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.
Liquid water is still the Holy Grail for planetary scientists.
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."
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.
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.