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.
