Seeking Life as We Know It

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.