
sky is blue." The rest of the poster proved to be the perfect rejoinder, for
beneath that simple question lay row upon row of complex equations, originally
published by Albert Einstein in 1911, that described in mathematical terms
precisely why the sky is blue.
When I looked beneath the surface of it, the question that opens this article
elicited a similar effect: I was surprised by how much I didn't know about why
water is thought necessary for life. Once I learned the particulars, it became clear
why planetary scientists on the lookout for life on Mars and elsewhere in the
solar system are on the lookout for water.
Of the essence
So why is liquid water the sine qua non of life as we know it? Liquid
water may sound redundant, but planetary scientists insist on using the
qualifier, for solid or vaporous water won't do. The biochemical reactions that
sustain life need a fluid in order to operate. In a liquid, molecules can
dissolve and chemical reactions occur. And because a liquid is always in flux,
it effectively conveys vital substances like metabolites and nutrients from one
place to another, whether it's around a cell, an organism, an ecosystem, or a
planet. Getting molecules where they need to go is difficult within a solid and
all too easy within a gas—vapor-based life would go all to pieces.
And why is water the best liquid to do the job?
For one thing, it dissolves just about anything. "Water is probably the best
solvent in the universe," says Jeffrey Bada, a planetary scientist at the
Scripps Institution of Oceanography in La Jolla, Calif. "Everything is soluble
in water to some degree." Even gold is somewhat soluble in seawater. (Before
you get any ideas about extracting gold from the oceans, I should add
that, according to Bada, the value of dissolved gold in a metric ton of
seawater comes to about $0.0000004).
Water plays another key role in the biochemistry of life: bending enzymes.
Enzymes are proteins that catalyze chemical reactions, making them occur much
faster than they otherwise would. To do their handiwork, enzymes must take on a
specific three-dimensional shape. Never mind how, but it is water molecules
that facilitate this.
Black sheep of the liquids
Water's ability to so successfully further the processes of life has a lot to do
with just how unusual a fluid it is. Not long ago, if I had to guess, I would
have said that water is about as normal a liquid as they come. In fact, despite
its ubiquity and molecular simplicity, H2O is abnormal in the
extreme.
For starters, while other substances form liquids, precious few do so under the
conditions of temperature and pressure that prevail on our planet's surface. In
fact, next to mercury and liquid ammonia, water is our only naturally occurring
inorganic liquid, the only one not arising from organic growth. It is also the
only chemical compound that occurs naturally on Earth's surface in all three
physical states: solid, liquid, and gas. Good thing, otherwise the hydrological
cycle that most living things rely on to ferry water from the oceans to the
land and back again would not exist. As science journalist Philip Ball writes
in his informative book Life's Matrix: A Biography of Water, "This cycle
of evaporation and condensation has come to seem so perfectly natural that we
never think to remark on why no other substances display such
transformations."
Liquid water is still the Holy Grail for planetary scientists.
Compared to most other liquids, water also has an extremely large liquid range.
Pure water freezes at 0°C (32°F) and boils at 100°C
(212°F). Add salt and you can lower the freezing temperature; natural
brines are known with freezing points below -50°F. Add pressure and you
can raise the boiling temperature; deep-sea vent waters can reach over
650°F. Water also has one of the highest specific heats of any substance
known, meaning it takes a lot of energy to raise the temperature of water even
a few degrees.
Water's broad liquid range and high heat capacity are good things, too. They
mean that temperatures on the Earth's surface, which is more than two-thirds
water, can undergo extreme variations—between night and day, say, or between
seasons—without its water freezing or boiling away, events that would throw
a big wrench into life as we know it. As it is, the oceans serve as a powerful
moderating influence on the world's climate.
Liquid water has yet another unusual property that means the difference between
life and essentially no life in cold regions of the planet. Unlike most other
liquids when they freeze, water expands and becomes less dense. Most other
frozen liquids are denser than their melted selves and thus sink. If it sank,
ice, being unable to melt because of the insulating layer of water above it,
would slowly fill up lakes and oceans in cold climates, making sea life in
those parts of the world a challenging prospect.
Waterless life
Could life as we don't know it have gotten a start without water? Some
planetary scientists have suggested that on certain very cold planetary bodies
liquid ammonia might serve in place of water to incubate life. But even though
it's the most common non-aqueous solvent, liquid ammonia would seem to have
several other things going against it as a medium for life. Its liquid range is
small, only about 30 degrees. Also, when it freezes, it sinks, and we know
what that would do.
Some have suggested that oceans of methane or other hydrocarbons on places like
Saturn's moon Titan could also serve the purpose. But, again, we're talking
temperatures so low that chemical reactions as we know them could only proceed
at a glacial pace. "At minus 150 degrees," says Bada, "most of the reactions
that we think about in terms of being important in the origin of life probably
wouldn't take place over the entire age of the solar system." Moreover,
compounds like amino acids and DNA would not be soluble in these other liquids.
"They would just be globs of gunk," Bada says.
For these and other reasons, liquid water is still the Holy Grail for planetary
scientists, who, based on what they know today, consider it likely that liquid
water is essential to all life, terrestrial and extra-. Says Neil de
Grasse Tyson, an astrophysicist and director of the Hayden Planetarium at the
American Museum of Natural History, "Given that life on Earth is so
dependent on water, and given that water is so prevalent in the universe, we
don't feel that we're going out on a limb to say that life would require liquid
water."
Just as a blue sky requires blue light to scatter far more than all the other
colors in the visible light spectrum—which, of course, is why the sky is
blue. Well, that's the simple answer anyway.
