Albert Einstein once famously wondered whether God had a choice in how he created the universe. His unanswered question drives physics to this day.
The same question could be asked about the biological universe -- especially now that the rover Opportunity has found signs of ancient standing water on Mars.
NASA's search for alien life is based on the strategy "follow the water," and for obvious reasons.
The only life we know is built on a scaffolding of carbon that floats in bags of water. Bacteria or brontosaurus, we're all made from the same basic recipe.
But did life have a choice? Could it have evolved from entirely different ingredients? In looking for water-based life in worlds beyond, are we making the mistake of peering into a mirror?
Why not life in ethanol? suggested Cornell University's Roald Hoffmann, a Nobel laureate in chemistry. Or ammonia?
"Now life in liquid ammonia, that would be colorful," said Hoffmann, explaining that metals can dissolve in ammonia, "giving bright blue solutions."
And why does the scaffolding have to be carbon?
Why not silicon, its neighbor on the periodic table of elements?
"We're so dumb about what life is because we only have one example," said astrobiologist Chris McKay of NASA Ames Research Center at Moffett Field, near the Bay Area city of Mountain View. "It may be true that we sail through the universe and everything we find is carbon and water, but I would hesitate to conclude that based on the one example we have."
As a practical matter, NASA's strategy of following the water makes good sense.
"We don't know how to do anything better," McKay said. "We're too stupid to look for things if we don't know what they are."
At $820 million, the twin rover missions have to look at what's most likely. "If you had to bet, what would you bet on?" asked Stanford chemist Richard Zare.
Still, one has to wonder what else might be out there.
The search is complicated by the fact that scientists aren't even sure what life is exactly. Bizarre new species are discovered on Earth all the time in the most unlikely places.
"We even have trouble understanding what's alive and what's dead," Zare said. "People still wonder what a virus is."
All life as we know it is spun from carbon-based threads swimming in water solutions. Both carbon and water have unique -- some say magical -- properties. Indeed, physics and chemistry strongly suggest that life might not have had a choice.
Water is the most eccentric of liquids. "It's this elusive, magical, mystery molecule," said James Garvin, lead scientist for the Mars exploration program at NASA headquarters in Washington.
On the face of it, water seems a rather silly molecule -- two hydrogen atoms attached to an oxygen atom in a way that looks like the head of Mickey Mouse. Even children know its chemical formula: H2O.
But the bonds it forms with itself and other molecules are anything but ordinary.
Atoms normally bond by sharing the negatively charged electrons that buzz around their positively charged nuclei, like people sharing popcorn at a movie.
In water, the oxygen shares one electron with each of its hydrogens, leaving four extras. These clump together as "lone pairs" that can grab onto other molecules like prehensile feet.
At the same time, the two positive hydrogen nuclei stick out the other side like arms. The "feet" of one water molecule grab the "arms" of the other, forming abnormally strong networks. Where one water molecule goes, the others tend to follow. Thus, water can climb tall trees -- hand over foot, as it were -- in defiance of gravity, carrying nutrients from the soil to the leaves.
Chemists say they would expect water to be a gas at room temperature because it's made up of just a few light atoms. But the strong bonds make the molecules stick together in a liquid form.
Luckily, the bonds aren't so sticky that they form a viscous gel -- something that Boston University physicist Eugene Stanley initially found perplexing. Water flows freely, he and others discovered, because water molecules stick to each other only briefly, let go, grab another partner -- whirling an ever-changing cast of partners around in a molecular square dance.
The upshot is that water stays watery over a remarkable range of temperatures (32 to 212 degrees Fahrenheit, to be exact).
This is a liquid bonanza for life, which seems to need some form of fluid to transport things from place to place. In solids, molecules stick together and can't go much of anywhere. In gases, the molecules don't get close enough to interact.
Water's unbalanced geometry -- positive charges on one side, negative on the other -- also gives it a distinctively schizophrenic personality (although chemists, like psychiatrists, prefer the term bipolar). This makes it an excellent solvent.